Why are taller people more intelligent?

By Razib Khan | April 11, 2013 1:51 am

Update: First, people coming to this weblog for the first time should know that I moderate comments. So if you leave an obnoxious one it’s basically like an email to me (no one will see it). Second, the correlation between height and intelligence is not that high. This association is probably not going to be intuitively visible to anyone, but rather only shows up in large data sets. So please stop offering yourself as a counter-example of the trend (also, the key is to look within families, because the signal here is going to be swamped by other factors when you compare across populations). Third, a friend has sent me another paper which does confirm that even within sibling cohorts there does seem to be a correlation between height and I.Q. The problem is that it is a very small one, so you need large data sets with a lot of power to see it.

End Update

One moderately interesting social science finding is that there is a positive correlation between height and measured intelligence (e.g., on an I.Q. test). Setting aside the possibility that I.Q. tests designs are culturally biased against shorter people, one wonders why this is so. Height is a highly heritable trait where most of the variation within the population is due to variation as numerous genes. In other words, there isn’t a “tall” or “short” gene, but thousands and thousands of variants which shape the variation of the trait across the population. When I say it is highly heritable, I mean to imply that most of the variation in height in developed societies is due to genes (80-90%). As it happens intelligence is somewhat similar in its genetic architecture, heritable due to small effects across many genes. In general estimates for the heritability of intelligence tend to be somewhat lower, on the order of ~50% rather than 80-90%.

It is due to the highly polygenic nature that both of these traits have been posited as candidates for a “good genes” model of sexual selection. Presumably individuals with a higher mutational load will have lower intelligence and be shorter, all things equal, because these traits have extensive genome-wide coverage and are big targets. Geoffrey Miller’s The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature, was predicated on this logic. If the mutational load argument holds then the reduced I.Q. of shorter individuals may simply be due to the same cause: “bad genes.”


Another scenario is that assortative mating between tall and intelligent people has generated a correlation between alleles which tend toward this end of the trait distribution. The phenomenon is simple enough to describe; height and intelligence are both attractive, and even if they are not due to the same genetic loci the pairing of tall and smart results in the correlation between the traits. My own assumption is that something like this, perhaps with a mutational effect at the bottom of the distribution (due to large effect deleterious alleles knocking people down in height and intelligence), generates most of the correlation. Part of this is due to my reading of The g Factor:

It is now well established that both height and weight are correlated with IQ. When age is controlled, the correlations in different studies range mostly between 0.10 and 0.30, and the average about 0.20. Studies based on siblings find no significant within-family correlation, and gifted children (who are taller than their age mates in the general population) are not taller than their non-gifted siblings.

Whenever people posit a pleiotropic relationship between traits I am always curious about the possibility that the traits may be correlated (or not) in siblings. Population structure of some sort can produce correlations, but patterns within families are often more informative of the genuine genetic basis of these correlations.

A new paper in PLOS GENETICS tackles this with more sophisticated techniques. They conclude:

Traits that are attractive to the opposite sex are often positively correlated when scaled such that scores increase with attractiveness, and this correlation typically has a genetic component. Such traits can be genetically correlated due to genes that affect both traits (“pleiotropy”) and/or because assortative mating causes statistical correlations to develop between selected alleles across the traits (“gametic phase disequilibrium”). In this study, we modeled the covariation between monozygotic and dizygotic twins, their siblings, and their parents (total N = 7,905) to elucidate the nature of the correlation between two potentially sexually selected traits in humans: height and IQ. Unlike previous designs used to investigate the nature of the height–IQ correlation, the present design accounts for the effects of assortative mating and provides much less biased estimates of additive genetic, non-additive genetic, and shared environmental influences. Both traits were highly heritable, although there was greater evidence for non-additive genetic effects in males. After accounting for assortative mating, the correlation between height and IQ was found to be almost entirely genetic in nature. Model fits indicate that both pleiotropy and assortative mating contribute significantly and about equally to this genetic correlation.

