In 1998, a kindly grandmother living in New Jersey wrote a book about child-rearing that created quite a stir. In “The Nurture Assumption: Why Children Turn Out the Way They Do,” Judith Rich Harris had the temerity to suggest that the most important influences on children were not their parents but genes and peers. This was heresy, and critics immediately attacked the book in reviews with titles such as “Parents Don’t Count!”

Nonetheless, Mrs. Harris had made a very convincing argument, and she stuck to her guns. Now, with “No Two Alike” (W.W. Norton & Company, 352 pages, $26.95), she has expanded her thesis and has attempted to formulate a new theory of personality formation – the first, in fact, since Sigmund Freud. More specifically, she has attempted to solve the mystery of why people are different.

Why are we the way we are? Why do identical twins, raised in the same house by the same parents, turn out to have such different personalities? For years, psychologists and other professionals thought they had the answers, but this grandmotherly, iconoclastic outsider may force us to revise our thinking about these basic questions.

One way to understand the great changes in our attitude about the formation of personality is to consider two well-known cases, one from the 19th century, one from the 20th. Alice James (sister of William and Henry) and John Cheever both suffered from depression. Each sought to explain their condition in terms of the prevailing ideas of their time. James blamed heredity; she believed she had inherited the affliction from her parents. Cheever, on the other hand, blamed his childhood environment. His mother worked away from home and neglected him, he claimed.

Two events caused this fundamental shift in attitude. First, the advent of Freud and his child development theories (based on infant sexuality), which became wildly influential and popular during the first half of the 20th century. Second, the complete discrediting of behavioral genetics following the Nazis and World War II (“eugenics”). Thus the blank slate (that is, the infinite malleability of the brain) became the dominant model, and environment the explanation of all human behavior.

This bitter debate still rages on today, despite the fact that Freud has been discredited completely and genetic explanations for human behavior are now widely accepted. Accepted, that is, by scientists in such fields as evolutionary psychology and behavioral genetics, but definitely not accepted by academic psychologists and child development experts. They continue to dispense advice about how we should parent our children (Benjamin Spock lives on!) and have continued to willfully underestimate the genetic role in human behavior.

This is where the indomitable Mrs. Harris has jumped into the fray. Here is how she describes the situation:

The developmentalists found that the children’s behavior was correlated with the parents’ behavior and attributed the correlation to the effects of the home environment. Though they realized that heredity might account for some of the correlation, they never considered the possibility that heredity might account for all of it. But that is exactly how it turned out. Once the effects of genetic similarities were estimated and skimmed off, the correlation declined to zero. The putative effects of the home environment disappeared.

This was not welcome news for developmental psychologists, and they responded with vitriolic attacks. Part of their problem was a lack of understanding of how genetics works, particularly with regard to its role in defining the behavior of the child’s parents. Good parenting itself is largely a genetic characteristic.

But the larger question Mrs. Harris seeks to address in this book is how to account for differences in human personalities.As she puts it: “My goal was to explain the variation in personality – the big and little differences among individuals – that cannot be attributed to variations in their genes.” This is not a simple matter. Her theory is not simple, either.

Basically, Mrs. Harris believes there are three “perpetrators” at work in the formation of the human personality, each associated with an aspect of a modular brain. One is the “relationship system,” designed to maintain favorable relationships in society.Another is the “Socialization System,” where the goal is to be a member of a group. The third is the “Status System,” where we compete with our peers for status.

The interplay among these systems accounts for the emergence of differences between individuals. So it is that even identical twins develop different personalities because the members of their community see them as unique individuals and treat them differently. Their individual striving for status propels them into different modes of competing, which in turn differentiates their personalities.

By combining inputs from so many scientific disciplines – social psychology, developmental psychology, psycholinguistics, neurophysiology, anthropology, primatology, and entomology, in addition to evolutionary psychology and behavioral genetics – Mrs. Harris has created a novel, holistic perspective. Even if it turns out the final explanation of human development is a long way off and still more complicated (“biological processes have turned out to be fancier and messier than anyone imagined,” she admits), this book clears out much dead wood and will shape the debate to come.

Mrs. Harris is an amazing woman. Now in ill health and largely confined to her home, she nonetheless has pursued her interest in these subjects in spite of her many physical handicaps and the fact that she is not a formal member of any of the scientific disciplines involved. Ironically, this has been her main strength. As an independent scholar, she took a broader view of the issue than was possible for many of the certified experts. Perhaps that is why she has been able to see the forest, as well as the trees.

Mr. Pettus last wrote in these pages on nuclear fusion.