The question of what shapes human behavior has become such a highly charged political issue that many people are eager to wish it away. Everyone now knows that heredity and environment play an intertwined role, they argue, so let’s just agree that the answer to the nature-nurture question is “some of each.”

Bad idea, says Pinker, a psychologist at Harvard University. It’s not even true that everyone acknowledges the role of heredity in human behavior. Some scientists cling to the theory of the mind as a blank slate, and postmodern thinkers in the humanities insist that virtually all human emotions and behavioral categories are “socially constructed.” More important, it’s not true that “some of each” is always the proper answer. Environmental influences provide 100 percent of the explanation for why people in different countries speak different languages, but these influences have been totally ruled out as a cause of certain psychopathologies, such as autism and schizophrenia. “Mothers don’t deserve some of the blame if their children have these disorders, as a nature-nurture compromise would imply,” Pinker notes. “They deserve none of it.”

It’s true that the expression of some genes is shaped by the environment, but that doesn’t mean, as some contend, that heredity is inconsequential. People taking this view often point to phenylketonuria (PKU), an inherited disease that causes mental retardation: Patients given a diet low in phenylalanine can avoid severe retardation. However, these advocates of the nurture perspective seldom note that “PKU children still have mean IQs in the 80s and 90s” and suffer other impairments, Pinker says. In fact, “genes specify what kinds of environmental manipulations have what kinds of effects and with what costs.”

Acknowledging and studying inborn proclivities can help us domesticate them. For example, humans seem to have a natural sympathy for others, but it’s normally limited to their “own”: family, clan, or village. In the right environment, however, that sympathy can be expanded to “clans, tribes, races, or even species.” Understanding what those circumstances are can reveal “possible levers for humane social change.”

One of the most startling findings in behavioral genetics is the revelation through research on identical twins that family environment has “little or no effect” on individual intelligence and personality. Yet twins do nevertheless differ in important ways. So now researchers are asking new questions: What is the role of peer culture in the development of personality? What is the role of chance events? “These profound questions are not about nature versus nurture,” Pinker writes. “They are about nurture versus nurture: about what, precisely, are the nongenetic causes of personality and intelligence.” And they might never have been asked if researchers had thrown up their hands and ended the nature-nurture debate by agreeing to split the difference.