The latest news about child rearing is that what parents do doesn't matter -- at least, not as much as we thought. The writer Judith Rich Harris argues in The Nurture Assumption: Why Children Turn Out the Way They Do, a recent best seller, that interaction with peers rather than with parents plays the key role in shaping a child's personality. She does give parents significant credit, but mainly for supplying genes. If Harris is right, good nurturing is a great way for parents to spend evenings and weekends, but nature does most of the real work.

In spite of a growing emphasis on genetic factors in shaping who we are, the pendulum that swings between the extreme positions in the nature/nurture debate still has plenty of momentum, fueled by facts on both sides. No one can argue seriously with the idea that genes make important contributions to personality. Animal breeders long have known that it takes only a few generations of controlled mating to influence such behavioral traits as fierceness or tameness in dogs. In people, as many researchers have shown, genetic factors influence how outgoing, fearful, or aggressive we are, as well as the likelihood that we will develop depression, anxiety disorders, or schizophrenia.

At the same time, the influence on a child's development of the environment in which he or she grows up is also so well substantiated, both scientifically and by common sense, as to hardly need elaboration. For example, Harris doesn't dismiss the role of environment, but rather the importance of parents in the child's environment. She notes that children raised in orphanages can turn out just fine, citing the fact that many of them marry and have children. It seems important to ask not whether they have families and children, but rather how those families do. In fact, Harris seems to avoid asking whether the way a child is raised influences the way that child will act as a parent. She admits that "children without parents are unhappier." But she goes on to say, "The things that make babies (or grownups) miserable do not necessarily have long-term consequences." This is clearly wrong: Stressful experiences can drastically alter important aspects of the brain's development and function.

One of the most important contributions of modern neuroscience has been to show that the nature/nurture debate operates around a false dichotomy: the assumption that biology, on one hand, and lived experience, on the other, affect us in fundamentally different ways. Research has shown that not only do nature and nurture each contribute (in disputable proportions) to who we are, but also that they speak the same language. Both achieve their effects by altering the synaptic organization of the brain.

Synapses are the connection points between brain cells that allow the cells to communicate with each other. Synapses are responsible for much of the brain's activity. The particular patterns of synapses in a person's brain, and the information that those connections encode, are the keys to who that person is.

We are born with a hefty dose of preprogrammed synaptic links -- this is why we can cry and wriggle around the moment we leave the womb. But experience alters synapses as well, either creating new ones or changing the strength of existing ones. Our ability to see the world the same way other humans do is programmed in us. But unless we have the right kinds of visual experiences at the right time, synapses in the visual system of the brain won't develop normally and, as a result, we will not see normally. For example, if a child's eyes are misaligned during a certain period in early life, the brain's visual system is deprived of depth cues and depth perception is impaired.

The process by which experience shapes synapses is referred to as "synaptic plasticity." Although a great deal of synaptic plasticity occurs during early childhood as the brain is developing, plasticity in the form of learning and memory continues to shape our synapses throughout our lives.

We are used to thinking of memory in terms of our ability to consciously recall past events. Recent advances in neuroscience and psychology have revealed that this explicit memory -- sometimes called declarative memory -- is only one of many forms of memory. Various other kinds are implicit, which is to say that they work unconsciously. They are what enable us to learn skills as diverse as how to ride a bike or play the piano, as well as to avoid stimuli previously associated with pain or danger.

Damage in the part of the brain that controls explicit memory can prevent you from remembering when you learned to ride a bike, for example, but not your ability to ride it. Damage to your implicit memory can interfere with your ability to ride but not your memory of having learned how.

Psychoanalysts have long believed that important aspects of our personality operate unconsciously. The traditional view was that unconscious processes reflected either innate (genetic) predispositions or repressed memories of unpleasant or traumatic experiences. However, many aspects of mind and behavior are managed by the brain unconsciously, not because of innate tendencies or repression, but simply because the systems involved are not part of the brain's conscious repertoire -- that is, they function implicitly.

For example, all animals (from bugs and slugs to fish, frogs, and humans) must be able to detect and react to danger, even if their brains do not have the ability to be conscious of those activities. It should not be surprising that animals that are conscious (humans being the example we are most sure of) can react to danger without having to rely on conscious processes. This ability to react to some dangers, such as snakes, can be innate. However, most of the stimuli that make us afraid are programmed into the brain not by evolution, but by our individual experiences. We are born with the ability to act afraid, but we usually have to learn precisely what to fear. This learning occurs as a result of synaptic plasticity.

Synapses between cells change constantly. Whenever we meet people and learn their names or remember their faces, synaptic plasticity is at work. Even patients with Alzheimer's disease, who have severe difficulties with explicit memory, are able to learn some new things through the synaptic plasticity in their systems of implicit memory.

Given the enormous capacity of the brain for synaptic plasticity throughout life, it is hard to imagine that children are shaped by their experiences with their peers, but not by what happens with their parents. What about those early years that children largely spend with their parents, instead of with their peers? Harris admits that the way parents act influences the way in which children behave when they are with their parents. When children become parents, don't many of them -- sometimes with chagrin -- hear themselves say to their children things that they remember their parents saying to them?

Accurately assessing these issues is complicated by the fact that our implicit, as well as explicit, memories register our experiences, yet we cannot consciously gain access to our implicit memories. At the same time, the complexity and importance of the issues involved demand that we correctly weigh the importance of genes, peers and parents, and other environmental factors, such as psychological stress, malnutrition, or physical deprivation. Even if, as Harris argues, research demonstrating the importance of parenting on children's behavior is lacking, that does not assure that parents have no effect. The absence of evidence for an effect is not evidence that the effect does not exist. The major impact of her book in the long run is not likely to be the beginning of the end for good nurturing, but rather the start of a more critical approach to research on the effects of parenting.

Further scientific study will surely reveal new areas of personality and behavior that are genetically predetermined; that is one of the promises, for instance, of the Human Genome Project. But it should be a comfort -- and a caution -- to us all that parents and peers, siblings and mentors, and triumphs and traumas, leave their marks not only in our explicit memories but also in the synapses of the vast and invisible networks of implicit memory.