Depression: The Olfactory Perspective

Depressed people have worse sense of smell, and people with worse sense of smell are more likely to get depressed. Kohli 2016 tries to figure out what’s going on.

They review six studies testing how well depressed people can smell things. Most use something called “The Sniffin’ Sticks Test” (really!) where people are asked to say which of two sticks has an odor; the strength of the odorous one is then decreased until the subject can no longer consistently get it right. This determines olfactory threshold – how sensitive the subject’s smell is. Depressed subjects did marginally (but significantly) worse on this test than controls (6.31 ± 1.38 vs. 6.78 ± 0.88; P = 0.0005) – I think this corresponds to an effect size of about 0.2. They also do a couple more tests to see if depressed people are worse at identifying odors and get similarly small results. Also, some neuroimaging studies directly correlate depression and olfactory bulb volume, and find that olfactory areas of depressed people’s brains shrink.

Next they look at three studies of people who have various known respiratory diseases that impair sense of smell, and see how many of them are depressed. The answer: lots! Normosmics (people with a normal sense of smell) have an average score of 5 on the Beck Depression Inventory. Anosmics (people with no sense of smell) have an average score of 14. Hyposmics (people with weak sense of smell) are in between. This seemed true independent of respiratory disease level (ie it’s not just that having a worse respiratory disease makes your smell worse and makes you more depressed).

The two most common diseases studied were chronic rhinosinusitis and post-upper respiratory infection olfactory dysfunction. One of the studies, Jung, Lee, & Park (2014), compares them. They find that 40% of CRS patients are depressed, vs. 76% of PURIOD patients, a significant difference (also, both much higher than the ~5-10% of depressives in the general population). They note that CRS involves a gradual loss of smell, and PURIOD a sudden loss of smell, and that maybe people adjust to gradual loss better than sudden.

We can easily come up with alternative hypotheses. You can never actually control for confounders properly, and having a respiratory disease sounds depressing. Also, the sorts of people who have respiratory diseases probably differ from the rest of the population in various ways. Biologically, they might have worse immune systems, or end up with worse oxygenation and chronic infections, or just be generally sickly. Sociologically, they might be poorer, or have worse diets, or more comorbidities. Sinus problems themselves are linked to depression for a bunch of reasons, probably relating to the sinus being so closely linked to the brain that sinus inflammation screws up your brain chemistry. Also, most chronic diseases have high depression rates – sickle cell anemia (chosen kind of at random) is 44%.

There really aren’t that many causes of anosmia that are 100% exogenous and have no chance at all of contributing to depression themselves. But animal researchers can take Gandhi’s advice and become the exogenous cause they wish to see in the world. Yuan and Slotnick (2014) discuss experiments where researchers remove the olfactory bulbs of rats. These rats tend to consistently become depressed. That seems like pretty strong evidence to me.

So why would depression reduce olfactory acuity? And why would reduced olfaction cause depression?

Maybe depression reduces olfactory acuity because it reduces sensory acuity in general. We already know that depression decreases visual contrast, causing the world to literally look washed-out and gray. If I’m reading this study right, it claims depression also decreases auditory threshold – ie depressed people are less able to hear very quiet sounds. I can’t find similarly good studies on taste or touch, although they should be easy to do. If anyone knows any studies on eg two-point discrimination in depressed vs. non-depressed subjects, let me know. If not, this would be an easy project that a college psychology student could do in a few weeks, and it would add to our understanding of this subject. But right now I think the evidence is consistent with a general decline in perceptual abilities. This fits my understanding of depression as a systemic disorder – the low mood is the most noticeable symptom, but you’re also getting everything from poor digestion to worse hearing. Probably this is because whatever is affecting the emotional centers of the brain is affecting the lower parts of the brain (and nervous system?) too.

Is the opposite of this true? Does any form of decreased sensory acuity cause depression? I’m leaning toward no. This study claims to find that myopia is linked to depression, but the association is so weak that I take it as a strike against the hypothesis. These two studies find hearing loss is associated with depression, but it’s still pretty weak and it could easily be because poor hearing hurts your opportunities to socialize. I’m most struck by discussion of monochromacy, ie total colorblindness, which absolutely fail to mention depression at all. If I’m going to be splitting hairs about how depressed people see slightly duller colors, the lack of any excess depression in people without color vision seems pretty important. Blind and deaf people have pretty high rates of depression, but being blind or deaf is really hard and I don’t want to draw too many conclusions there. Overall it doesn’t seem to me like decreased sensory acuity causes depression full stop, though I could be wrong. This would mean that loss of smell is unique in its emotional effects.

The sense of smell is pretty emotionally salient. Everyone always says that “smell is the sense most linked to memory” – though I can’t figure out exactly which study discovered it and whether it considered (for example) how reliably seeing a picture of my mother reminds me of my mother. Body odor seems to be closely linked to who we’re attracted to. Smell is responsible for all taste sensation beyond sweet/salty/sour/bitter/umami, and eating food is one of life’s most visceral pleasures. From

Anosmia sufferers often talk of feeling isolated and cut-off from the world around them, and experiencing a ‘blunting’ of the emotions. Smell loss can affect one’s ability to form and maintain close personal relationships and can lead to depression. An important issue here is the fact that smell loss is invisible to all but the patient; how would you know that you had met an anosmia sufferer unless they themselves told you? This is one of the reasons, alongside the general lack of understanding of the impact that smell has on our lives, why anosmia has never received much attention – you really do not know what you have got until it is gone.

But also, Yuan and Slotnick’s rat paper gives a more biological explanation. The olfactory bulb is the beginning of pathways that stimulate many other parts of the brain. When it’s removed (and presumably also when it just never gets any incoming stimuli) it stops doing that, and the downstream parts of the brain shrink. For some reason this also causes decreased brain-wide synthesis of serotonin, maybe because the olfactory bulb is a net positive stimulus on the raphe nuclei. Sure, sounds like the kind of thing that might cause depression.

What does this imply about treating depression?

SSRI antidepressants probably decrease sense of smell as an immediate side effect. Awkward. But they seem to improve sense of smell long term as part of their general treatment of depression. In studies fluoxetine (Prozac) causes nerve growth in the olfactory bulb after a few weeks. Does that suggest a story where SSRIs work by improving smell? Probably not – more likely they work by [a cascading system of effects involving] causing nerve growth more generally, and the olfactory bulb benefits along with everything else.

