I recently came across this article at Quilette about twins studies.
In 2000, the psychologist Eric Turkheimer concluded that the evidence from behavioral genetic data was consistent enough to summarize in three laws. The first law holds that all human traits are heritable (i.e., genetic differences account for phenotypic differences) to some degree. This assertion may not seem all that surprising today, although the word ‘all’ is still considered provocative by some. However, twin studies have produced copious data demonstrating that almost every trait is heritable to some degree or another.
The remaining two laws concern environmental influence. The second of these holds that the effect of being raised in the same family is smaller than the effect of genes. Judith Harris called attention to this theory in her 1998 book The Nurture Assumption, and was subsequently defended by Harvard psychologist Steven Pinker in The Blank Slate. Pinker encouraged parents to stop fretting about what they had or had not done in order to turn their offspring into wonderful individuals. Parents, he argued, do not hold their children’s future in their hands, only their present. Pinker emphasized that parenthood remains an awesome ethical responsibility, and that it is important to give one’s child a childhood worth remembering. But parents cannot shape a child’s personalities and IQ as a sculptor fashions clay. As Dr. Nancy Segal has put it, homes do matter, but they do not make people alike.
The third law holds that a substantial portion of the variation in complex human behavioral traits is not accounted for by either the effects of the genes or families. In other words, while about 50 percent of the variation is due to the environment, this environmental effect does *not *come from the family. Instead, it may be produced by the wider culture, society, the neighborhood, school, peer-groups and friends, but also simply chance: random encounters or openings in the social hierarchy, cosmic rays that damage a piece of DNA, neurons that go zig instead of zag, and so on.
Twin studies have uncovered the enormous importance of genetics. They have laid to rest the notion that parents are omnipotent sculptors, and a child is a piece of clay. They have hammered another nail into the coffin of the Freudian guilt complex, where everything that goes wrong in an individual’s life may be attributable to poor parenting.
This has some interesting consequences. The most important is when you attach this to the observed fact that genetically distinct populations exist, no matter how fuzzy the boundaries. Sure, some aspects are less genetically determined/forced than others, and in nearly all cases there is some variance, and the variance within a population grouping is wider than the difference between them. Nevertheless, if traits like conscientiousness, the “warrior gene”, etc. are heritable, and we know for a fact that the prevalence or tendency towards certain personality traits and mannerisms and behaviors are genetically rooted, that means that stereotypical behaviors and trends for various ethnic groups are grounded in scientific fact, and their existence is not merely a social construct, even if their expression may be.