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Brain Size and IQ


  1. Brain Size Correlates With IQ

In the last ten years, three meta-analyses on over 100 studies on the relationship between brain size and intelligence have been done. All three showed that the larger a person’s brain is the higher they are likely to score on an IQ test (McDaniel 2005Rushton and Ankey 2009, and Pietschnig et al 2015). These studies produced correlations ranging from .24 to .40, meaning that a person with a brain size 1 standard deviation above average would, on average, be predicted to have an IQ score .24-.40 standard deviations above average. (Or, put yet another way, brain size explains between 24% and 40% of differences in IQ. No, this is not what r^2 means.)

Rushton and Ankey (2009) provided the largest review of this evidence by comparing how well brain size predicts intelligence when brain size is measured via MRI as opposed to being estimated from the size of the head. They found that brain size is a better predictor of IQ when measured by MRI. When an MRI is used, the correlation between brain volume and IQ is 0.38, while it is only 0.20 when non-MRI techniques are used.

Another interesting comparison was made by Pietschnig (2015), who showed that brain size is a better predictor of IQ among healthy people than among clinical samples (r = .26 vs .21). This makes sense as we’re looking at more “standardized” brains and not the effects of various pathologies.

Also noteworthy: an analysis of six studies showed that there is a .63 correlation between how well an IQ sub-test correlated with brain size and how well that IQ sub-test measures general intelligence as opposed to a specific intellectual skill (Rushton and Ankey 2009).

  1. Brain Size Differences Cause IQ Differences

Now, the fact that brain size correlates with IQ does not show that difference in brain size cause difference in IQ. A correlation is evidence of causation, but far from proof. However, there are several other pieces of evidence suggesting that brain size and IQ are causally related:

  1.  Several studies have shown a genetic correlation between IQ and brain size. This means that the same genes which explain variation in IQ also explain variation in brain size (Posthuma et al. 2002Gignac et al, 2003, and Hagenaars et al, 2016.) The simplest explanation for this finding is that some genes influence IQ by influencing brain size.
  2. People’s brain size changes through out life. Studies looking at the same people over several years show that changes in brain size predict changes in IQ (McGill 2011) . Moreover, brain size and IQ follow the same life span pattern of increasing until a person’s mid 20’s and sharply declining in old age (Rushton and Ankey 2009). The simplest explanation for these findings is that changes in brain size over time cause changes in IQ over time.
  3. Breeding experiments which breed animal populations to be more intelligent also end up breeding bigger brained animals. For instance, one study bred mice for 12 generations to be better at finding their way through mazes and said mice ended up with with brains 2.5 standard deviations larger than the first generation had (Hamilton 1935).
  4. In the 20th century IQs around the world have risen. So has brain size (Thompson 2015).
  5.  Humans have evolved through several species, each more intelligent than the last. We have also evolved larger brains with each passing species (Roth and Dicke 2005).
  6. Five studies have looked at the relationship between brain size and IQ within families (Rushton  and Ankey 2009). Four of them found that smarter siblings also tend to have larger brains. This suggests that the brain size – IQ correlation cannot be explained by factors live poverty and access to nutrition, which are generally equal within families and between siblings.

Of course, no single piece of evidence on its own is strong enough to justify the claim that variation in brain size causes variation in IQ. But, taken together, I think they make a decent case. I can’t think of any better single scientific explanation for all 6 points.

  1. Common Objections

That being said, there are also a series of common objections to this idea. I will try to list each and then explain why I don’t think it shows that brain size does not impact IQ.

The most common argument goes like this: men have bigger brains than women, but men are not smarter than women. Therefore, larger brains much not cause people to be smarter. I think this argument would work well against the view that brain size is the only factor that influences intelligence. However, I’ve never heard of anyone who actually holds that view.

Rather, I think that brain size is one of many factors that influence IQ. So, if men have larger brains than women but are not smarter than women then I would assume that women make up for this deficit by having an advantage over men in some other way. For instance, some autopsy data has suggested that women’s neurons are more tightly packed than men’s are. Thus, men and women may have a the same neuron count even though they have different brain sizes (Lipton 2002 page 193). Or, there may be some other difference that we, or at least I, don’t know about. Either way, this is not a very compelling argument.

It’s also worth noting that the assumption behind that argument, that men and women are equally intelligent, may not be true. Depending on which meta-analysis or large study you look, you can find that men have higher IQs than women or that the sexes have equal IQ (Jackson and Rushton 2006Flynn and Case 2011, and Irwing and Lynn 2005). If there is an IQ advantage for men, then this argument against brain size and IQ becomes even weaker.

In addition, men and women differ in their patterns of intelligence (Parsons 2003Contreras 2007Kaufman 2006) and the weightings of skills on IQ tests don’t necessarily have a 1:1 ratio with how much brain area each skill uses.

Another argument sometimes made is that people with megalencephaly, a neurological disease, have huge brains but do not have high IQs. Therefore, large brains must not cause high IQ. Though this case was made by various well known academics in Pietschnig et al (2015), frankly, I think this is a pretty silly argument. If having a large brain causes people to be more intelligent it is probably because having large IQ-related brain regions, all else being equal, causes you to be smarter. But having really big IQ-related regions of the brain does not cause brain diseases like megalencephaly. More generally, people with megalencephaly have brains that do not work like normal people’s do. Given this, it isn’t safe to conclude anything about the general relationship between brain size and IQ based on this diseased population.

An analogous argument to this line about megalencephaly would be to say that bigger hearts don’t pump more blood because people with enlarged hearts pump less blood.

It is worth mentioning the extremely dishonest case against the brain size and IQ connection given by the psychologist Richard Nisbett in his popular 2009 book “Intelligence and How to Get It“. On page 219, Nisbett writes “The correlation found within the white population probably does not indicate that greater brain size causes higher IQ. Within a given family, the sibling with the larger brain has no higher IQ on average than the sibling with the smaller brain”. To justify this claim Nisbett cites the single study that found this null result and does not tell his reader that 4 other studies have found that brain size does predict IQ within families.

Moreover, 4 of these 5 studies have small to moderate samples with questionable quality. Berhvall et al (2006) is the major exception. This study found the within family IQ relationship in a representative sample of over 90,000 Swedish males. Thus, it is a well established fact that siblings with larger brains also tend to be smarter. Nisbett just cherry picked the single study to argue that this was not true in a shockingly obvious way, and basically got away with it.

Finally, some people make the argument that some animal species have larger brains than humans, but no animal species are smarter than humans. Therefore, brain size doesn’t cause IQ. The response to this argument is two fold. First, if one species has a larger body than another it may require a larger brain to manage that body equally well. This is important because many of the animals with larger brains than humans, such as certain kinds of whales, are also huge.

Secondly, brains in different species are organized differently than human brains are. This difference in organization is what causes them to be less intelligent in the first place and there is no reason whatsoever to think that brain size, even if it is one causal factor in intelligence, could make up for this. Should we abandon the idea that there is a connection between computer size and disk space if someone shows us a really big computer from 1970 with less disk space than an Iphone?

And while this is not a direct response to the argument – whales, dolphins and elephants are generally considered some of the smartest non-human animals.