Could you treat depression through improving olfactory sensitivity? Maybe, but I have no idea how to do that. Yuan and Slotnick suggest directly stimulating the olfactory regions of the brain, but this is pretty invasive, and there are probably already better treatments for the small minority of patients who are going to let you directly stimulate brain regions. Maybe before we worry about this problem, we should investigate the more general question of sensory enhancement for depression. What would happen if you made people wear glasses that enhanced the color saturation of everything they saw?

Finally, what about just exposing depressed people to really strong smells? You will be excited to know that real scientists have studied this ridiculous idea, and it seems promising, at least in extremely sketchy experiments on mice (1, 2, 3). I have never seen any studies done on humans unless this is actually how “aromatherapy” works, which would be hilarious. Aromatherapy seems to get positive results for depression with the same kind of bad studies that let all quackery generate positive results for everything. I can’t say more than that and I’m pretty skeptical here.

This entry was posted in Uncategorized and tagged . Bookmark the permalink.

100 Responses to Depression: The Olfactory Perspective

  1. a real dog says:

    Wait, you might be on to something.

    I have personal experience with atypical depression. Depresssive episodes are perceived in the part of the mind responsible for “telling the story” – the thing meditation is supposed to help you quiet down. Consciousness seems to travel on a spectrum between storytelling and sensory immersion during daily activity, and – this is important – for me it is NOT POSSIBLE to be immersed in perception and feel depressed simultaneously!

    Kinesthethic perception works best for this – I pretty much avoided killing myself once by moving my hand, spontaneously focused on the perception of movement, and realized that I actually have free will and can act from “non-diseased” part of my mind, whatever that is.

    Now, the long-term depressive state will return once you lose focus, but mental states seem to be feedback loops where they slowly fade out if you stop reinforcing them, so this is actually sort of useful as a coping tactic.

    So do the various non-CBT depression therapies (mindfulness-based, behavioral activation…) simply force you into the sensory state often enough that it shifts your baseline, and you stop falling deep into depressive feedback loops? What about Japanese forest bathing? Can we use a shortcut and sign up all the depressed people for whatever strong sensory inputs break their mental filters, aromatherapy, massages, make them learn skills that force them to use their senses (e.g. rock climbing for kinesthethics)?

    Also: self-mutilation. Teens worldwide have been doing this sensory focusing thing without even being told about it. This is REALLY suspicious.

    Is the rising tide of depression in the modern world caused by people being understimulated as they sit in a building and read from a screen all day?

    On the other hand, we’re working off research on mice. Does a mouse have an analog of the human’s non-sensory, storytelling mind state? Are mice even valid models for depression in the first place?

    • Error says:

      Now, the long-term depressive state will return once you lose focus, but mental states seem to be feedback loops where they slowly fade out if you stop reinforcing them, so this is actually sort of useful as a coping tactic.

      I wonder to what extent this might work on other mental states. I don’t have depression (anymore), but I do have other forms of…”brainlock,” I suppose you could say…that I could do without. Feels kind of like being trapped on tvtropes, but with a train of thought instead of a website. Or sometimes both.

  2. noyann says:

    We already know that depression decreases visual contrast, causing the world to literally look washed-out and gray.

    Could it also be the reverse causation? That could explain seasonal affective disorder.

    Does any form of decreased sensory acuity cause depression? I’m leaning toward no.
    To cross-check the idea — is sensory acuity increased in manic patients? Does it change in bipolar disorder?

    • That could explain seasonal affective disorder.

      Potential complication: SAD exists both in winter and summer forms, although the summer form is much less common.

      Slight bit more on this: I have summer-SAD. As far as I can tell, the problem is that there is too much light (i.e. there is not enough atmosphere between the sun and me, due to the angle) – it feels like it drowns out my thoughts – and a secondary problem is that my metabolism is prone to easily overheating. B12 supplements have helped me counter former effect a little bit, making me no longer dimly suicidal during summer months, ‘just’ unhappier and less productive. I try to hide in a cave as much as possible during the months May, June, July and August, but it only helps so much. (Just to clarify, though, there’s no need to worry about me, I anticipate I’ll be fine, I’ve got plenty coping mechanisms as this point. I’m sharing this purely for data; no pity required. I promise I’ve got a handle on it!)

      • noyann says:

        I have the same, except that brightness of mood correlates with brightness levels. No sadness in summer but hyper-vigilance and a state of ‘too much’, everything is too intense (sensory, emotional, stresses of all kinds, cognitive load); I curb some of it with H1-antihistaminics, shady indoors, quiet environment. Winter feels like hibernation: sofa, carbohydrates, little exercise, a certain dullness.

        Tha B12 helps is new to me, maybe B12 is why I sometimes feel better after eating meat; I’ll give some tablets a try. Thank you for that!

        • babarganesh says:

          i definitely get sensory overload in the summer sun, which makes me more uncomfortable all around. the thing is i LIKE the experience of seeing bright light a lot and i like the heat. but it’s overwhelming.

          i wouldn’t call it depression either. more like the wires are oversignaling and all my filters are uncoordinated. my tinnitus gets worse!

      • Purplehermann says:

        Why don’t those affected by summer SAD just wear sunglasses?

        • Majuscule says:

          I’ve wondered for years if sunglasses might have something to do with rising rates of depression from the mid-20th century to now. Sunglasses have existed for centuries, but only in the last 50 years or so have they become ubiquitous. I wear mine a lot, but I’ve started taking them off periodically because I feel like seeing the true color spectrum has an immediate effect on my mental clarity and overall wellbeing.

        • noyann says:

          Maybe it’s really more than the photon flux. More qualia getting too much input?

    • JRG says:

      I have lost count of the number of people prone to mania that I work with who do really well on lithium but complain that they hate it because colors seem less vibrant. I have not heard comments about olfactory acuity, possibly because what people tend to notice if anything is a weird metallic taste.

      People having intense perceptual experiences at high rate for transition to psychosis (i.e. “prodromal schizophrenia”, not that schizophrenia is really a thing) often talk about all of their perceptual experiences being much more intense than usual. It often goes along with intense anxiety and agitation but sometimes a global, constant, perpetual bewilderment (i.e. “perplexity”) and sense of everything being so richly textured and interconnected as to surpass all understanding. This is not the same as bipolar disorder obviously but we now tend to think about people having these experiences as being in a pluripotent risk state. It could break in a number of ways, basically, and sometimes it leads to bipolar I, often a psychotic bipolar I.

      This all makes me think of older dopaminergic theories of depression and how some people only really seem to respond to medications that affect dopamine in some way and a cursory search suggests the olfactory bulb has many dopamine receptors that play a significant role in odor processing, at least in rat.

