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Dermatology Pop Quiz

My dermatology lecture this morning presents: one of those Two Truths and a Lie games. You choose which two you think are true and – special house rule – give explanations for why. The explanations do not require specialized medical knowledge beyond the level of a smart amateur. Answers tomorrow-ish.

1. Significantly more Americans get skin cancer on the left half of the face than on the right half.

2. People who had acne as children live on average four years longer than those who did not.

3. In very early studies, Botox has shown great promise as a treatment for depression.

(this is one of those shorter posts I was threatening)
(judging from the comments, this was way too easy. Grumble.)

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60 Responses to Dermatology Pop Quiz

  1. Ishaan says:

    1) Gehr, orpnhfr gur qevire’f frng vf ba gur yrsg fvqr bs gur pne (fb fha rkcbfher)

    2) Snyfr, orpnhfr V pna’g guvax bs n tbbq ernfba jul gung fubhyq or gehr.

    3) Gehr, orpnhfr … vzcebirq nccrnenapr? Sbepvat snpvny zhfpyrf gb punatr cbfvgvba? abg fher.

    [These are my answers. rot13 to decode]

    • Qiaochu Yuan says:

      As a general comment, rot13 on “true” and “false” is not so helpful at disguising them.

      • Daniel H says:

        That seems more like a specific rule. How about the general rule of “When there are only a limited number of options, especially when they don’t all have the same number of characters, rot13 isn’t so helpful at disguising them”.

        I’m good enough at skipping over what my brain thinks of as “gibberish” instead of “a language I recognize” that I wouldn’t actually notice this. I can see how that would be the case though, and will attempt to avoid that kind of situation.

      • Ishaan says:

        I noticed that when I was reading over my comment and considered editing it out but then I decided it was not necessary. I figured that anyone who *truly* didn’t want to know the answer before answering wouldn’t be squinting at the rot13 that closely.

        I might have been wrong though…after all, this hypothetical person *is* scrolling through the comments to begin with, and if they want to remain ignorant then that implies that they are the type of person who tends to do ignorance-preservation wrong.

        • I was, indeed, not squinting too closely such that I didn’t notice; unless I’m intending to unrot it my eyes slides over rot13’ed text specifically to avoid accidentally picking things like that up.

          Then I read the first reply and accidentally looked at the rot before my brain could catch up and say “DON’T DO IT”.


        • Ishaan says:

          Ha. So, ironically, Qiaochu Yuan is more to blame than I for destroying your ignorance – though really, it was pretty inevitable that it would happen once you started scrolling through the comments, since not everyone ROTs 🙂

  2. kappa says:

    1. I cannot think of a single earthly reason why this would be… hang on a sec… do these people drive?

    2. I cannot think of a single earthly reason why this would be the case and unlike the above I didn’t spontaneously think of one in the middle of saying so.

    3. I’m reminded of that thing I heard once where making or mimicing a particular facial expression has an effect on your mood pushing it in the direction of that expression. Botox is that thing that unwrinkles your forehead, isn’t it? Maybe people with artificially slackened foreheads get a defrowning effect. I wouldn’t think that effect would be strong enough to qualify as “great promise”, but it’s vaguely conceivable, I guess.

    So… I think 1 is true because I thought of a good reason for it, I tentatively think 2 isn’t because I can’t think of a good reason for it, and I tentatively think 3 is true because I thought of a reason for it but I don’t think it’s that good a reason.

    • Benquo says:

      This is the exact same thought process I went through, with the same conclusions, at the same levels of confidence.

      • Anonymous says:

        Almost the same for me… I thought acne might correlate, via diet or something, to wealth and hence probably to longevity, but thought this was kind of flimsy and 4 years sounded too big. Driving for 1 and mimicking smiles, or increasing satisfaction with body image (or possibly something else–it did say “very early studies”) for 3 seemed possible enough, so I went with 2 as the fake.

      • kappa says:

        Hello, brain twin! Nice to meet you! XD

  3. St. Rev says:

    1) Gehr, orpnhfr Nzrevpna qeviref fvg ba gur yrsg, rkcbfvat gur yrsg fvqr bs gur snpr gb zber fhayvtug.

    2) Snyfr, ol ryvzvangvba.

    3) Gehr; gur bcrengvat ulcbgurfvf vf gung, nf sbepvat n snxrq fzvyr pna (va fbzr pnfrf) vzcebir zbbq, erzbivat gur novyvgl gb znvagnva n jbeevrq/fnq rkcerffvba znl vzcebir qrcerffvba.

