Intelligence Largely Rooted in the Genes

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There seems to be lots of misunderstandings about this study and the science related to it.
Some clarifications:
- Heritability refers to the proportion of trait variation between individuals that is explained by genes. If the heritability of IQ is, say, 50%, it does NOT follow that 50% of your IQ is determined by genes and 50% by environment or culture. Heritability indicates the extent to which differences between individuals in a population are due to genetic differences. Thus, if a population consists of genetic clones, the heritability of differences in IQ or any other trait between them is 0 percent (because clones are genetically identical). Heritability does not say anything about why a particular individual has a certain level of IQ (okay, it does say something but not in the sense most people would think).
- This new study indicates that 40-50 percent of variation in intelligence can be explained by genetic differences. However, as the authors say in their paper, this establishes only the lower bound of the heritability of intelligence. Twin and family studies indicate that the heritability of intelligence is as much as 80% in adults in the developed world. More studies are needed to discover all genetic sources of IQ variation. Much variation may be due to rare genetic variants that the common variant DNA data the researchers had did not include.
- All forms of intelligence are intercorrelated in a population. This means that people who are good at math tend to be good at, say, learning foreign languages, too, while those who are bad at one thing are usually bad at other things, too. This is what is referred to as general intelligence. There are also specific, narrower types of intelligence, but they are less important than general intelligence. To illustrate this, think of idiot savants who have some specific intellectual gift but a low IQ. Such savants can accomplish extraordinary feats using their specific intellectual gift, yet due to their low general intelligence (as measured by IQ) they usually cannot even support themselves and live independently.

Engineer Dad of CA 12:51AM August 10, 2011