      The trouble with linking depression and any other kind of finding is that “depression” isn’t really a thing but a vague label for many different states, probably with wildly different etiologies, and often without very similar subjective/phenomenological experiences. Once we decided to lump together everyone who was feeling low and didn’t enjoy things as much for a prolonged period of time, psychiatry set off down an unproductive road.

      • noyann says:

        People having intense perceptual experiences at high rate for transition to psychosis (i.e. “prodromal schizophrenia”, not that schizophrenia is really a thing) …

        How would you unpack (or define or describe) what is usually labelled ‘schizophrenia’?

        …often talk about all of their perceptual experiences being much more intense than usual.

        Should I be worried?

        • JRG says:

          I would describe it as a loose cluster of syndromes or patterns of intense, unusual experience typically comprising alterations in sensory perception at variance with consensus reality, strongly held beliefs that are difficult to justify with publically available information alone, a leaping-to-conclusions cognitive style, and difficulty generating linear or globally coherent narrative reports. They all also in some way significantly impact or impair the attempts of the people in question to pursue normative occupational and social goals or carry out activities of daily living.

          Shorter version, we can talk about schizophrenia spectrum conditions or primary psychotic conditions but “schizophrenia” is not a unitary phenomenon.

          Not going to give medical advice, but I will say if more intense colors are the only thing about all that that strikes you as resonant with your own experience I don’t think it conveys all that much information per se

      • a real dog says:

        perpetual bewilderment (i.e. “perplexity”) and sense of everything being so richly textured and interconnected as to surpass all understanding.

        Sounds like the psychedelic experience, with psylocybin in particular. Compare with Huxley’s Doors of Perception (mescaline).

  3. chaosmage says:

    A friend of mine lost his sense of smell in a pretty horrific car crash. He cannot be the only one, since the nose is right in front and first to go. Maybe by talking to cranial and cosmetic surgeons it would be possible to get a big-enough sample of such people, and compare them to others who also had horrific car crashes but did not lose their sense of smell?

    • Decorinius says:

      Seems like traumatic brain injury would be an impossible to remove confound, so I’m not sure how useful that would be. If you are in a car crash severe enough that you permanently lose your sense of smell, it could easily have caused permanent depression due to trauma – mental or physical – unrelated to the loss of smell.

      • clipmaker says:

        There is a known type of injury sometimes seen in car crashes, that kills the sense of smell. Some bundle of nerves inside the nose gets sheared off. I saw a diagram of it as explained by an ENT. I unfortunately don’t remember the specifics.

  4. Capt. Adama says:

    Interesting. The moment I read the title of this post I assumed memory stimulation would be the primary driver, but the biological hypothesis is quite neat.

    My experience with this: my wife once went to great lengths to identify and locate the source of the fragrance that permeates certain Disney hotel lobbies. Not long after succeeding, our home began to smell a lot like the lobby of the Grand Floridian, and, I have to say, it had a substantial effect on the spirits (good family memories there).

    As for the importance of fragrance/smell in general, my guess is that Barnes and Noble would be considerably less successful if the aromatic combination of paper and coffee wasn’t so widely appealing.

    • Deiseach says:

      Coffee is definitely something that smells a heck of a lot more enticing and appealing than it tastes.

      • arch1 says:

        Paper, too.

      • webnaut says:

        Glad to see you’re back Deiseach!

        I will be in bread production mode soon. I hear the scent of freshly baked bread is compelling.

      • Beans says:

        This is what I thought before discovering what varieties I like. In particular, common dark roasts smell fine to me but taste like cigarette butts. Light-to-medium roasts are more likely to taste how they smell and avoid the acrid bitterness, I find.

      • AC Harper says:

        Although no longer so acceptable culturally I always found the smell of a (good) cigar more appealing than the taste.

      • Gerry Quinn says:

        I love the smell of real coffee roasting. But what I like to drink is instant. (I’ll take a cappuccino too, but I suspect the flavour of the actual coffee is pretty much drowned out in that.)

        Strawberries are another thing that smell better than they taste.

        • Bugmaster says:

          @Gerry Quinn:
          If you’ve never tried wild strawberries (they are little oval-shaped berries about the size of your pinkie fingernail), you’re missing out. The taste is absolutely amazing.

      • Robin says:

        I once dug out dandelion roots, which is fun in itself, because it made me feel like a herbalist witch.
        Then I roasted them, and the smell is absolutely gorgeous! It kind of smells like chocolate.
        Finally, I made dandelion root coffee, and I’m sad to say that it tasted only bitter, nothing else.
        But I’m tempted to dig out a few more roots, just to smell those roasting roots again. And to feel like a herbalist witch.

    • Error says:

      I just looked for a book-smell air freshener cartridge. Nobody seems to make one, but I do see candles for it…hrm.

    • sharper13 says:

      You can’t go with that story and then not tell us the scent she found!

      • Capt. Adama says:

        My apologies, you’re right.

        I think the consensus is that Disney used/uses a system called Enviroscent, and that their go-to fragrance at hotels is primarily green clover and aloe. In fact, there are a few Disney-themed candles that go with that combo. If memory serves, my wife went with “Walt’s Wonderful World” from Columbia Fragrance Co. It’s pretty spot on to me.

  5. Well... says:

    There’ve been times where I injured my mouth or tongue in a way that muted my ability to taste for a few weeks, and it definitely turned my world a bit gray for that time. I’ve always said that permanently losing my ability to enjoy food would be a huge blow to the value I derive from living. Taste is closely related to smell…

  6. convie says:

    Any chance smoking could be a confounder?

    • sflicht says:

      I have been a smoker and I currently have covid-induced anosmia. I can tell you that actual anosmia is night-and-day different from other things that people commonly associate with reduced sense of smell. I am not even sure that smoking even affects the sense of smell in a genuine way (although it certainly seems plausible that smokers have qualitatively different olfactory preferences than non-smokers, insofar as they likely find the scent of tobacco less offensive). Certainly what I’ve experienced with covid is nothing at all like “stuffy nose making smells less potent” or “lingering tobacco smoke blocking other smells”. It’s really a question of the olfactory nerve cells just not firing. (As my smell has started to return, I’ve experienced a “smell and taste extinction” phenomenon, where my presumably reduced population of functional olfactory nerve cells uses up their mojo quickly — so I can smell coffee if I stick my nose in it at first, but then it just stops working again.)

