单项选择题

The term "g" (general intelligence) represents a measure of overall cognitive ability across a variety of tests. It’s not the same as IQ but it does tend to correlate. Everyone agrees that "g" tends to run in families. But is this down to genetics or to environmental influences
However, no single gene has yet been conclusively linked to intelligence. Rather it appears to be a case of complex interactions on many levels between many different genes.
Identical twins have exactly the same genes, while non-identical twins share about half their genes. Another feature of twins that makes them an ideal choice for studies is that they tend to be raised in pretty much the same environment. If a particular feature is the same in identical twins, but not m non-identical twins, then chances are it’s mainly genes that are controlling that feature.
So what do the twin studies show Well, first degree relatives tend to have "g" correlation of about 0.4-0.5. (Perfect correlation is 1; correlation of 0 means that the two things in question are totally unrelated). Identical twins have a correlation of 0.85, while for non-identical twins it’s about 0.6. Which suggests that genes play a very important role, but are not the only factor, since if they were, the correlation between identical twins would be 1.
Identical twins reared apart are almost as similar in "g" scores as those reared together. Adopted children and their adoptive parents have a "g" correlation of zero, while adopted children and their biological parents tend to have the same correlations as any parent-child pair. So although genes don’t seem to be the only thing affecting intelligence, their effects seem to be constant and apparently not overridden by environment.
Does heritability of intelligence alter over a lifetime Remarkably, it appears so. "g" heritability climbs gradually from 20% in babies to 40% in children, peaking at 60% in adults. Why this should be is still a matter of speculation. It’s been suggested that as our cognitive abilities become more complex, new genes may come into play that were not needed when brain functions were less sophisticated. Or individuals may be drawn towards environments that fit with their genetic makeup, as time goes by and genetic effects that started out small in childhood build up together during adulthood. All of the following statements about "g" are true EXCEPT______.