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 in 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. it can be concluded from the passage that ______.
A. it is easy to identify what determines our intelligence
B. genes are a more important determinant of "g" than environment
C. identical twins have exactly the same IQ
D. IQ depends more on environment than on genes