      • Alethenous says:

        I have suffered the same thing and I can affirm that it’s a remarkably different sensation from the sort of reduced or eliminated smell one experiences during, for example, a cold. At its worst I could smell literally nothing, although my airways were fairly clear (no mucus but noticeable inflammation and mild pain); it’s been five weeks since onset and my sense of smell is maybe halfway back to normal. n=1, it did severely negatively affect my mood, and I’m extremely glad it doesn’t seem to be permanent; a nasal steroid spray I acquired the other day seems to be helping.

      • Sp0keShave says:

        Smoking definately affects sensitivity. As as ex-smoker i can detect cigarettes from a long way away, Much further than when I used to smoke, or before I smoked.

        • sflicht says:

          I think there are psychological factors at play that affect the threshold for scent discernibility. Clearly attention pays some role, and clearly tobacco aroma has powerful emotional connotations that differ for current and ex-smokers, which I’d imagine influences one’s attentiveness.

  7. alwhite says:

    The peanut butter test has been used to detect Alzheimer’s for a few years now.

    • rocoulm says:

      Is this taken seriously in the field? A couple paragraphs into your article and it says there’s only one small study in which it worked, and it failed to replicate after that…

  8. relative-energy says:

    Do we have evidence that lets us rule out this hypothesis: depressed people lack the motivation to strain to hear/see/smell differences between two very similar things?

    Suppose I’m participating in a study on this topic, and I’m having a good day. I’ve just had some coffee, and I’m firing on all cylinders. The researcher plays two subtly different tones, and although I can’t quite articulate why they seem different, I make an effort and really try to listen for something distinguishing.

    Now suppose I’m tired, not having a good day, and not looking forward to doing this study. The researcher gives me two sticks that are supposed to smell like different things. “Eh, they’re both cinnamon or whatever,” I say. “Can I go home?”

    • Anteros says:

      I reacted with a similar thought to this. When I’ve been depressed, the effort to make subtle discriminations seems huge and I would take whatever easy route I could find to avoid them. And yes – ‘Can I go home now?’

      • rocoulm says:

        This doesn’t explain the prevalence of depression in Anosmics, does it?
        “Eh, I just don’t feel like smelling anything this lifetime…”

    • JRG says:

      It’s a valid point, but usually in these sorts of discrimination tasks people are not being asked to explain anything or even supply verbal labels. They just press a button to indicate “same” or “different” or, in the stick experiment example, saying “A” or “B” is the stick with the odor. There are definitely measures you could use to assess effort or at least the degree to which people are foveating on the relevant stimuli if you were really concerned about this.

      Also, remember, people who are depressed in these studies are people who, when asked by strangers whether they wanted to sniff stuff for an hour and answer stupid questions about them, said “sure, let’s do it.” Either they’re outpatients, in which case they have the motivation to show up in the first place, or they are inpatient, in which case they have the motivation to do something other than lie in bed all day.

  9. BlindKungFuMaster says:

    I find it probable that loss of smell leads to depression in rats because smell is super-important to rats in a way that can’t really be compensated. So I don’t know how evidence from mice or rats should be weighted.

  10. onyomi says:

    Random observation: fasting, even for just 24 hrs, makes my sense of smell temporarily stronger (subjectively, at least). This is not surprising; presumably your body upregulates whatever it is when you’re hungry to increase your incentive to find food. But maybe in the process it upregulates everything, or at least more than one thing sense-acuity related? I feel like colors also sometimes seem brighter, sounds clearer, etc. when I’m fasting and for a couple days after. I also think periodic fasting has a positive effect on my mood, in part because it makes me feel more “in touch” with the world, which seems to be sort of the opposite of depression on some level.

    • Beans says:

      I’ve also experienced something of this sort. After not eating for enough time, hunger passes and it feels like something kicks in that makes me very “with it” and ready to do things. As you say, I figured this must be a mechanism that arose to facilitate food-getting rather than laying around starving.

    • a real dog says:

      Yeah, the smell amplification of fasting is insane. After three days I was smelling things like having my face right in them, despite standing three meters away.

      My friend in similar circumstances smelled some guys’ chips from across the street.

  11. noahmotion says:

    Those Kohli results strike me as a nice illustration of a set of effects that are statistically significant and real-world meaningless. In the quoted example – 6.31 (1.38) vs 6.78 (0.88) – the two groups are less than half a standard deviation different on average. The relatively large Ns make the standard error very small, though, so you get a teeny-tiny p-value.

    • @jaralaus says:

      0.43 of the pooled standard deviation to be precise. So, if Im not completely off, the effect size (measured in Cohens D) is not 0.2 but 0.43 which is not that bad (close to medium sized). Though it depends on what you want to use it for. A good way to think of an effect size is how detectable a phenomenon is. A Cohens D of 0.43 means that if we run 100 studies comparing 83 depressed individuals to 83 healthy individuals we should reach a p<0.05 in 80 of these studies (G*power).

      As a biomarker to identify depression it is thus not very useful, but compared to most other effects in psychiatry it is pretty big. It allows us to speculate about the biology behind depression which is something.

  12. TheMadMapmaker says:

    Smell is linked to taste, so I wondered if just eating more very fragrant food wouldn’t lead to less depression ? Finland has notoriously bland food, and notoriously high depression.

    However, looking at a map of spicy food:

    … and a map of depression rates:

    … shows no obvious correlation between the two, so, if there’s a relationship it’s not just intensity of taste.

    How about the idea that eating tasty food is enjoyable, so if you taste it less, you enjoy yourself less, and thus are more likely to get depressed ?

    • eric23 says:

      Spicy != tasty. In general, though, lower latitudes have more diversity of plants growing, which means more interesting tastes as well as more possibility of spicyness. But lower latitude also seems to lead to lower depression due to factors like the presence of sunlight.

      To me, the map of depression rates shows no obvious correlation to anything. I suspect every country has its own definitions and diagnostic criteria, so rates cannot meaningfully be compared between countries.

  13. Nancy Lebovitz says:

    Could magnetic stimulation be used to crank up the olfactory bulb?

  14. Deiseach says:

    unless this is actually how “aromatherapy” works, which would be hilarious

    You laugh, but – !

    I don’t know if I have a strong or weak sense of smell (I’ve never taken a test, and how do you compare yourself to other people?) but I do know when I feel depressed.

    And lately I’ve been burning a heck of a lot of scented candles. Also when I smell things I really like (like certain flower scents) or wear perfume when I’m in a certain frame of mind, it does help a bit.

    Some of that is probably merely(?) “things smell nice, this means the environment is pleasant, that’s better than a poke in the eye with a sharp stick” but yeah – this could be how aromatherapy helps; it may well be that massaging peppermint oil into your skin does Sweet Fanny Adams for the general problem of an upset stomach, but the smell helps settle your mind (be that from the association formed from “every antacid and upset stomach problem smells and tastes of peppermint”) and that has a downstream effect on the gurgling tummy.