  4. mimosomal says:

    Same instinct as the others – driving -> unequal sun exposure, botox -> stiffened face and possibly less variable emotions, on the other hand acne causes and seems to be increased by stress, so I’d be surprised if it gave any long lasting health benefits.. And 4 years? Unless there’s some random genetic correlation where groups with higher acne rates have lower mortality for other reasons I find that implausible.

  5. Protagoras says:

    1) I’ll say false because it’s not surprising enough, given the driving explanation.

    3) True, because something being highly toxic means we know for sure it has a dramatic impact on human physiology, so I can easily believe that it has other effects in less than toxic doses.

    2) True, because I’ve already chosen my candidate for falsity, and also because this one would be very surprising and I’m being very meta in my approach to this quiz.

  6. Kolya says:

    2) I think is true (but sneaky/misleading use of statistics) – acne strikes late in childhood so part of the population of people who never experience acne are people who died pre-puberty and ‘did not have acne as children’, pushing down the average life expectancy.

    • St. Rev says:

      Oh, that’s sneaky. Looking at, there’s about a 99% chance of surviving to age 12, and the cohort who die before age 12 have a mean lifespan of about 2 years (most deaths are in the first year after birth). So that 1% are losing about 74 years of life expectancy. Backing that out with the 4 year mean loss, one can infer that if there’s no actual protective effect from acne, about 20% of the adult population never suffered from acne in childhood. Which…doesn’t sound that far off. Huh.

      It would be hilarious if Scott made it up as a ringer and it turned out to be true.

    • St. Rev says:

      Aaaaaaaaaaand Wikipedia says acne vulgaris has a lifetime prevalence of ~85-90%, close enough. That’s just silly. Scott, stop warping reality please.

      • Raoul says:

        After adjusting for the likely sex of those who’ve never had acne (“Approximately 95% to 100% of adolescent boys and 83% to 85% of adolescent girls aged 16 to 17 years are affected by this condition”, which I would guess adds about four years to their life expectancy, 90% seems to fit quite well.

        (Although Scott doesn’t specify, I’m assuming that we’re restricting to developed countries.)

    • Daniel H says:

      That… actually seems reasonable. I could easily see Scott excluding people who died before the age of 1 or 2; I think that’s common in actuarial statistics for that reason [citation needed]. I would have expected him to look up that kind of thing (or copy the quiz wholesale from the hospital, which I would expect to look this up).

    • Benquo says:

      Aaagh I am so stupid I was expecting a direct causal relationship.

      This seems so obviously true now but I’m not sure which of my just-so stories for the other two has to be false.

      1 also has a plausible causal story, and I saw a picture illustrating something related going around the internet (the trucker with just one side of his face super wrinkled) and in this case I think that’s negatively correlated with the truth. because tricky puzzle.

      That leaves 3, and now that I think about it, it seems likely to be true less because of the plausible causal mechanism, and more because very early studies will show nearly everything.

      This comment supersedes my earlier one.

    • RCF says:

      Now I’m wondering which group has higher average lifespan: people who had Alzheimer’s, or people who didn’t have it.

  7. Kiboh says:

    1) I’m ashamed to admit that I couldn’t come up with the driving explanation before skimming through the comments, but actually I think that’s it, yeah. Also, I feel that it’s too blandly impossible-sounding to be anything other than the truth.

    2) I can think of at least half a dozen various-levels-of-crackpot explanations for this (Gene X causes both acne and longer life! Children with rich parents get to eat a lot of acne-inducing sweets, and also get support from them which allows them to live longer! Popping zits removes toxins from the body which would otherwise fester in it ad infinitum! etc etc) but none of them quite ring true. Oh and also it’s written with more detail and certainty than the other two, which a fake would be. I’m guessing this is the lie.

    3) ~95% sure this is true just because of the phrasing used: unless they were being VERY tricky/meta/contrary, I don’t think someone constructing a false statement would go to the trouble of mentioning the “very early” nature of the relevant studies. Also, I can provide a mechanism: Botox makes people smile more, smiling causes happiness (no, really, it works at least as reliably this way around!), happiness counters depression.

