[T]he findings of behavioral genetics are highly damaging to the Blank Slate and its companion doctrines. The slate cannot be blank if different genes can make it more or less smart, articulate, shy, happy, conscientious, neurotic, open, introverted, giggly, spatially challenged, or likely to dip buttered toast in coffee. For genes to affect the mind in all these ways, the mind must have many parts and features for the genes to affect [i.e., the mind must be modular]. Similarly, if the mutation or deletion of a gene can target a cognitive ability as specific as spatial construction or a personality-trait as specific as sensation-seeking, that trait may be a distinct component of the psyche.
Moreover, many of the traits affected by genes are far from noble. Psychologists have discovered that our personalities differ in five major ways: we are to varying degrees introverted or extroverted, neurotic or stable, incurious or open to experience, agreeable or antagonistic, and conscientious or undirected.... All five of the major personality dimensions are heritable, with perhaps 40 to 50 percent of variation in a typical population tied to differences in their genes. The unfortunate wretch who is introverted, neurotic, narrow, selfish, and undependable is probably that way way in part because of his genes, and so, most likely, are the rest of us who have tendencies in any of those directions as compared with our fellows.
It's not just unpleasant temperaments that are partly heritable, but actual behavior with real consequences. Study after study has shown that a willingness to commit antisocial acts, including lying, stealing, starting fights, and destroying property, is partly heritable.... People who commit truly heinous acts, such as bilking elderly people out of their life savings, raping a succession of women, or shooting convenience store clerks lying on the floor during a robbery, are often diagnosed with "psychopathy" or "antisocial personality disorder." Most psychopaths showed signs of malice from the time they were children. They bullied smaller children, tortured animals, lied habitually, and were incapable of empathy or remorse, often despite normal family backgrounds and the best efforts of their distraught parents. Most experts on psychopathy believe that it comes from a genetic predisposition, though in some cases it may come from early brain damage. (Blank Slate, 50-51)If behavioral genetics is largely correct, than Rand's tabula rasa view of human nature is wrong. Man is not a being of a self-made soul; his volition is in fact saddled with tendencies; and his emotions are not entirely the product of his conclusions. Now the evidence for behavior genetics is very compelling. "The effects of differences in genes in minds can be measured, and the same rough estimate — substantially greater than zero, but substantially less than 100 percent — pops out of the data no matter what measuring stick is used," writes Pinker. Whether we're talking about identical twins raised apart or experiments involving isolation of genes, it all points to one conclusion: that genes influence (though they don't determine!) the mind and the behavior that emerges from it.