    It really would be hilarious if all the quackspeak about how the oils have an effect is just verbiage but the folk-wisdom aspect of “lavender helps you sleep” really does work.

    • webnaut says:

      I am convinced that Hinoki oil works – it boosts production of natural killer cells.

      If you live in Dublin and the pandemic is over I rec to visit the MUJI store. They have a wonderful collection of essential oils and the world’s most perfect teaspoons. I can’t explain it – you have to visit it.

      • Lambert says:

        Seconding Muji.
        You can find them in most big cosmopolitan cities.
        I go there for all my highly minimalist japanese homeware needs.

        • webnaut says:

          I have a wall of their coloured glycerin soaps in my bathroom – brings me joy, their cutlery of course – to the uninitiated to rave on teaspoons sounds insane but you can’t understand their magnificence from images on the internet.

          I love their stainless saucepans or pots – I have been importing them from Japan through Buyee because they don’t sell them aboard – but I was not so happy with their stainless steel utensils – I have found a very similar looking but more robust version of the utensils – if you look on wayfair for a brand called vinters you’ll see them.

      • Deiseach says:

        Thank you for the scent recommendation, I will keep it in mind.

        As for spoons and saucepans, I can’t say, but I did buy a new trivet for the kitchen about six months ago and it still gives me pleasure to look at and use it 🙂

  15. Homo_erectus says:

    Interesting. My wife suffers from severe depression and it’s been a running joke between us for almost two decades that dog poop is the only thing she can smell.

  16. kulibali says:

    Also has an effect on fat metabolism?

  17. keaswaran says:

    I’ve been with my partner for 15 years, and in that entire time it’s been clear that he has a very strong sense of smell. After many years of him pointing out various smells, and distinctions among smells, I’ve definitely gotten much better at noticing and detecting these things. So I think that at least some aspect of smell sensitivity can be trained. (Though I don’t know if this actually increases the acuity of the receptors in the nose, or the transmission along the nerves to the relevant part of the brain, or if this is “only” an increase in discrimination on the awareness side, whatever this distinction might mean.) It would be interesting to see if this sort of training could be used to mitigate depression.

    Also, this podcast episode was a great set of metaphorical ruminations on the connection between smell and a sense of planning for the future:
    It was amazingly timely, in that the person must have created the episode before it became clear that we were entering a pandemic, because her examples of predicting some dreadful future all involved climate change, or work, or relationships, or a personal illness – but it clearly felt relevant to my emotional state about the pandemic on March 21, when I listened to it.

  18. jasoncrawford says:

    I have full anosmia, and as far as I know it’s congenital—I don’t remember ever smelling anything in my life. (For what it’s worth, I’ve never had depression.) My father seems to have hyposmia (reduced smell), so maybe it’s at least partially genetic.

    People who lose their sense of smell report missing it, as if color has gone out of the world. But congenital anosmiacs (CA) like myself don’t know what we’re missing. I didn’t even *know* I had full anomia until I got checked out at a clinic in my early 20s—for my entire childhood I thought I just had a “weak” sense of smell. (It turns out that when you breathe in, you can taste the air, because you have taste buds down the back of your throat, not just on your tongue. That, plus any other sensations I got when I breathed in, such as temperature, moisture, or tingling from alcohol or other irritants, are what I interpreted as smell.)

    Imagine today you found out that everyone else can sense the earth’s magnetic field: an inner compass (like maybe some birds have?) Your reaction would probably be something like, “Huh, that’s cool. No wonder I have a poor sense of direction. I guess it would be neat to have that.” And then you just sort of go back to living your life, and you memorize which way is north in your neighborhood using landmarks, and you use a compass when you need to. That’s kind of what it’s like to be diagnosed with CA.

    It would be interesting to compare CA from acquired anosmia and see if one is more associated with depression.

    PS, one very minor thing I don’t understand above is about the sticks test: I thought it was important to start with the *weakest* smells, literally undetectable, and increase gradually until the subject can smell something. It was explained to me that strong smells temporarily desensitize you, so if you want to see how sensitive someone is, you start from the bottom. (Also, for what it’s worth, I was tested with little vials of liquid instead of sticks. This was once in 2004 and multiple times around 2008.)

    • Hansi says:

      PS, one very minor thing I don’t understand above is about the sticks test: I thought it was important to start with the *weakest* smells, literally undetectable, and increase gradually until the subject can smell something. It was explained to me that strong smells temporarily desensitize you, so if you want to see how sensitive someone is, you start from the bottom.

      That’s definitely a thing I experience personally. Whenever I’ve been in an area with strong unpleasant odors, and someone new comes to the scene and tells me how bad it smells, I’m like “wth are you talking about? I don’t smell anything”. And wearing a dust mask for even a short while desensitizes my sense of smell, like it was reset to a base sensibility level.

      So yes, it makes sense to start with the weakest smell. Maybe it’s unrelated but that’s how hearing tests are conducted as well. For any frequency you start at a low volume that is steadily increased until the subject hears it and has to press a button.

  19. Maxander says:

    Snark about aromatherapy aside, it boggles my mind that this article didn’t end with a list of studies testing strong smells as a treatment for depression. It would help elucidate the whole smell/depression connection, it’s cheap to test, and no IRB committee only a very strict IRB committee would object to endangering subjects with a risk of encountering unpleasant smells.

    • Deiseach says:

      only a very strict IRB committee would object to endangering subjects with a risk of encountering unpleasant smells

      To get around the IRB, just bring your test group to a candle shop and get them to stick their noses into the Yankee Candle or Jo Malone or whatever brand jars, then record the results 🙂

  20. arch1 says:

    Scott, I’m scratching my head about your paragraph which begins

    Is the opposite of this true? Does any form of decreased sensory acuity cause depression? I’m leaning toward no.

    because later in that paragraph you seem to imply that a couple of scenarios:
    -poor hearing hurting opportunities to socialize which may in turn cause depression
    -blindness or deafness making things “really hard” which in turn causes depression
    are non-examples of this, which I would naively consider to be examples of this.

    So can you clarify what you mean by “cause” in this context? E.g. if I suddenly lose my sense of smell, would you consider any of the below pathways to be an example of that loss causing depression?:
    1) I’m depressed because losing 1.5 senses makes my ongoing life experience less enjoyable
    2) I’m depressed because I realize this loss may be permanent
    3) I’m depressed because I’m concerned that the loss may reflect an underlying serious health problem (tumor, etc.)