Pleiotropy here means that the same gene is impacting different traits (height and I.Q.). The additive genetic correlation between height and I.Q. was 0.08 and 0.17 in males and females respectively. These are small correlations obviously, but it’s what we’d expect.

After the statistical modeling using a twin design there’s a lot of talk about sexual selection and the long arc of evolutionary genetics (e.g., additive genetic variation being exhausted by selection). This is what you have to do in discussions, but I’m not sure it really adds any value. How strong is sexual selection for intelligence and height? The data show that taller men have more sexual partners, but the problem here is that taller men have taller daughters, and these daughters are not necessarily so reproductively fit. Whenever you are talking about sexual selection you need to take into account the antagonism that might entail because of the differential value of a trait between the sexes (e.g., masculine men may have masculine daughters). As for I.Q., I’m not sure about the long term distribution of fitness for this trait. I have a suspicion that the “sweet spot” for mating is to be only somewhat smarter than than the average, but not so clever so as to be obnoxious.

In the end I’d really like to see a massive number of siblings compared. I think that’s doable with this data set, but I didn’t see it in the paper (tell me if I’ve missed something). At some point we’ll have accurate high coverage whole genomes for many pairs, and we can ascertain whether it’s mutational load and pleiotropy more directly when it comes to correlations like this. Since pedophiles tend to be shorter and less intelligent I’m willing to accept deep biological connections across many traits. But I feel that the whole area is somewhat of a muddle right now. And talking a lot about sexual selection strikes me as excessive hand waving.

Citation: Keller MC, Garver-Apgar CE, Wright MJ, Martin NG, Corley RP, et al. (2013) The Genetic Correlation between Height and IQ: Shared Genes or Assortative Mating? PLoS Genet 9(4): e1003451. doi:10.1371/journal.pgen.1003451

CATEGORIZED UNDER: Anthroplogy, Genetics, Genomics, Select
MORE ABOUT: Height, Intelligence
  • https://delicious.com/robertford Robert Ford

    Almost 700 retweets? Wow!

  • Dmitry Pruss

    Could it be partly explained by both height and intelligence being metrics of childhood being relatively free of nutritional deprivation and of communicable diseases, i.e. a correlate of the societal status of the parents (which is both heritable and intelligence-dependent, and which is also sought by their children’s marital partners too) ? Within sibships, availability of parenting resources and their selective allocation to individual siblings is highly environmentally / temporally variable, so a correlation between siblings would have been frayed. But between sibships, whose daddy’s rich and whose mommy’s smart makes a lot of difference for childhood growth.

    • razibkhan

      re: height. it seems pretty clear in western societies there’s no nutritional problem for height. also, re: sibships, this was a twin design. shouldn’t the temporal noise cancel?

      • http://twitter.com/ThatOneNuge Conor Nugent

        Well, unless you have undiagnosed nutritional disease. My brother has celiac disease and we didn’t know until early puberty; consequently, he’s shorter and (I think) less intelligent than the rest of us.

        • razibkhan

          you don’t know if it’s a consequence. you’re speculating on really small sample sizes. though you may have a point. though i suppose it could be a genetic effect that comes through a mediating disease of malabsorption or some such thing.

      • Dmitry Pruss

        Read your update, thanks, Razib. Yes, given that the correlation between intelligence and height is relatively subtle (and may be difficult to assess in sib pair studies simply due the insufficient power of the cohorts), I think it is still too early to dismiss the possible nutritional / lifestyle mechanism whereby the children (and perhaps especially the fav children) of the smart and successful parents face fewer growth-stunting environmental risks such as imbalanced nutrition, childhood diseases, and perhaps earlier onset of puberty.

        “Growth acceleration” is document in most developed countries, and is commonly explained by a correlation of environmental and genetic factor (the latter in the general hybrid vigor category of gene pool-mixing effects). Since the acceleration has been rather dramatic, it may very well have residual heterogeneity even decades later.

