What do your fingers, palms, ears, ankles, and elbows have to do with your intelligence?
Every once in a while researchers report something about intelligence that is startling and weird: symmetrical people tend to score higher on IQ tests than people whose fingers, palms, ears, ankles, and elbows are differently sized. Of all the things associated with intelligence, being symmetrical does not usually leap to mind. Now, if an unexpected finding comes from only one study with a small sample, it is probably a fluke. However, if three different research teams with 5 different samples all report the same thing, the finding deserves serious attention. This is now the case with the correlation of intelligence with symmetry (Bates, 2007).
It is no small feat of engineering that our genes are able to get our body parts that come in pairs (arms, legs, ears, eyes, etc.) to be roughly the same size and stay that way throughout our development. It is true that developmental processes fail to get it exactly right in everyone all the time, but it is amazing that it works at all. When one side of the body is substantially larger than the other, it is not usually due to abnormal genes but is instead the result of an environmentally caused disturbance of development such as exposure to a parasite, a toxin, a stroke, or any number of stressors that strike at random. For each individual, we can measure the size differences between body part pairs and calculate an overall measure of asymmetry. This overall measure, called fluctuating asymmetry, is hypothesized to be a rough index of how much biological stress an individual has endured during development, particularly during pregnancy and infancy. The logic behind the connection between intelligence and fluctuating asymmetry goes something like this: the kinds of stressors that result in asymmetry might also have an effect on the brain. Therefore, when we see a very asymmetrical person, it is likely that the processes that disrupted development in the person's body, have have also (but not necessary) disrupted aspects of brain development that lead to lower IQ test scores.
Before we get carried away with this finding and starting using calipers to make hiring and college admission decisions, it is important to remember that the kinds of correlations found in these studies, although substantial enough (between .2 and .4) to be scientifically interesting, are not sufficiently strong to suggest that fluctuating asymmetry measures should be used for decisions about individuals, especially in the absence of other information. In the Bates (2007) study, it appeared that highly symmetrical people tended to score higher on intelligence tests but could also score low. In contrast, highly asymmetrical people almost always scored lower on IQ tests. Thus, it appears that asymmetry is a signal that something may have gone wrong with brain development but being symmetrical does not necessarily mean that a person is highly intelligent.
- Bates, T. C. (2007). Fluctuating asymmetry and intelligence, Intelligence, 35, 41-46. (click here to view)
- The general factor of mental ability (g) may reflect general biological fitness. If so, g-loaded measures such as Raven's progressive matrices should be related to morphological measures of fitness such as fluctuating asymmetry (FA: left–right asymmetry of a set of typically left–right symmetrical body traits such as finger lengths). This prediction of a negative correlation between FA and IQ was confirmed in two independent samples, with correlations of −0.41 and −0.29, respectively. Head size also predicted Raven's scores but this relationship appeared to be mediated by FA. It is concluded that g along with correlated variables such as head size are in large part a reflection of a more general fitness factor influencing the growth and maintenance of all bodily systems, with brain function being an especially sensitive indicator of this fitness factor.
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