    • Sniffnoy says:

      I mean, in this context it’s pretty clear that in these posts he’s talking the possible phenomenon of sensory loss causing dpression via internal biological causes; that is, ways to affect the mind that don’t respect our abstraction of it as “receive inputs, make decision, send outputs”. That having one’s senses dulled makes one’s life worse goes by the usual means of “receive inputs” and so isn’t interesting here; even if “go into depression” doesn’t really fit the abstraction above, we still already know that this is a thing that can happen. The things you describe are, in this context, confounders.

      I think asking for a general definition of “cause” here is a mistake. In other contexts one might say that it causes depression by these means. It’s a question of what one cares about at the time. One could use more precise language to disambiguate but that takes time and in this context it’s pretty clear so there’s not a lot of need.

      • arch1 says:

        Thanks much Sniffnoy, sounds right. (Nit: I didn’t ask for a general definition of cause, but rather a definition of cause “in this context”).

  21. steve3920 says:

    Seems reasonably consistent with my theory that depression is a vicious cycle where a chronically understimulated midbrain creates chronically negative valence, and chronically negative valence causes the neocortex to issue weak predictions (including, in particular, sending weak signals to the midbrain!). See my discussion here (although after thinking about it more, I have a lot of edits that I would want to make to that article…).

    The implication would be that the quantity of midbrain stimulation could well be important (although not all stimulation is created equal or has the same downstream consequences), but the associated sensory acuity probably is not important. Vision information does get dumped into the midbrain (besides the visual cortex, there’s a little-known second vision system in the superior colliculus), but myopia shouldn’t matter, it’s the same amount of input whether it’s blurry or not. In fact, I believe the midbrain vision system is quite low-resolution anyway. As for monochromacy, the literature is unclear (AFAICT) on whether the midbrain vision system gets any color information at all. Massage has slight positive effects for depression (allegedly…), and I imagine it would be better if you could get massaged 24/7. Sound information certainly goes to the midbrain; does white noise help with depression? I dunno. All stimulation is not created equal, it’s possible that the midbrain just ignores white noise after a while.

    Smell and taste definitely go get processed in the midbrain, otherwise we couldn’t find smells and tastes innately pleasing or repulsive. (I think that the neocortex can’t have those kinds of innate associations, only the midbrain can.)

  22. Reasoner says:

    >Aromatherapy seems to get positive results for depression with the same kind of bad studies that let all quackery generate positive results for everything. I can’t say more than that and I’m pretty skeptical here.

    How do I tell if a study is bad

  23. AnthonyC says:

    Thank you for writing this. Really.

    I was depressed for several years. The onset was very gradual, also taking many years, to the point where I didn’t even notice the gradual loss of almost all emotions (frog, boiling water). And from the outside, no one could tell b/c I was able to keep up with a normal daily routine and keep my relationship and my job (until I almost couldn’t, and started therapy).

    What I did notice was the gradual loss of perception in multiple senses. I wouldn’t notice that the garbage needed to get taken out or the litter box needed cleaning. My cooking got worse because I kept overseasoning things. Hearing and vision were affected too. I spent years thinking it was probably a neurological problem. Then I started bupropion, and all my senses came back three days later. Not a typical response, I know, but that day I opened a jar of peanut butter, smelled it, and just started crying. Except for one nurse, no medical professional I spoke to had ever heard of such a response before.

    • BlackboardBinaryBook says:

      Not a medical professional, but definitely had that that response! It heightened all of my senses, and I think that’s a big part of why it worked so well for me.

    • Hansi says:

      Hm, I got treated for depression with Bupropion for a few weeks, but I didn’t notice any heigthened senses. But interestingly the medication didn’t help with my depression either.

  24. AC Harper says:

    I don’t know how we could test the effect of smells on depression, but you could test for visual impact.

    Two sets of people fill out the relevant tests to assess their mood. One set views a number of short films on normal computer screens, the other set views the same films on HDR computer screens (more contrast, greater saturation of colours). Both sets then redo the tests to assess their mood.

    Would a richer visual experience alter mood?

  25. J says:

    This canine study has low N but found startling results post nasal planectomy.

  26. webnaut says:

    My sense of smell sucks and I’d like a way to improve it.

  27. kai.teorn says:

    This might also be part of the how seasonal affective disorder works. Sense of smell is suppressed by low temperatures (at least for me). Winter is smellless.

  28. dariuou says:

    I’m completely anosmic, have been for as long as I can remember, so most likely congenital. I’ve experienced almost constant depression since I was around 11 years old, 25+ years. The anosmia could be a significant part of it, but there are certainly a fair number of other things contributing.

    The idea of lack of sensory stimulation being linked to depression is what really stood out to me. Strongly flavored foods affect my mood a lot, often while eating something sugary, like cookies or ice cream, the world seems brighter and I’ll feel like everything is okay. Then I run out of ice cream and the clouds descend again. I’m sure there are a lot of things that could contribute to that but the stimulation seems to be an important aspect of it. Also, sometimes I’ll get really unhappy, and have a strong craving for any sort of sensory stimulation, crunchy food that’s very strongly flavored, loud music, fast moving colorful video games, preferably all at the same time.

    This article has given me some good stuff to think about in managing this. Maybe I could find some other sort of sensory stimulation that’s more healthy than 1000 calories worth of ice cream.

    • Nancy Lebovitz says:

      I think of sweet flavors as being very good at cutting through depressive haze.

    • arch1 says:

      I hope that your management efforts get things moving in a good direction, dariuou.

    • Betty Cook says:

      My husband likes to say that kimchee is the food with the highest taste-to-calorie ratio. Would spicy food work, or does it have to be sweet?

      • dariuou says:

        That’s actually an interesting thing. When I tell people that I’m anosmic they typically say that I must really like spicy food. I obviously don’t know what it’s like for other people, but for me more spicy typically means less flavor. A lot of things that are very spicy taste fairly bland to me, nothing but uncomfortable burning. But medium level spicy things can taste pretty good. For example, I really like Tabasco Jalapeno sauce, lots of flavor, and not too spicy, but actual jalapenos have almost no flavor, just the burn.

        I usually tend to favor sugar, but I’ve never tried spicy stuff. I’ll have to try that next time I get a hankering for stimulation. Thanks for the suggestion.

