When psychology emerged as a science in the early 20th century, it focused on nurture, the environmental causes of behavior. Environmentalism—not the ecological kind, but rather the view that we are what we learn—dominated psychology for decades. From Freud onward, the family environment was assumed to be the key factor in determining who we are. In the 1960s geneticists began to challenge this view. Psychological traits such as mental illness clearly run in families, but there was a gradual recognition that family resemblance could be due to nature (genetics) rather than nurture (environment) alone, because children are 50 percent similar genetically to their parents.

During the past four decades, scientists have conducted long-term studies on special relatives like twins and adoptees to test the effects of nature and nurture. This research has built a mountain of evidence showing that genetics contributes importantly to all psychological differences between us. In fact, inherited DNA differences account for about 50 percent of the differences between us, in our personality, mental health and illness, and cognitive abilities and disabilities.

The word “genetic” can mean several things, but here it refers to differences in DNA sequence, the 3 billion steps in the spiral staircase of DNA that we inherit from our parents at the moment of conception. It is mind-boggling to think about the long reach of these inherited differences that formed the single cell with which we began life. They affect our behavior as adults, when that single cell with which our lives began has become trillions of cells, all with the same DNA. They survive the long and convoluted developmental pathways between genes and behavior, pathways that meander through gene expression, proteins and the brain. The power of genetic research comes from its ability to detect the effect of these inherited DNA differences on psychological traits without knowing anything about the intervening processes.

Understanding the importance of genetic influence is just the beginning of the story of how DNA makes us who we are. Studying genetically informative cases like those of twins and adoptees led to some of the biggest findings in psychology because, for the first time, nature and nurture could be disentangled.

One of the most remarkable discoveries is that even most measures of the environment that are used in psychology—such as the quality of parenting, social support and life events—show significant genetic impact. How is this possible when environments have no DNA themselves? Genetic influence slips in because the environment is not randomly “out there” independent of us and our behavior. We select, modify and even create our environments in line with our genetic propensities. Correlations between such so-called environments and psychological traits don’t necessarily mean that the environments cause the traits. For example, parental negativity correlates with their children’s antisocial behavior, but this doesn’t mean that the parents cause their children’s antisocial behavior. Instead, this correlation is substantially caused by parents responding negatively to their children’s genetically-driven propensities.

A second crucial discovery is that the environment works completely differently from the way environmentalists thought it worked. For most of the 20th century, environmental factors were called nurture because the family was thought to be crucial in determining environmentally who we become. Genetic research has shown that this is not the case. We would essentially be the same person if we had been adopted at birth and raised in a different family. Environmental influences are important, accounting for about half of the differences between us, but they are largely unsystematic, unstable and idiosyncratic—in a word, random.

The DNA differences inherited from our parents at the moment of conception are the consistent, lifelong source of psychological individuality, the blueprint that makes us who we are. A blueprint is a plan. It is obviously not the same as the finished three-dimensional structure. The environment can alter this plan temporarily, but after these environmental bumps we bounce back to our genetic trajectory. DNA isn’t all that matters, but it matters more than everything else put together in terms of the stable psychological traits that make us who we are.

These findings call for a radical rethink about parenting, education and the events that shape our lives. It also provides a novel perspective on equal opportunity, social mobility and the structure of society.

The nature-nurture war is over. Nature wins, hands down.