What makes one person socially insightful but mathematically challenged, and another musically gifted yet devoid of a sense of direction? Individual differences in general cognitive ability are thought to be mediated by “generalist genes” that affect many cognitive abilities similarly without specific genetic influences on particular cognitive abilities. In contrast, we present here evidence for cognitive “specialist genes”: monozygotic twins are more similar than dizygotic twins in the specific cognitive ability of face perception. Each of three measures of face-specific processing was heritable, i.e., more correlated in monozygotic than dizygotic twins: face-specific recognition ability, the face-inversion effect, and the composite-face effect. Crucially, this effect is due to the heritability of face processing in particular, not to a more general aspect of cognition such as IQ or global attention. Thus, individual differences in at least one specific mental talent are independently heritable. This finding raises the question of what other specific cognitive abilities are independently heritable and may elucidate the mechanisms by which heritable disorders like dyslexia and autism can have highly uneven cognitive profiles in which some mental processes can be selectively impaired while others remain unaffected or even selectively enhanced.
Here’s some more from ScienceDaily:
For the study, Liu and his colleagues recruited 102 pairs of identical twins and 71 pairs of fraternal twins aged 7 to 19 from Beijing schools. Because identical twins have 100 percent of their genes in common while fraternal twins have just 50 percent, traits that are strongly hereditary are more similar between identical twins than between fraternal twins. (Identical twins still show variability because of the influence of environmental factors.)
Participants were shown black-and-white images of 20 different faces on a computer screen for one second per image. They were then shown 10 of the original faces mixed with 20 new faces and asked which ones they had seen before. The scores were more closely matched between identical twins than fraternal twins, and Liu attributed 39 percent of the variance between individuals to genetic effects. Further tests confirmed that these differences were specific to face recognition, and did not reflect differences in sharpness of vision, general object recognition abilities, memory or other cognitive processes.
In an independent sample of 321 students, the researchers found that face recognition ability was not correlated with IQ, indicating that the genes that affect face recognition ability are distinct from those that affect IQ. Liu and Kanwisher are now investigating whether other cognitive abilities, such as language processing, understanding numbers, or navigation, are also heritable and independent from general intelligence and other cognitive abilities.
I’ve blogged about prosopagnosia before, which is a much more extreme manifestation of lack of face recognition than they’re talking about here. These “face blind” individuals are shockingly common within the general population, 1 out of 50 humans. I haven’t heard it reported that these individuals are particularly unintelligent, so it stands to reason that there’d be no relationship between intelligence and the ability to recognize faces. I happen to know some individuals with this issue, and they’ve developed “tricks” to compensate, so I know firsthand that they’re not necessarily a dull lot.
So how to reconcile the heritability of IQ with domain specific competencies that exhibit modularity? I think the model in terms of genetic architecture is pretty simple. General intelligence is a quantitative trait characterized by a normal distribution; the bell curve. This is because variation on the trait is due to many genes of small effect; i.e., each genetic variant which controls variation of IQ has only a very small effect on the trait. By contrast, imagine that face recognition is controlled by a few genes of large effect, for example, five genes controlling most of the variance of the trait. Even if these five genes which control face recognition ability also affect intelligence, their contribution to the variance on the latter is going to be trivial because any specific gene has only a small effect, at least in the normal range. I assume that this sort of dynamic could characterize many domain specific cognitive traits.
Citation: Heritability of the Specific Cognitive Ability of Face Perception, doi:10.1016/j.cub.2009.11.067