  29. Rick Jones says:

    I’m an enologist of over 30 years, so olfaction is mostly what I do. I also have genetic disposition to depression from my fathers side which manifests mostly as dysthymia. I manage my condition without medication usually although I have been on medication 2-3 times for depressive episodes, fluoxetine mostly.
    My initial reaction to the post is that olfaction requires mental concentration above all else. Especially in the perfume strip test, which sounds like a type of arrangement test, I can tell you that these tests are hard and require a lot of mental focus. The second smelly sticks test sounds like an ID threshold test and these are even more difficult. Pros screw them up all the time.
    When I’m not feeling great mentally, it often find lack of mental concentration as the first sign. So Scott’s idea of depression as a “global brain syndrome” resonates with me.
    I never experienced Prozac as having any effect on my olfactory acuity. The post made me consider the possibility that I picked winemaking as a profession because it was therapeutic to my condition which was pretty cool!
    I’m interested in how you evaluate depression in mice. Sounded funny to me.

  30. SEE says:

    Hi, I’m your confounder, someone who’s been seriously depressed (partially alleviated by medication) for thirty years and who has high sensitivity to smells.

    Of course, I’m autism-spectrum, so I’m highly sensitive to other sensory input, too. And if we reasonably assume “depression” is a symptom of multiple underlying disorders, there’s perfectly possibly one form of depression connected to anosmia while others are not.

  31. sflicht says:

    Thought I would share an interesting piece by a journalist who wrote about his viral anosmia for *New Scientist* in 2005. (That page has a link to the original article at the very bottom.) The relevance to this post is that his treatment at The Taste And Smell Clinic in Washington, D.C., involved anti-psychotic medications (haloperidol).

  32. zeleza says:

    I’m more struck by the possible link to olfactory function maybe being a proxy for adult hippocampal neurogenesis?

    Both the hippocampus and the olfactory epithelium are the only brain regions that continue to produce new neurons in adulthood (though the existence of new neuron production in the hippocampus in adults is disputed). All known antidepressants increase new neuron production (neurogenic) both in the hippocampus and in the olfactory bulb (at least in mice). Neurogenesis in the hippocampus is required for antidepressant efficacy (at least in mice, again).

    Could decreased olfactory function reflect a decrease in olfactory epithelium neurogenesis that also indicates a lack of hippocampal neurogenesis, which leads to depression? Could antidepressants fix both problems at the same time? I don’t have a good theory for why damaging the olfactory epithelium/bulb would lead to depression, except that maybe the subventricular zone that produces the new neurons gets damaged or depleted? I don’t know the exact neurodevelopmental/anatomical pathways well enough to speculate on that.

    (Previous SSC discussion of adult human neurogenesis)

  33. MikeJohnson says:

    “Depression” is a concept/diagnosis that bundles a lot of things together. To drastically simplify our position, QRI conceptualizes depression as a lack of harmony in the brain, with ‘harmony’ being operationalized as consonance (vs dissonance & noise) within connectome-specific harmonic waves (CSHWs).

    Hypothesis: regular stimulation of the olfactory bulb is a small but non-trivial contributor to the maintenance of this sort of overall rhythmic activity in the brain. This would predict that hyposmics and anosmics will have more dissonant and/or noisy CSHWs than normosmics.

    Further prediction: other conditions which we would expect to ‘fuzz up’ CSHWs for similar reasons (e.g. tinnitus, inflammation, hearing loss, poor vagal tone) will also lead to affective blunting or depression.

    Further prediction #2: anosmics may get outsized protective benefits from activities that perform ‘rhythmic maintenance’ of CSHWs via alternative sensory routes: listening to music, singing, dancing, sleeping near rivers, etc.

    Connectome-specific harmonic wave (CSHW) paradigm, from Selen Atasoy / Oxford:

    QRI’s method for quantifying the ‘CDNS’ (consonance-dissonance-noise signature) of CSHWs:

    QRI’s extensions of the CSHW paradigm:

  34. Lauro says:

    The family goes to the beach. Everybody is a bit cranky. At the beach, one feels the sand, hears the noise of the sea, smells the water, feels the temperature of the sun, the sand and the water, and so on. Everybody’s mood improves immediately. (n=8)

    • eric23 says:

      In the car, everyone feels the road vibrations, hears the motor and radio, smells the sweat of the person next to them, feels the temperature of the car AC, and so on. But they’re still cranky.

  35. PeterPan says:

    There is a link between decreased adult neurogenesis and depression, and the olfactory bulb is one of the few areas of the brain where adult neurogenesis occurs.

  36. Douglas Knight says:

    “smell is the sense most linked to memory” – though I can’t figure out exactly which study discovered it

    VLGEMP (1913, 1919, 1920, 1921a, 1921b, 1922, 1923, 1925, 1927)

  37. PDV says:

    So, N=1, but I am a nearly confounder-free example.

    I am allergic to mold. It makes my nose stuff up, inhibiting my sense of smell. And, due to seriously amateur remodeling of my childhood home by the previous owners, every wall in the basement of that house was absolutely stuffed with mold, behind the drywall. We did not learn this fact until I was about 10 years old, when we got the basement professionally remodeled to usable as something other than storage space. The drywall was knocked out, the mold was discovered, dishonor was heaped upon the previous owners and upon their cow, and for the first time in my life I had a sense of smell. (This is also how I discovered I was allergic to mold.) For quite a while I hated pretty much all scents whatsoever.

    This was a couple years after I became depressed. It did not noticeably help; my depression stuck around for another decade and more.

  38. Thomas says:

    Decreased hearing might cause depression, because people are jerks. Ask someone to repeat what they mumbled and they say something completely different. What I want is the person repeat what they said, when I am facing them and paying attention. If you think it was important for me to hear, say it in a way I can hear. If you don’t, don’t get mad if I seem not to be paying attention.
    Ask someone who KNOWS you are hard of hearing to at least face you when they speak to you, or speak a bit louder than normal, and they may remember for 10 seconds. Next time they say something it’s facing away or low tones again, so eventually you simply stop trying to listen to them. Then they get mad at you. It’s endless.

    • Hansi says:

      Ugh, I feel ya. I have trouble understanding speech in noisy environments, other than that my hearing is actually pretty good. So this impairment makes me more sensitive to how people react when being asked to repeat what they said, not only when I ask them. And there’re these people who tend to be like that, who give someone a faintly dismissive look when asked to repeat what they said, and they think it’s some psychological trick or they’re being judged for what they just said, so they give a passive-aggressive answer instead of just repeating the exact words.

      Having trouble hearing in noisy environments and having experienced this behavior first hand definitely made it easier to me to weed out these individuals as… jerks. It’s the same people who will make fun of you after you repeated the exact same words when they asked you to repeat yourself, because they obviously asked in a rhetorical sense. They are totally unaware that people with this kind of invisible disability exist, or they chose to not think about it.