  • William Mayes

    I’ve never known a tall person who was as intelligent as people seemed to think he was, Tall people have a psychological advantage and get preferred over short people in things like jobs and promotions. I remember walking down the street one day and and passed some men working on the street. There were three men standing around overseeing four men digging a hole. The short men were in the hole. the tall men were supervising. I doubt very seriously that the tall men were any more intelligent than the short men. Tall men don’t even realize they are being given preference because of their size. Being kind of short makes you more aware this.

    • razibkhan

      Tall people have a psychological advantage and get preferred over short people in things like jobs and promotions

      men. not women, from what i recall. this effect seems to be valid for both sexes in this study.

  • Patrick Mays

    I’m a 5’5″ male who is significantly more intelligent than almost (not all) my friends who are all taller than me. I’ve also had more partners than most of them as well which has nothing to do with my height or intelligence imho.

    • razibkhan

      your friends must be rather dull. not a high sign of intelligence to assume that you falsify a small correlation by your N = 1.

    • Tim Jackman

      I am 6’4″ and when i think about it, i am smarter than every last person that i have ever met who is shorter than me. This study goes to show that if your not over 6’1″ there is no chance that you can have any intelligence what so ever. So if your under 6 feet i am amazed that you can even read this. Good for you.

  • http://twitter.com/Animelee Andrew Selvarasa

    Handsomeness (face and body) definitely correlates with the most reproductive success overall, right?

  • http://www.facebook.com/karl.zimmerman Karl Zimmerman

    Can most of the statistical correlation between height and adult male earnings be explained by the relationship between height and IQ? I’d guess not, since Judith Rich Harris notes height at age 16 is more strongly correlated with adult earnings than final height, but this was just one study which she cited, and even though I tend to agree with most of her arguments, she can call some things wrong.

    • razibkhan

      Can most of the statistical correlation between height and adult male earnings be explained by the relationship between height and IQ?

      i doubt it. the correlation between height and iq for males was of pretty significant magnitude last i checked. this is not a high correlation.

  • http://www.facebook.com/karl.zimmerman Karl Zimmerman

    Oh and another thing I was considering…

    I am of somewhat above average intelligence (127-133 IQ), and 5’8.” My growth was almost certainly stunted however, and not due to improper nutrition. I was an ADHD child, and on several different medications during my childhood. During the period I was on Cylert from age 5-7, I did not grow at all, and weighted only 45 pounds. When I went off it I did have a mini-growth spurt (and gained 20 pounds within a few months, going from severely underweight to chubby, as I was as a small child), but the doctors estimated I should have been around 5’10″ if I was not medicated as a child. Studies of the effects of ADHD on child growth have generally borne my personal experience out.

    Given ADHD is so widely diagnosed now (20% of high school kids have this diagnoses), and statistically speaking, most ADHD kids are of below-average intelligence, this might (at least within a U.S. context) be not insubstantial reason why the less intelligent tend to be shorter overall.

    • Emil Kirkegaard

      Testable. Get a sample of ADHD people. Measure them (height, g), and see which of them had ADHD drugs. Look for correlation.

  • http://www.facebook.com/karl.zimmerman Karl Zimmerman

    An interesting way to test this would be to see if more intelligent people within naturally shorter populations are on the tall side of their local ethnic group.

    After all, the relationship between height and IQ, if it’s linked due to both having sexual selection, means that on average smarter people should be taller than the norm within their population, but not always taller than the norm on a global scale.

    • razibkhan

      yes. this is ethnicity controlled.

      • http://www.facebook.com/karl.zimmerman Karl Zimmerman

        This would also suggest that the height-IQ relationship would be weakest in multiethnic societies like the U.S., and much stronger in places like Europe or Japan, where everyone is of similar background.

        Thinking back to childhood all the honors kids who were short either shot up in college, or were part of a generally slightly shorter ethnic group (Ashkenazi, Italian, South or East Asian, etc).