  8. Leah says:

    1) Gehr – crbcyr qevir naq gurve yrsg fvqr vf zber rkcbfrq gb gur fha guebhtu gur jvaqbj

    2) Snyfr – orpnhfr gur bgure gjb whzcrq bhg nf cebonoyl gehr

    3) Gehr – gung guvat jurer jura lbh znxr n snpr, lbh srry gur rzbgvba (gung ubyqvat n crapvy va lbhe grrgu–sbepvat n fzvyr–znxrf lbh unccvre fghql gung V’z gbb ynml gb ybbx hc). Fb, ceriragvat sebjaf vapernfrf unccvarff.

  9. Anonymous says:

    One is true. There are several candidate reasons, such as driving. My anecdotal evidence is that I’m much more tan on my left side, and I don’t drive.

    I suspect two is also true. I suspect acne might have some correlation with higher rates of involuntary celibacy, higher nutrition, and different hormonal makeup.

    Three is probably false. It’s a paralytic poison.

  10. misha says:

    My guesses:

    1 is true. The reason: Driving. The driver in a car sits on the left hand side so the left side of their face gets more sunlight on average.

    2 is true, but is a statistical artifact similar to how infant mortality distorts life expectancy numbers: People who die before they have a chance to get acne get counted as “did not get acne while children”.

    3. is false.

    • misha says:

      Elaboration after reading the other comments:

      I think everyone agrees on 1 which kind of makes me want to doubt it out of contrariness but I’ll stick with my answer.

      2, at least one other person agrees with and Rev seems to have crunched at least a few numbers.

      If 3 is true, I think it’s more likely that the reason is the early studies were badly done or later proved false. If the facial expression reasoning is correct then later studies would’ve confirmed it. So the mechanism has to be something different than the one obviously guessed by smart laypeople.

  11. Drew Hardies says:

    1. Gehr, orpnhfr bs qevivat
    2. Cebonoyl abg. Zra trg zber npar naq qvr rneyvre?
    3. Gehr? Zl thrff vf fryrpgvba-ovnf. Crbcyr frrxvat obgbk ner znxvat punatrf

  12. Ian Osmond says:

    True, false, true.

    Americans often drive by themselves, and sit on the left when we do so. Therefore, we get more sun exposure on the left side of our face, from the time we’re sitting in the drivers’ seat.

    False, because the other two seem true.

    Emotions aren’t one-way. Our facial expressions are the result of our emotions, but they’re also the cause of them. If we lose the ability to express an emotion, it dulls our ability to experience it. If you can’t express sadness, you feel it less intensely.

  13. Sid K says:

    (I haven’t looked at the other answers yet.) My answer:

    1 is false. 2&3 are true.

    1 just seems completely implausible. 2 because of your immune system getting trained while you’re a kid or something? 3 because you simply look better with Botox, improving your perceived self-image and maybe also your social life.

  14. gwern says:

    1. Significantly more Americans get skin cancer on the left half of the face than on the right half.

    Is this ‘significantly’ in the useless p-value sense, or in the way everyone will misunderstand it as? That knowledge would change my answer, as I recall hearing about a national-level study finding more skin cancer on the left arm than right, which implies (1) a p-value < 0.05 and (2) a completely trivial effect size since it took a nation-sized sample to detect.

  15. Vaniver says:

    I enjoy when one rot13s ‘true’ and ‘false’, as if the two did not have different numbers of characters, and that when it was specified ‘two are true and one is false,’ it would not be trivial to decode which was which by finding the odd one out.

    My thought processes appear to be similar to the others in this thread. I think it’s more likely that 3 is true than 2, and think 1 is likeliest to be true. However, I am skeptical of the claim that exactly two are true- surely it would be interesting to give this prompt for three true statements, and notice which one people think is least likely (and thus claim to be false) and how they rationalize it away? But one of the statements is tricky to verify.

  16. R. Jones says:

    Well this paper was published on April 1st of this year.

    So I think 3 might be false.

  17. Izaak Weiss says:

    I’m definitely going with 2, because I remember reading something about how children who played with bare feet had stronger immune systems growing up, because of exposure to mostly harmless bacteria. Maybe something similar is going on here?

    1 seems outlandish, and at most a statistical fluke and at worst an outright falsehood, but part of me thinks that this means that it probably is true.

    3 seems like it’s a decoy. But who knows. I’m only putting this here so that I have proof that I’ve made an opinion, and I certainly wouldn’t bet any money on being right.

  18. Platypus says:

    I believe (1) is true, because once while wandering the Web I came upon a picture of a truck driver. The left side of his face, exposed to the Sun through his cab window, showed the effects of age; the right side looked much different.

