were sepa- rated at birth, had already concluded that genes alone are responsible for perhaps 50 percent of a childs personality and abilities. So if nature accounts for half of a childs destiny, what accounts for the other half? Surely it must be the nurturing-the Baby Mozart tapes, the church sermons, the museum trips, the French lessons, the bargaining and hugging and quarreling and punishing that, in toto, constitute the act of parenting. But how then to explain another fa- mous study, the Colorado Adoption Project, which followed the lives of 245 babies put up for adoption and found virtually no correlation between the childs personality traits and those of his adopted par- ents? Or the other studies showing that a childs character wasnt much affected whether or not he was sent to day care, whether he had one parent or two, whether his mother worked or didnt, whether he had two mommies or two daddies or one of each? These nature-nurture discrepancies were addressed in a 1998 book by a little-known textbook author named Judith Rich Harris. The Nurture Assumption was in effect an attack on obsessive parenting, a book so provocative that it required two subtitles: Why Children Turn Out the Way They Do and Parents Matter Less than You Think and Peers Matter More. Harris argued, albeit gently, that parents are wrong to think they contribute so mightily to their childs personality. This be- lief, she wrote, was a "cultural myth." Harris argued that the top- down influence of parents is overwhelmed by the grassroots effect of peer pressure, the blunt force applied each day by friends and school- mates.     The unlikeliness of Harriss bombshell-she was a grandmother, no less, without PhD or academic affiliation-prompted both won- der and chagrin. "The public may be forgiven for saying, Here we go again, " wrote one reviewer. "One year were told bonding is the key, the next that its birth order. Wait, what really matters is stimulation.