January 25, 2017

The Heritability of Brain Size

A review of 14 twin studies in which brain size was measured via MRI found that total brain volume was 82% heritable (Pepper et al 2007). In other words,  the vast majority of why some people’s brains are bigger than others is explained by genetic differences between people.

Some people may find this surprising because they’ve read about various extreme circumstances, such as child abuse or very bad poverty, which are associated with smaller brains. However, these findings are not contradictory. Such environments are rare and, because of this, explain little of brain volume variation in the general population.

Consider a garden of 500 plants. Suppose that most of these plants have basically the same soil, but 5 of them have really bad soil and this makes them shorter than the other plants. Even though this effect is real and practically significant, it will explain very little of the total height variation within this garden because this deprived environment impacts so few plants. The same thing goes on with these rare environments that impact brain size.

This is also an instance in which we should be mindful of gene-environment correlations. It may be that parents who are abusive or extremely poor have small brains for genetic reasons and then pass on their small brain genes to their kids. Or, people with genes that cause small brains may also tend to be the kinds of people who put themselves in bad environments (intentionally or unintentionally). Studies which merely show a correlation between poor environments and small brains cannot rule out either of these possibilities.

A particularly noteworthy study on the brain changing with the environment is Boas (1912). Boas, who was a founder a modern cultural anthropology, looked at the American children of European immigrants and found that their mean brain size was very different from that of their parents and more similar to the brain size of non-immigrant American children. This study has gone on to be cited endlessly and pointed to as the first crack in the armor of genetic determinism. The problem? Boas forget to account for brain size differences that are a function of age. I kid you not.

Sparks and Jantz (2002) re-analyzed Boas’s data after accounting for the age difference between parents and children. The results? The brain size of immigrants and there children did not differ in any statistically significant way and the brain size of immigrant children did significantly differ from those of Americans. In fact, based on the correlations between parent and children brain size, the heritability coefficient derived from Baos’s own data is greater than 50%. To end this post, I’d like to quote Sparks and Jantz at length:

“Some 10 years before the immigrant study, Boas was one of the most (if not the most) statistical and quantitatively oriented anthropologists, as seen in publications from the period predating the immigrant study. In the final report presented to congress, Boas’ statistical fluency tends to disappear, perhaps in the face of such a large data set and the lack of proper statistical tests. For the period in which this study was published, the results were presented in a manner making the data look as convincing as possible. We also must consider the attitude of Boas toward the scientific racism of the day. Evidence of Boas’ disdain for the often typological and racist ideas in anthropology have been reviewed previously and are evident also in his later publications. Boas’ motives for the immigrant study could have been entwined in his view that the racist and typological nature of early anthropology should end, and his argument for dramatic changes in head form would provide evidence sufficient to cull the typological thinking. We make no claim that Boas made deceptive or ill-contrived conclusions. In Fig. 1 it is evident that there are differences between Americanand European-born samples. What we do claim is that when his data are subjected to a modern analysis, they do not support his statements about environmental influence on cranial form.”


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