    …Even allowing for the uncertainty that that website might have been a hoax, it makes sense to me that most drivers will get more solar exposure on the left side of their faces.

    I have no idea about (2) and (3).

  19. Andrew says:

    My guesses are 1 and 3.

    For 1 I immediately thought sunlight predominately hitting the left side of your face while driving. Then I remembered something about car windows filtering UV light, so I googled that, but it turns out that’s a special laminate that’s only done to windshields. So, theory still seems good.

    For 3, my guess is that botox improves people’s appearance, and everybody knows that people treat good looking people better than not so good looking people. There’s probably an entirely internal self-esteem boost associated with looking better too.

    I can’t think of any common sense reason why 2 would be true. I wouldn’t put it past it to have some genetic connection, but that’s nothing a smart layman would know about.

  20. Anonymous says:

    1. True. I didn’t think of the driving until I read the comments. I selected this because I knew #3 was true. #2 seemed somewhat plausible as something related to immune system function or some other handwavy explanation, and #1 sounded absurd. So naturally I guessed #1 was true, because why would you give us such an absurd thing that’s false?

    2. False.

    3. True. I remembered reading this somewhere.

  21. ckp says:

    1) false. it can’t be due to driving because glass blocks UV radiation.

    2) true. fighting off the infection strengthens the immune system?

    3) true. people’s appearance has a lot of effect on their mood, so improving appearance would improve mood.

    EDIT: aaand looking at the comments I’m probably wrong on all 3 counts 😡

  22. Pingback: The Appearances and The Things Themselves | Benjamin Ross Hoffman's personal blog

  23. Symmetry says:

    1) Sounds likely, given how much outside time peopel spend in cars.

    2 and 3) Both sound suprising to me, but since by the rules of the game they can’t both be false I’m going to guess that they’re both true, therefore locking in 2/3 accuracy.

  24. Mark Dominus says:

    Sbe #1, V vzntvar n ybg bs crbcyr erzrzore gung ARWZ negvpyr sebz 2012
    nobhg Ovyy ZpRyyvtbgg, jub qebir n zvyx gehpx sbe 30 lrnef naq jubfr
    snpvny jrngurevat jnf fgevxvatyl nflzzrgevp. Vg znqr gur fbpvny zrqvn
    ebhaqf ng gur gvzr.

  25. anon says:

    I’m going to stick with my original answers despite your edit, which threatened to ruin my fun. I’ll try to justify my answers in the context of other popular answers here.

    1. False. Most people don’t have noticeable differences in their skin color on one side of their face. I think people would intentionally compensate if one side of their face got more tanned than another. Driving is a plausible explanation, but I don’t think it’s big enough. I wouldn’t expect skin cancer rates to be significantly effected unless there was also a change in appearance, though maybe cancer’s just more subtle than I suspect.

    2. True. The comment that talks about infant mortality is probably right. My original thought was in the same indirect mindset – I thought that maybe most people who don’t get acne have some sort of disease or disfigurement precluding it. In retrospect that probably wasn’t sufficient to explain four years gap though. I don’t really have any idea how many children avoid acne, so this one was tricky.

    3. True. I think you specified “very early” studies for a reason. I think Botox increases happiness among people who sign themselves up to receive treatment, as a consequence of improving their appearance. I doubt there’s any more sophisticated relationship going on here. Someone else speculated in the comments that Botox forces you to smile which helps with depression – I guess that’s a possibility, but I think it’s a weak one.

  26. Error says:

    Before reading comments:

    #1 sounds plausible. Sunlight causes skin cancer, and drivers in the U.S. could get more sun on the left side of the face (because closer window).

    #2 ….not sure here. No explanation comes to mind.

    #3 The only thing I know about Botox is its use as an anti-wrinkle agent. I suppose I could see it helping people who are depressed because they think they look ugly, but it feels like a stretch and I don’t think it’s what you mean anyway.

    I’m going to go with #1 probably true, unsure about #2 vs #3. If I had to put money on it I’d put it on #3.

    *after checking comments*

    Seems I agree with the crowd on 1. I think the expression-altering effects of botox mentioned elsewhere in the thread (which I did not know about, but should have guessed) are a better explanation than mine. And the statistical effects of early death on #2 make that plausible too. So, hrm. I’m still going to say 1 is true. I’ll still guess 2 is false because I don’t think Scott would sneak that sort of trick in, but I am less certain now.

