Spent by Geoffrey Miller

Author:Geoffrey Miller
Language: eng
Format: epub, mobi
Publisher: Penguin Group US
Published: 2010-02-25T16:00:00+00:00

For example, I was involved in a couple of studies that examined relationships between intelligence and brain size, and intelligence and body symmetry. The psychologist Lars Penke and I reviewed all fifteen studies we could find on the relationship between general intelligence (as measured with reliable, valid IQ tests) and brain volume (as assessed by MRI brain imaging). In a total sample of 935 normal adults, general intelligence correlated +.43 with brain size—much higher than the correlation previously found between intelligence and external head size (about +.2). We also reviewed eight studies on the heritability of brain size among twins and families, which showed an average heritability of .91 in a total sample of 2,494 normal adults—a heritability as high as that found for any other human trait (such as height), and even higher than the .5 to .7 heritability found for intelligence in mature adults. (Heritability measures the proportion of a trait’s variation across individuals that can be explained by genetic differences between individuals, so it can range from 0 to 1.) Recent twin research has also found that there is a positive genetic correlation between intelligence and brain size, meaning that many genes have similar positive or negative effects on both intelligence and brain size. In other words, bigger brains are associated with higher intelligence not just because the same environmental factors (nutrition, education) help or harm both, but because the same genes help or harm both. So, general intelligence and brain size are highly heritable, and they are moderately correlated at both the trait level and the genetic level. This is just one piece of evidence suggesting that intelligence is a genuine individual-differences trait with a deep biological basis.

In another study, psychologists Mark Prokosch, Ron Yeo, and I studied the relationship between intelligence and body symmetry. We recruited seventy-eight male college students, and used digital calipers to measure their left-right symmetry at ten points on their bodies (such as ankle width, elbow width, ear width, ear length, finger lengths). Body symmetry is often measured in biology as an index of physical health, condition, genetic quality, and/or fitness. We also gave the students five mental tests each (an excellent intelligence test called Raven’s matrices, two decent intelligence tests based on vocabulary knowledge, and two tests of number memory that are reliable, but that are not good measures of intelligence). We found that, across individuals, scores on the best intelligence test (Raven’s) correlated about +.39 with overall body symmetry. We also found that the better a mental test was at measuring intelligence (the higher its “g-loading,” in technical terms), the more highly correlated its scores were with body symmetry across individuals. Other work by Ron Yeo and colleagues shows that higher body symmetry is also associated with lower risks of neurodevelopmen tal disorders such as mental retardation and lower risks of mental illnesses such as schizophrenia. This suggests that the closer a mental test comes to measuring general intelligence, the closer it comes to measuring general health, fitness, and genetic quality.

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