Judith Harris, author of ''The Nurture Assumption.'' (Shonna Valeska – Newsweek)

Surely there is no more cherished, yet humbling, idea than the conviction that parents hold in their hands the power to shape their child's tomorrows. And the evidence for it is as impossible to ignore as the toddler throwing a tantrum in the grocery store when Daddy refuses to buy him M&Ms: setting reasonable, but firm, limits teaches children self-control and good behavior, but being either too permissive or too dictatorial breeds little brats. Giving your little girl a big hug when she skins her knee makes her feel loved and secure, which enables her to form trusting relationships when she blooms into a young woman. Reading and talking to children fosters a love of reading; divorce puts them at risk of depression and academic failure. Physical abuse makes them aggressive, but patience and kindness, as shown by the parents who soothe their child's frustration at not being able to play a favorite piano piece rather than belittling him, leaves a child better able to handle distress both in youth and in adulthood. Right?

Wrong, wrong and wrong again, contends Judith Rich Harris. In a new book, "The Nurture Assumption: Why Children Turn Out the Way They Do; Parents Matter Less Than You Think and Peers Matter More" (462 pages. Free Press. $26), Harris is igniting a bonfire of controversy for her central claim: the belief "that what influences children's development... is the way their parents bring them up... is wrong." After parents contribute an egg or a sperm filled with DNA, she argues, virtually nothing they do or say--no kind words or hugs, slaps or tirades; neither permissiveness nor authoritarianism; neither encouragement nor scorn--makes a smidgen of difference to what kind of adult the child becomes. Nothing parents do will affect his behavior, mental health, ability to form relationships, sense of self-worth, intelligence or personality. What genes don't do, peers do.

Although Harris's book lists some 750 scientific papers, articles and books as references, maybe all she really had to do to reach this conclusion was keep good notes about the goings-on in her own suburban New Jersey colonial. Harris and her husband, Charles, had one daughter, Nomi, on New Year's Day, 1966, and adopted a second, Elaine, almost four years later. The girls grew up in the same home "filled to overflowing with books and magazines, where classical music was played, where jokes were told," recalls Harris. Both girls took ballet lessons; both learned the crawl at Mrs. Dee's Swim School. Both were read books by their parents and both delighted in birthday parties with homemade cake. Both experienced the sorrow and stress of a sick mother (Harris developed a mysterious autoimmune illness, part lupus and part systemic sclerosis, when Elaine was 6 and Nomi 10, and was often confined to bed). Yet Nomi was a well-behaved child who "didn't want to do anything we didn't want her to do," says Harris over iced tea in her kitchen. Elaine, adopted at 2 months, was defiant by the age of 11. She angrily announced to her parents that she didn't have to listen to them. When they grounded her once, at 15, she left for school the next morning--and didn't come back that night. Nomi was a model student; Elaine dropped out of high school.

It made Harris wonder. Why was she having about as much influence on Elaine as the fluttering wings of a butterfly do on the path of a hurricane? And it made her mad. "All of these studies that supposedly show an influence of parents on children--they don't prove what they purport to," she fumes. Having floated this idea in the scientific journal Psychological Review in 1995, she has now turned it into a book that is becoming the publishing phenom of the season. This week Harris is scheduled for morning television shows, radio interviews and network magazine shows. The Free Press has gone back for a third printing after an initial run of 15,000, and her publicists say every author's dream--Oprah--may be in her future.

This petite, gray-haired grandmother hardly seems the type to be lobbing Molotov cocktails at one of the most dearly held ideas in all of child development. Harris, 60, has no academic affiliation and no Ph.D. In 1961, she was thrown out of Harvard University's graduate department of psychology because her professors believed she showed no ability to do important original research. She got a job writing psych textbooks. Yet in August, Harris shared a $500 prize from the American Psychological Association, for the paper that best integrates disparate fields of psychology. And she has some big guns on her side. Neuroscientist Robert Sapolsky of Stanford University says her book is "based on solid science." John Bruer, president of the James S. McDonnell Foundation, which funds education programs, praises it as "a needed corrective to this belief that early experiences between the child and parents have a deterministic, lifelong effect." And linguist Steven Pinker of the Massachusetts Institute of Technology predicts that "The Nurture Assumption" "will come to be seen as a turning point in the history of psychology."

So far, though, that's a minority view, and many scientists are nothing short of scathing. "I am embarrassed for psychology," says Harvard's Jerome Kagan, arguably one of the deans of child development. "She's all wrong," says psychologist Frank Farley of Temple University, president of the APA division that honored Harris. "She's taking an extreme position based on a limited set of data. Her thesis is absurd on its face, but consider what might happen if parents believe this stuff! Will it free some to mistreat their kids, since 'it doesn't matter'? Will it tell parents who are tired after a long day that they needn't bother even paying any attention to their kid since 'it doesn't matter'?" Psychologist Wendy Williams of Cornell University, who studies how environment affects IQ, argues that "there are many, many good studies that show parents can affect how children turn out in both cognitive abilities and behavior. By taking such an extreme position, Harris does a tremendous disservice."