    So still 1 and 3, but my confidence in 3 over 2 is reduced somewhat.

  27. Before reading the comments, I would have guessed 2 was false. 1 is true because of the driving explanation. Couldn’t figure out such a clear reason to pick between #2 and #3, but #2 just seemed like an implausibly large effect. Like, I can imagine acne being correlated with better health outcomes, but four years? Unless the infant mortality thing is true.

  28. Rob says:

    1. For left/right differences, the two things that came to mind were handedness and driving. The driving mechanism seems plausible enough, and the fact that you specified “americans” where “people” would be more natural suggests that this varies from country to country, which re-enforces the driving hypothesis since people drive on different sides. So I say True.
    2. My first thought was early immune system development, but the statistical trickery suggestion in comments is very highly plausible. The specificity of the phrase “on average four years” suggests stats, and I know you like those kinds of things. So true.
    3. I’m aware of theories about mood being so tightly tied to facial expression that you can reverse the causality, which provides a mechanism. But a few problems: firstly I think of depression as being more about flat affect than sadness per se, so if facial expression can drive affect, paralysing facial muscles ought to make things worse? Secondly how do you practically speaking run that study? What ethics board approves it and who volunteers to participate? “Very early studies” makes this more plausible, but it also reads like you hedging or trying to handwave our not having already heard about this. So false.

    It’s interesting how much of my reasoning comes from the wording of the questions and knowledge of how you operate, rather than anything object-level.

    Edit: Also, I would very much like you to install a “Notify me only on replies” plugin for wordpress. I’d like to get an email if somebody replies to my comments, but not get an email for every comment on the post, which is the only option currently available. Thanks 🙂

  29. Ken Arromdee says:

    My first thought on the acne question is that foods associated with acne are eaten more by rich people and rich people live longer. But for the actual foods in question I’m not sure if, for instance, rich people or poor people eat more chocolate–there are plausible reasons why it could be either way.

    There’s also the possibility that acne has a racial or gender bias and so does mortality. I wouldn’t have any idea what this bias would be, either.

  30. Timothy says:

    I’m confused by the first one. Everyone else has suggested that driving is the culprit (which I’ll admit didn’t occur to me). But glass is mostly opaque to UV light. So as long as everyone is driving with their windows up, this shouldn’t make a difference. Have you ever been sunburned while driving?

  31. moridinamael says:

    3. Botox can be a fairly effective treatment for chronic migraine. Chronic migraine sucks and can cause depression. Control your study poorly, and it looks like Botox cures depression.

  32. Paul Torek says:

    Before I read any un-rot13d answers,

    Bar, orpnhfr qevivat
    Ger, orpnhfr pnapryvat sebja-jevaxyrf.

    OK now let’s see what the consensus is. Yeah, I’m in the majority.

  33. Jonathan Weissman says:

    3. In very early studies, Botox has shown great promise as a treatment for depression.

    I find this somewhat plausible on the basis that very early studies can show just about any stupid thing.

  34. Eric Rall says:

    1. False. I couldn’t think of any reason for this to be true before reading the other comments, and I’m recording my independent guesses.

    2. True-ish. I wouldn’t be surprised if acne diagnosis rates were a reasonable proxy at the population level for socioeconomic status, and socioeconomic status correlates with life expectancy. I’m not very confident of this and only adopted it because you said two of the statements were true; I’m also considering the possibility that your “two truths” claim was merely a ruse.

    3. True. Very early studies show all sorts of things merely due to statistical flukes and the file-drawer effect.

    • Eric Rall says:

      Even after reading the other comments, I still doubt the window-glass explanation for #1, due to two line of argument:

      1. Window glass blocks almost all UV-B, so the effect would only show up on people who regularly drive with the window rolled down and the sun shining in from that side. In my experience, that is very rare.

      2. I can’t recall ever noticing anyone being more tanned or sunburned on the left side of their face.

  35. Shmi Nux says:

    My initial guess: T,F,T. Also Gehr and Snyfr should be names for a Janus-like creature in Norse mythology.

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  37. Alexander Stanislaw says:

    I’ll guess that 2 was the lie.

    I don’t think this quiz is easy. I think that selection bias caused mostly the people who knew the answers to post.

    If Botox paralyzes muscles then maybe it has something to do with neurotransmitters? If 1 is true, maybe its because people who sunbathe lie on their right side.

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