Since people, not to mention families, are so infernally complicated, consider the rat. As soon as their wriggly little pups are born, rat mothers lick and groom them, but like mothers of other species they vary in how obsessive they are about getting every one of their offspring's hairs in place. Pups whose mothers treat them like living lollipops grow up different from pups of less devoted mothers: particular genes in the pups' brains are turned on "high." These brain genes play a pivotal role in behavior. With the genes turned up full blast, the rats churn out fewer stress hormones and, as adults, are more resistant to stress, finds Michael Meaney of McGill University. These rats don't startle as easily, are less fearful in the face of novel situations and braver when they have to explore an open field. In rats whose mothers did not lick them so much, the brain genes are not turned up so high (though they are very much present), and the pups grow up to be jumpy, angst-ridden and stressed-out--Woody Allens with whiskers. All because of how much Mom licked and groomed them.
Thanks to experiments like these, the age-old nature-nurture controversy--is the person we become shaped more by the genes we inherit from our parents, or by our life experiences?--is growing up. Now the consensus is that we are shaped by both nature and nurture, also known as heredity and environment. But while this wishy-washy conclusion offends neither naturists nor nurturists, it hasn't explained much either. That is changing. An ambitious study of 720 pairs of adolescents with different degrees of genetic relatedness (from identical twins to step-siblings) reconciles nature and nurture by explaining how genetic tendencies are encouraged, or stifled, by specific parental responses. "Biology is not destiny," psychologist David Reiss of George Washington University writes in the new book "The Relationship Code," which describes his 12-year study. "Many genetic factors, powerful as they may be in psychological development, exert their influence only through the good offices of the family." And that means that how parents raise their children actually does matter.
To understand why, take a trait like shyness, which seems to be partly heritable. But it is not heritable the way, say, eye color is. If parents coddle and overprotect a shy toddler, she will probably remain shy; if they encourage, even force, her into spending time with other tykes, she has a good chance of overcoming it. Exactly how these different parental responses encourage or suppress, at the molecular level, genes that predispose for shyness remains a great unknown. But the general message is clear: gene expression is not foreordained. To have any effect, genes must be turned on. (A quick glance at your hand will show this: the cells in the skin contain the same 80,000 or so genes as the cells in your pancreas, but the gene that codes for insulin is turned on in your pancreas yet silenced in your skin.) Whether, and how strongly, genes that underlie complex behaviors are turned on, or "expressed," says Reiss, depends on the interactions and relationships a child has with the important people in his or her life.
Already scientists who have been foursquare in the nature camp are having second thoughts as a result of the study by Reiss, Robert Plomin, Mavis Hetherington and colleagues. "It's a tour de force," says David Rowe of the University of Arizona, who has long argued that genes, not families, determine how children turn out. "It will be very influential and, in the long run, very important." Behavioral geneticist Kenneth Kendler of the Medical College of Virginia praises the study for "going beyond what anyone has ever done before in trying to bridge family research and genetics." Building that bridge would tax any scientific construction crew. On the one side, many psychologists believe that environment is all: children of abusive parents often become antisocial and sometimes delinquent, children of cold parents struggle to form emotional bonds. On the other side, behavioral geneticists conclude that a child's fate is in his genes. Their evidence: identical twins reared together are no more alike than identicals reared apart. Apparently, being raised in the same way by the same parents added nothing to the twins' similarities in self-esteem, intelligence, personality, anxiety and other traits. And unrelated children who are adopted by the same family are no more alike than two kids chosen at random from a community, again casting doubt on the power of environment.
The scientists reach a different conclusion. Yes, genes matter: any new parent can see that children are born with innate temperaments. These traits--cuddly or cold, cranky or calm--"cause their parents... to respond to them in a certain way," says Reiss. If a baby is unresponsive, most parents show her less affection; if the child is a holy terror, most parents scream, punish, hit or otherwise lash out. Reiss and his colleagues argue that it is these very responses that determine whether the gene underlying the trait is expressed or silenced, turned up or turned down. "Genetic factors initiate a sequence of influences on development, but certain social processes are critical for the expression of these genetic influences," says Reiss.
With a problem kid, who is perpetually disobeying, acting out, threatening, hitting, parents typically meet threat with threat, violence with violence and coercion with coercion. That is likely to exaggerate the child's innate proclivities and even increase the chances that he will become seriously antisocial and even criminal. "Family is like a catapult," says Kendler. "Kids with a difficult temperament can be managed and set on a good course, or their innate tendencies can be magnified by the family and catapulted into conduct disorder. This is a concrete, testable model of how genes and environment interact." On a happier note, genes seem to have an early effect on verbal IQ. A highly verbal toddler will likely elicit hour after hour of reading from her parents; that probably stimulates her innate cognitive tendencies. Later, she also develops self-confidence in her academic abilities, seeks out challenges and gains a reputation as a studious, bright kid. All this may act to maintain and even amplify her cognitive gifts into adolescence and adulthood until she leaves her peers in the academic dust--even if her (hypothetical) IQ genes are only the slightest bit smarter than those of the other kids.
Figuring out which environments turn up a gene and which turn it down is like trying to match the right soil to a delicate flower, only harder. Still, scientists have identified some links. Besides intellectual achievement, genetic factors seem to influence the first stirrings of sociability and antisocial behavior. These glimmerings typically evoke parental responses that reinforce the nascent trait. A child with a difficult temperament--irritability, aggressiveness--brings on parents' harsh discipline, verbal abuse, anger, hostility and relentless criticism. That seems to exacerbate the child's innate bad side, which only makes parents even more negative, on and on in a vicious cycle until the adolescent loses all sense of responsibility and academic focus. "Heritable characteristics of the child shape the level of parental hostility toward that child," says Reiss. "Parental hostility then intensifies the child's characteristics, making antisocial behavior more likely." But a sociable baby brings out a mother's affection and encouragement, which reinforce those initial tendencies toward sociability.
The controversial 1998 book "The Nurture Assumption" argued that the way parents treat their children matters not a whit to how their sons and daughters turn out; whatever traits genes don't determine, peers do. Although the book was roundly criticized for its weak science, it did have one thing going for it: it let parents off the hook. Now parents are right back on it. But if Reiss and Plomin's explanation of how children develop is correct, it offers hope that with appropriate parenting, a child's sociability, sense of responsibility and cognitive development will bloom, and his tendency to antisocial behavior, academic failure and emotional problems wither.
Such "parenting against type"--pushing your shrinking violet into the social whirl rather than coddling her, reading and reading to a child who shows little interest, cuddling a baby who doesn't cuddle back--won't come easy. But it can happen. So-called intervention programs, which teach parents to respond to their infuriating child not by escalating the anger but by firm discipline, have brought about remarkable changes in children, slowly making them less impulsive and hostile. Similarly, parents can be taught not to close their hearts to their fussy baby but to soothe her, which improves both attachment and the child's ability to soothe herself. "We're close to designing these interventions," says Reiss. "If they work, we'll be changing a genetic trait by changing the family environment." Mothers and fathers would be doing gene therapy on their children simply by raising them.