Progress Daily

People who share genes tend to have similar IQs. The correlation between parent and child is 0.42. If you average both parents’ IQ scores, then the correlation rises to 0.72, indicating that children tend toward the average intelligence of their two parents. Siblings’ IQs correlate by a factor of 0.47, similar to the parent/child correlation of 0.42, which fits with the fact that both pairings share 50% of their genes.

The IQs of adopted children and their biological mothers correlate by a factor of 0.22, about half as much as when they are reared by them. Similarly, biological siblings adopted into different homes show a 0.24 correlation. Doubling these numbers, to correct for the fact that siblings or parent-child pairings share only half of the same genes, allows us to estimate that genes account for 40 to 50% of a person’s intelligence.

The IQs of identical twins reared apart correlate by a factor of 0.72. Fraternal twins show considerably higher IQ correlation than ordinary siblings, even though both pairings have the same proportion of genes (50%) in common, presumably because of their shared experience in the womb. Researchers believe that prenatal experience accounts for up to 20% of IQ variance. Subtracting this effect from the IQ correlation of identical twins reared apart (0.72) leaves genes once again accounting for about 50% of a person’s IQ.

The highest correlation is for identical twins reared together — a whopping 0.86 correlation. Fraternal twins reared together show a correlation of 0.60. The difference between these two numbers, 0.26, represents the additional IQ correlation that identical twins share because of their extra 50% of shared genes, so doubling the number gives us 52% heritability.

IQ tests emphasize spatial skills, like map-reading and mental rotation, which show the greatest dependence on genes, and verbal skills, which are not far behind. Mental speed appears somewhat less heritable, and memory skills are the least heritable of the mental abilities that behavioral geneticists have assessed.

Intelligence actually grows more heritable with age. About 15% of the variance in babies’ IQs can be explained by genes. The figure rises to 40% by the early school years, 50% by adolescence, and a few additional points by adulthood. By adolescence, adoptees’ IQs come to resemble that of their biological parents much more than their adoptive parents’.

Since the beginning of IQ testing, each generation has scored higher than the last. American IQs are rising about 8 points per generation. The average American today scores as well as the top 2% a century ago. IQ gains are considerably greater for visual-spatial skills than for verbal ones, suggesting that the increase may be largely attributable to the explosion of visual media.

What’s Going On in There?, by Lise Eliot, Ph.D.

This entry was posted on Wednesday, May 31st, 2006 at 11:14 am and is filed under Demographics, Genetics, Cognition. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.