  • razibkhan

    please note that this paper focuses on people from developed societies. #3 probably not an issue for height. #2 is the one that i can think of is plausible, and i noted it in the post. but also note that i want to see huge comparisons of siblings and see if this shows up. previous work did not suggest it.

  • razibkhan

    it is a low correlation. why i’m cautious. not sure how robust this is… if it’s robust though the low correlation can still be informative.

  • razibkhan

    yes, but we’ve hit diminishing marginal returns for height & nutrition. people get wider, not taller.

    • Richard Seiter

      I think it’s important to distinguish between macronutrient and micronutrient status in discussions like this. I think it is fair to say that much of the population eating the Standard American Diet is overnourished in a macronutrient sense (e.g. calories), but may be undernourished with respect to specific micronutrients. A particular example relevant to brain development is the omega 3 fatty acids EPA and DHA. Do you have any sense of the relative importance of different nutrients in height? Is that primarily a matter of calories?

  • Street Anthropologist

    This is the kind of empirical study that should be investigated in anthropology. Does the socio-cultural archetype tall-dark-handsome have a biological basis? Is it related to health, intelligence, and sexual/reproductive desirability? Unfortunately, cultural anthropologists are allergic to numbers, genetics, and statistical correlation. The IQ test alone is a fertile subject for their postmodernist and deconstructivist babbles.

    • Emil Kirkegaard

      Cultural anthropology is garbage.

  • Karch_Buttreau

    My take at an executive summary is as follows. There are probably thousands of SNPs affecting intelligence, and there are probably at least hundreds of SNPs affecting height. A tiny fraction of these SNPs affect both in the same direction, so you have your small correlation.

  • Stephan Guyenet

    I’m not sure height is a reflection of good genetic quality. Physiological dysfunction such as genetic melanocortin deficiency or acromegaly can increase linear growth. I think selection based on (male) height is related to finding a partner who was well nourished during development and is physically powerful.

    Ironically, our large stature today (by evolutionary/historical standards) is probably at least in part due to overnutrition during development.

  • Street Anthropologist

    I wonder if Harvard freshman students are taller than USC freshman students, considering most students of both schools are economically advantaged so malnutrition/poor childcare won’t be an issue. Can SAT score plus high school GPA be a good measurement for intelligence?

    I don’t think tall height-high intelligence correlation will hold true if race is involved; short Asians are far more intelligent than tall Africans. If Math evaluation can measure intelligence, short people of Hongkong, Singapore, Japan, South Korea lead the international pack.

    Are tall Massais more intelligent than short Batwa pygmies? Both groups are in Kenya. How do we measure their intelligence? Without digging existing nutritional studies, I suspect Massais’ height is due to their high consumption of milk/calcium.

    If high calcium intake explains tall height, I wonder if Dutch’s consumption of milk is high, considering they are the tallest people in the world. Are they also more intelligent than Swedes or French or Germans?

    • razibkhan

      1) i have stated, this is not a large effect.

      2) also, it’s pretty obvious that you have to control for population stratification. that’s why you focus within families.

      • Street Anthropologist

        I don’t think focusing within families is a good idea. Parental resources and household economy affect children’s growth and development. Early born children enjoy more resources than late born.

        http://ije.oxfordjournals.org/content/37/6/1408.full

        • razibkhan

          first, for this paper, you are aware this is a twin design, right? the mz vs. dz comparison is crucial. second, within-family dynamics are usually far less important that between-family/population. so i’m not sure how you think the latter is easier to correct for than the former, even if the former is a factor. don’t be vague, i’m curious about your technical ideas about model design. tell me.

          • Street Anthropologist

            Yes, I’m aware, but twins can have siblings and can be later-born children. My logic is simple: the lesser the factors affecting height involved, the more plausible the genetic explanation. You have these people here using themselves as samples because they want to extend the factors affecting height/growth/development to weaken the role of genetics.

  • Street Anthropologist

    Sorry for double posting. As a street anthropologist, I’m interested if there is a grain of truth to the notion that smart rich kids go to Harvard and dumb rich kids go to USC. Can the investigation of their average heights, SAT scores, and GPA’s prove that?

