One of the more likely causes of this height–intelligence association, according to the available science, is genetic. Intelligence is known to be heritable to a certain degree. To clarify, heritability is the extent to which a property or trait of a person varies due to genetics. Something with a heritability of 1.0 means all possible variation of a trait is due to genes, and a heritability of 0.0 would mean none of the variation is genetic.
For example, your species is purely a result of your genes, so “species” would have a heritability of 1.0. If your parents were pigs, you’ll be a pig, no matter what happens as you grow and develop. There are no environmental factors that will turn a pig into a cow. By contrast, if you are currently on fire, this is purely the result of the environment, so has a heritability of 0.0. There are no genes that cause people to burst into flames; your DNA doesn’t cause you to burn constantly and produce little burning babies. However, countless properties of the brain are the result of both genes and environment.
Intelligence itself is heritable to a surprisingly high degree; a review of the available evidence by Thomas J. Bouchard13 suggests that in adults it’s around 0.85, although interestingly it’s only about 0.45 in children. This may seem odd; how can genes influence adult intellect more than children’s? But this is an inaccurate interpretation of what heritability means. Heritability is a measurement of the extent to which variation among groups is genetic in nature, not the extent to which genes cause something. Genes may be just as influential in determining a child’s intelligence as an adult’s, but with children it seems there are more things that can also influence intelligence. Children’s brains are still developing and learning, so there’s a lot going on that can contribute to apparent intelligence. Adult brains are more “set”; they’ve gone through the whole development and maturing process, so external factors aren’t so potent any more, so differences between individuals (who in typical societies with compulsory education will have roughly similar learning backgrounds) are more likely to be due to more internal (genetic) differences.
All of this may giving a misleading idea about intelligence and the genes, implying it’s a far simpler and more direct arrangement than it is. Some people like to think (or hope) that there is a gene for intelligence, something that could make us smarter if it was activated or strengthened. This seems unlikely; just as intelligence is the sum of many different processes, so these processes are controlled by many different genes, all of which have a part to play. Wondering which gene is responsible for a trait such as intelligence is like wondering which piano key is responsible for a symphony.*
Height is also determined by numerous factors, many of them genetic, and some scientists think that there might be a gene (or genes) that influences intelligence that also influences height, thus providing a link between being tall and being intelligent. It’s entirely possible for single genes to have multiple functions. This is known as pleiotropy.
Another argument is that there’s no gene(s) that mediates both height and intelligence, but rather the association is due to sexual selection, because both height and intelligence are qualities in men that typically attract women. As a result, tall intelligent men would have the most sexual partners and be more able to spread their DNA through the population via their offspring, all of whom would have the genes for height and intelligence in their DNA.
An interesting theory, but not one that is universally accepted. Firstly, it’s very biased towards men, suggesting that they only need to have a couple of attractive traits and women will be inexplicably drawn to them, like moths to a gangly, witty flame. Height is far from the only thing people are attracted to. Also, tall men tend to have taller daughters, and a lot of men are put off and intimidated by tall women (or so my tall female friends tell me).
Same goes for intelligent women (or so my intelligent female friends tell me, which for the record is all of them). There’s no real actual evidence to suggest that women are invariably attracted to intelligent men either, for various reasons; for instance, confidence is often considered sexy and, as we’ve seen, intelligent people can be less confident overall. This isn’t to mention the fact that intelligence can be unnerving and off-putting; the terms “nerd” or “geek” may have been largely reclaimed these days, but they were insults for much of their history, and the stereotype is of them being typically dreadful with the opposite sex. These are just a few examples of how the spread of genes for both height and intelligence could be limited.
Another theory is that growing tall requires access to good health and nutrition, and this may also facilitate brain and therefore intelligence development. It could be as simple as that; greater access to good nutrition and a healthier life during development may result in both increased height and intelligence. It can’t be just that though, because countless people who have the most privileged and healthy life imaginable end up being short. Or an idiot. Or both.
Could it be to do with brain size? Taller people do have typically bigger brains, and there is a minor correlation between brain size and general intelligence.14 This is quite a contentious issue. The efficiency of the brain’s processing and connections plays a big part in an individual’s intelligence. but then there is also the fact that certain areas, such as the prefrontal cortex and the hippocampus, are bigger and have more gray matter in people of greater intelligence. Bigger brains would logically make this more likely or possible just by presenting the resources to expand and develop. The general impression seems to be that a bigger brain is maybe yet another contributing factor, but not a definite cause. Big brains perhaps give you more of a chance of becoming intelligent, rather than it being an inevitability? Buying expensive new running shoes doesn’t actually make you faster at running, but they might encourage you to become so. The same can be said of specific genes, actually.