There are two kinds of special families: those with twins and those with adoptees. If you want to disentangle the effects of nature and nurture, one approach is to compare identical twins, who share all of their genes, to fraternal twins, who share only half. Another approach is to compare adoptees to members of their adoptive families. If identical twins are more similar than fraternal twins, we have strong reason to believe that the cause is nature. If adoptees resemble members of the families they grew up with, we have strong reason to believe that the cause is nurture.
By using — and refining — these twin and adoption methods, behavioral geneticists have produced credible answers to the nature-nurture controversy. To put it simply, nature wins. Heredity alone can account for almost all shared traits among siblings. "Environment" broadly defined has to matter, because even genetically identical twins are never literally identical. But the specific effects of family environment ("nurture") are small to nonexistent. As Steven Pinker, a professor of psychology at Harvard University, summarizes the evidence:
"First, adult siblings are equally similar whether they grew up together or apart. Second, adoptive siblings are no more similar than two people plucked off the street at random. And third, identical twins are no more similar than one would expect from the effects of their shared genes."
The punch line is that, at least within the normal range of parenting styles, how you raise your children has little effect on how your children turn out...
Recent scholarship does highlight some exceptions [but] the fact remains that people tend to greatly overestimate the power of nurture.
If family environment has little effect, why does almost everyone think the opposite? Behavior geneticists have a plausible explanation for our confusion: Family environment has substantial effects on children. Casual observers are right to think that parents can change their kids; the catch is that the effect of family environment largely fades out by adulthood. For example, one prominent study found that when adoptees are 3 to 4 years old, their IQ has a .20 correlation with the IQ of their adopting parents; but by the time adoptees are 12 years old, that correlation falls to 0. The lesson: Children are not like lumps of clay that parents mold for life; they are more like pieces of flexible plastic that respond to pressure, but pop back to their original shape when that pressure is released.
Bleak news indeed for the SES warriors.
This is why we see substantial IQ gains for some preschool programs whose effects fade by the end of elementary school. This is also why time should not be wasted in elementary school doing "developmentally appropriate" nonsense. There is a brief window of opportunity that needs to be taken advantage of.