  • Emil Kirkegaard

    The usual thinking is that a larger body requires a larger brain, so if taller people have larger brains, then one has to adjust for body size. It is possible that the brain size increases more than the height increase, so that your hypothesis works out.

    Cf. http://www.psychologytoday.com/blog/the-scientific-fundamentalist/200901/why-men-are-more-intelligent-women and the follow-up post.

    Also keep in mind that that guy is a dodgy researcher.

  • Olga Musayev

    What about epigenetics? Just because height isn’t affected by nutritional deficiencies *today* doesn’t mean that it didn’t affect our grandparents. Poor food= shorter and less intelligent -> epigenetic markers on kids propagating the effect.

    • http://www.facebook.com/people/Katie-Rappold/68138143 Katie Rappold

      good point. Grandparents nutrition passed down could effect intelligence and height even if the grandchild’s diet sucked. hmmm.

  • http://www.facebook.com/matt.angelucci Matt Angelucci

    It would be interesting to see how intelligence and height correlation might be distributed across small variations in height. For example, are 6’1″ tall people just a bit more intelligent than 6′ tall people? It could be that if one is short enough to be perceived as “really short,” or tall enought to be considered “really tall,” environmental factors become more important than genetics.

  • Douglas Knight

    Presumably individuals with a higher mutational load will have lower
    intelligence and be shorter, all things equal, because these traits have
    extensive genome-wide coverage and are big targets.

    I don’t mean to put words in your mouth, but it seems to me that a naive reading of that sentence is incorrect. The error I see is reifying mutational load. If you compare people from different populations with different mutational loads, then, yes, maybe IQ predicts population load predicts height. But if you pull a sample from a single population, the particular mutations are independent. The individual’s height load tells you nothing about the individual’s IQ load (excluding pleiotropy). Inside a single family, the situation might be different yet again because of linkage. But maybe Geoffrey Miller says something more sophisticated to salvage this.

    • razibkhan

      The individual’s height load tells you nothing about the individual’s IQ load (excluding pleiotropy).

      don’t exclude it. miller claims that ~30% of the coding genes express in the brain. so the likelihood that the load impacts IQ is supposedly large. the same model is applicable to height. not sure if the numbers work out…but they aren’t ipso facto implausible.

      • Douglas Knight

        Sure, pleiotropy is a plausible explanation for the correlation. Indeed, the greater the number of genes that control IQ and height, the more reason there is to expect pleiotropy. If that’s the only role that load is playing, sure. But the quoted sentence seems to claim load as an alternative explanation for correlation, rather than as evidence for the existence of pleiotropy.

        • razibkhan

          i assume that the load has to work through pleiotropy. how else would it work? the two refer to different categories of phenomena, not alternative ones.

  • http://www.facebook.com/people/Tom-Jensen/525076086 Tom Jensen

    I demand a re-test!

  • facefault

    A larger body *does* require a larger brain; your nerves need to cover more area, so you need more of them. Raw brain size doesn’t correlate nearly as well with intelligence as encephalization quotient does (which is why whales and elephants aren’t as smart as humans). Even encephalization quotient doesn’t capture variations in brain density, though; my understanding is that the reason women and men have the same intelligence by most metrics (though on average men have better spatial and women have better verbal) despite men having larger brains on average is because women have somewhat denser brains than men on average. And it’s unclear whether a tall person would necessarily have equally dense IQ-relevant centers of the brain as a short person does.

  • Anthony_A

    As for I.Q., I’m not sure about the long term distribution of fitness for this trait. I have a suspicion that the “sweet spot” for mating is to be only somewhat smarter than than the average, but not so clever so as to be obnoxious.