  39. Synonym Seven says:

    I’m most struck by discussion of monochromacy, ie total colorblindness, which absolutely fail to mention depression at all.

    People are born colorblind. You can’t get depressed by the absence of something you’ve never sensed.

  40. inkter says:

    I took Adderall for a brief time, and quit because the crashes were terrible. During them, I’d basically be depressed beyond any level I’ve experienced in real life (having had diagnosed clinical depression most of my life, I have good reference values) and one feature of that was *extreme* loss of visual contrast. My vision would become flat and colorless for the 30 minutes to 2 hours it took to recover from the Adderall crash, and I could tell the crash was ending because color seemed very literally to be seeping back into the world.

    Makes me wonder if the visual processing functions of the brain rely on some combination of chemicals which become scarce when depressed and/or crashing off a stimulant.

  41. Hansi says:

    Makes me wonder if there’s a link between self-medication with psychoactive drugs like THC or microdosed LSD for depression and anxiety, and the heightened sensory awareness these drugs cause.

  42. AJCB says:


    I went from having very strong smell to having next to none at about the age of 15. Simultaneously I started having various inflammatory auto immune symptoms (later diagnosed as Behćet’s disease). This presaged, perhaps not coincidentally, severe chronic depression.

    Since then my sense of smell is comparable to my young self having an intense cold or sinus infection. I can detect very strong scents if I get very close. Specifically; If I get within a couple inches of an aromatic candle or freshly microwaved buttery popcorn I can usually pick up a slight sensation, but I have difficulty discerning between similar smells.

    (As far as I know, loss of smell is not normally considered part of Bećhet’s symptoms, although there’s some observations pointing in that direction, see

    Like many of the comments so far, I went without noticing I couldn’t smell for longer than I would have expected. After a few years I realized while eating Thanksgiving dinner that I had forgotten what cranberries smell like, which cascaded to noticing I couldn’t smell anything I was eating, and finally realizing I couldn’t remember the last time I could smell food from a distance.

    My dad mentioned that he had no sense of smell when I was around 25 years old. Until then neither of us had brought up anything olfactory to each other! He does not have Bećhet’s, though we do have depression in common.

    All these mixtures of factors! I’ve been curious about which leads to what. Treatment for depression and mitigation of Bećhet’s symptoms have left me with a smell-shaped hole left unresolved.


    I often wake up from intense dreams with the sensation of a smell fresh enough to simulate in my mind for a few minutes. The papery-gluey smell of a magazine, the citrus of orange juice, even the sick rotten smells. In my dreams they are as strong or stronger than they ever were.

  43. InfiniteRand says:

    From my perspective as someone who has chronic depression (or maybe type 2 bipolar tending toward the depressive side), although who has no claim to be typical in my depression (whatever that might mean). I find this connection between senses and depression interesting. I can’t really comment on the biology of the matter, but one caution I have is that for me, and potentially others, it is hard to determine whether when I am depressed how acute my senses are because typically I lose trust in my senses. Part of this is the occasional mild auditory hallucination (which I think is pretty atypical for standard chronic depression), but there’s also a general sense of losing trust in myself to distinguish between sounds, smells, and other senses. For instance, I’m not sure if my eyesight gets less acute, but I do become less sure if that guy I saw is really someone I recognize or just a stranger that kind-of looks the same. One thing with senses is certainty that you can distinguish between things is self-reinforcing, as you think two things are different, the differences stand out clearer, likewise uncertainty is self-reinforcing, if you are unsure if two pairs of jeans are really different maybe the perceived color difference is just an a quirk of the light. I’ve even found eyecharts exercises in uncertainty when feeling depressed, but relatively straightforward when in good spirits.

    This introduces a real problem with studying the phenomenon if you are measuring sensory perception by asking people to distinguish things. It might be useful to try to measure acuteness by attempting to measure reaction times and other unconscious reactions, but that might be tainted by the depressive tendency to get locked into obsessive thought spirals.

    This also introduces a counter-point. Often depressive thought-spirals are introduced by obsessing over small sensory phenomenons that others ignore, a background scent that might be easily ignorable by most, but to a depressed person might be oppressively obvious and signifying the general terribleness of the world. Of course, this is a fairly weak counter-point because of the fact that the depressive thought-spirals are generally irrational and distort perspective.

    I guess what I’m getting at is that it is difficult to get an objective sense of someone’s sensory acuteness when the processing and reporting side of the senses is compromised, which is often the case with chronic depression.

    On the flip side of this is acute sensations do pull people out of their head, which can be disruptive to depressive thought spirals (I tend to favor the cognitive behavioral therapy school of treatment, without 100% accepting all of its premises) which tilts my perspective here).

    I wonder if we could get some insight by comparing depression that has a known physical cause as a side effect, ie certain diseases that affect the brain, certain medications, vs a classic presentation of chronic depression. Firstly, it would be good to gauge whether the two present in generally the same manner, which would point toward a potentially common intermediate cause, but secondly, if they present differently, it would allow us to compensate for some of the distortion effects that depression has on the symptom reporting process (to a degree, since it is also possible that the patterns in the distortion might differ as well). Probably there’s already some research in this direction, I would be interested if anyone knew any links.

    So like 99% of our research on depression, I find this line of research interesting, suggestive, but unlikely to bring short or medium term benefits, although could be useful in the long term.

  44. inedibill says:

    Semi-relevant touchpoint via Nova’s “Remarkable Rabbits” Did you know that Canadian white hares leverage their anxiety to keep the predatory Lynx population in check?

    This hare will remember the stress of this hunt, as well as the multitude of other times it must run for its life. New science is showing how these traumatic memories affect the balance of power between hares and their predators. As their predators reach peak levels, the hares react to the chronic stress by significantly slowing their reproduction. Instead of 4 litters a year, they’ll have only 2. Instead of the usual 20 babies, they will have only 7 or 8. Just enough to sustain their population’s long-term survival while starving out their predators.

    It seems that the hare has evolved to dial up its anxiety & dial down reproduction when it is of benefit to the species. This made me wonder if some past incarnation of great apes may have been sufficiently hunted that it became advantageous for progeny to pass on their anxiety/depression, which diminishes senses and tamps down on the reproductive rate to starve out predators?

    That’s veering pretty far into speculation, but still thought this worth sharing since it’s the first incidence I’ve encountered where anxiety is passed between generations and used to advantageously suppress short-term population in order to maximize long-term population growth. Nature is nothing if not parsimonious.