On the effect of quality time: 'Parenting has been oversold. You have been led to believe that you have more of an influence on your child's personality than you really do.'

In fact, neither scholars nor parents have always believed that parents matter. Sure, today rows upon rows of parent-advice books fill stores, parenting magazines clog newsstands, and new parents know the names Penelope Leach and T. Berry Brazelton better than they do their newborns'. But a leading tome on child development published in 1934 didn't even include a chapter on parents. It was only in the 1950s that researchers began to seek the causes of differences among children in the ways that parents raised them. Now Harris is part of a growing backlash against the idea that parents can mold their child like Play-Doh.

With an impish wit and a chatty style, Harris spins a persuasive argument that the 1934 book got it right. Her starting point is behavioral genetics. This field examines how much of the differences between people reflect heredity, the genes they inherit from their parents. Over the years, researchers have concluded that variations in traits like impulsivity, aggression, thrill-seeking, neuroticism, intelligence, amiability and shyness are partly due to genes. "Partly" means anywhere from 20 to 70 percent. The other 30 to 80 percent reflects "environment." "Environment" means influences like an encounter with a bully, a best-friendship that lasts decades, an inspiring math teacher. It also includes, you'd think, how your parents reared you. But Harris argues that "environment" includes a parental contribution of precisely zero (unless you count Mom and Dad's decision about which neighborhood to live in, which we'll get to later). When she says parents don't "matter," she means they do not leave a lasting effect--into adulthood. (She accepts that how parents treat a child affects how that child behaves at home, as well as whether the grown child regards the parents with love, resentment or anger.)

On divorce: 'Heredity . . . makes the children of divorce more likely to fail in their own marriages. Parental divorce has no lasting effects on the way children behave when they're not at home.'

To reach her parents-don't-matter conclusion, Harris first demolishes some truly lousy studies that have become part of the scientific canon. A lot of research, for instance, concludes that divorce puts kids at greater risk of academic failure and problem behavior like drug use and drinking. Other studies claim to show that parents who treat their kids with love and respect, and who get along well with others, have children who also have successful personal relationships. Yet neither sort of study "proves the power of nurture at all," Harris says emphatically. Why? They do not take into account genetics. Maybe the reason some parents are loving or competent or prone to divorce or whatever is genetic. After all, being impulsive and aggressive makes you more likely to divorce; both tendencies are partly genetic, so maybe you passed them on to your kids. Then it's their genes, and not seeing their parents' marriage fail, that explain the kids' troubles, Harris claims. And if being patient and agreeable makes you more likely to be a loving and patient parent, and if you pass that nice DNA to your kids, then again it is the genes and not the parenting that made the kids nice.

Do your own eyes tell you that being a just-right disciplinarian--not too strict, not too easy--teaches children limits and self-control? Not so fast. Harris points out that children, through their innate temperament, can elicit a particular parenting style. For example, a little hellion will likely make her parents first impatient and then angry and then resigned. It isn't parental anger and resignation that made the kid, say, a runaway and a dropout. Rather, the child's natural, genetic tendencies made her parents behave a certain way; those same tendencies made her a runaway and a dropout. Again, argues Harris, not the parents' fault. By this logic, of course, parents don't get credit, either. You think reading to your toddler made her an academic star? Uh-uh, says Harris. Maybe kids get read to more if they like to get read to. If so, liking books is also what makes them good in school, not listening to "Goodnight Moon."

Studies of twins seem to support Harris's demotion of parents. "[I]dentical twins reared in the same home," says Harris, "... are no more alike than identical twins separated in infancy and reared in different homes." Apparently, being reared by the same parents did nothing to increase twins' alikeness. Same with siblings. "[B]eing reared by the same parents [has]little or no effect on [their] adult personalities," writes Harris. "The genes they share can entirely account for any resemblances between them; there are no leftover similarities for the shared environment to explain." By "shared environment," she means things like parents' working outside the home, battling constantly, being dour or affectionate. A son might be a cold fishlike Dad, or react against him and become a warm puppy. "If children can go either way, turning out like their parents or going in the opposite direction," says Harris, "then what you are saying is that parents have no predictable effects on their children. You are saying that this parenting style does not produce this trait in the adult."

What Harris offers in place of this "nurture assumption" is the idea that peer groups teach children how to behave out in the world. A second-grade girl identifies with second-grade girls and adopts the behavioral norms of that group. Kids model themselves on other kids, "taking on [the group's]attitudes, behaviors, speech, and styles of dress and adornment," Harris says. Later, a child gravitates toward the studious kids or the mischief makers or whomever. Because people try to become more similar to members of their group and more distinct from members of other groups, innate differences get magnified. The jock becomes jockier, the good student more studious. This all begins in elementary school. Harris's bottom line: "The world that children share with their peers determines the sort of people they will be when they grow up."