    As intelligence specialize at higher levels – people with high IQs do tend to be noticeably stronger in a few sorts of cognitive processing, with the specific sorts being different across the population of smart people – I suspect that the sweet spot for mating is much higher for those high-IQ people whose intelligence is strongest in social interaction. (Possible confounding variable to test this – someone who is really smart in a socially-adept way will probably have a higher income than someone equally smart whose main cognitive strength is mechanical skill. Higher income, regardless of sources, increases reproductive fitness.)

  • http://www.facebook.com/robertdyoung Robert Young

    Studies show that short people live longer. And while moderately taller than average may be an advantage in a physical altercation, the “very tall” like Yao Ming tend NOT to live as long…by a LOT. The tallest man on record died at just age 22, and his death was related to his height. And he is not just one example..google gigantism and acromegaly.

    • Street Anthropologist

      Is it related to tall height or to his abnormal physiology? Endocrine problem, maybe?

  • http://www.facebook.com/mike.kelly.583671 Mike Kelly

    As we discuss tall and short here, what height range is which?

  • http://www.facebook.com/mike.kelly.583671 Mike Kelly

    Well, that’s the long and the short of it, I guess.

  • http://profiles.google.com/j2saret john sarette

    looks like the study considered men and women separately which removed gender bias, however I wonder if there was some sort of nutritional bias here. A better diet will express it self in a taller population and a better diet is often associated with income which associates with educational and early parenting opportunity. I think this is an indication not an outcome.

  • http://www.facebook.com/AlanMGross Alan M. Gross

    I have heard that sibling order is correlated with IQ because, it is said, the average IQ in a family environment decreases with increased numbers of children, so the older siblings get more intellectual stimulation. I believe it is also true that improved nutrition increases average height. The current result of IQ being correlated with height makes me wonder whether the earlier born children are not only (slightly) more intelligent, but also (slightly) taller on average due to more food being available per person.

  • EquusMtn

    I’m sure someone has already made this observation, so apologies for
    restating: do the studies that show a correlation between height and
    intelligence correct sufficiently for overall health and nutrition?
    Obviously both height and intelligence are positively related to health
    and nutrition, and health problems and poor nutrition negatively impact
    both height and intelligence. Even subtle differences in
    health/nutrition can have statistically significant effects on height
    and intelligence. I’m highly suspicious of any claim that adjustment
    has been made for these factors; how can you know what subtle health
    problems (including digestive issues that equate to poor nutrition) are
    present in anyone? I don’t think you can convince me that you’ve truly
    corrected for these factors, but if you can I’ll be a believer.

  • Mark Page

    Just a layman’s question, does this mean that in years to come and Mars is populated by humans, (with the lower gravity), people would evolve taller, does this also mean they would evolve more intelligent than their earth bound cousin?
    By the way – I’m 180cm tall (5ft 9in) – average – so I suppose this would be classed as an average question.

  • quidnunc1

    A quick google scholar search revealed this:

    http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1003451#pgen.1003451-Davies1

    “Correlations between spouses indicate that individuals mate assortatively on both height and IQ, and a cross-trait spousal correlation indicates that smart women partner with tall men (r = .18) and that smart males partner with tall women (r = .11).”

    The correlation isn’t a proxy for the spousal choices/pairing as it could be hitchhiking on some other traits which might help explain the latter correlation

    later, in the discussion:

    “Furthermore, as with almost all twin studies, the conclusions of our study rest on the assumption that environmental influences affecting IQ and height do not cause greater similarity in MZ twins than DZ twins. However, the possibility that this assumption is violated for these traits is increasingly unlikely in light of recent findings, also showing very high levels of additive genetic variation in height [42] and IQ [54], that are based on genomic similarity among unrelated individuals who are unlikely to share environmental factors”

    I don’t think that it necessarily contradicts the environmental theory but supposing the environmental story is true it would seem to predict something like: the average set of genes that would explain the correlation are a subset of genes that tend to have a more indirect effect on iq and have a trait dependent expression which would either complement (i.e. diminish) or discount the assortative mating theory (illusory cause). I haven’t read enough into the topic to know whether that’s plausible.

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Gene Expression

This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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