In it, she argued that parents play less of a role than they think they do in shaping their children. Many of the questions that frazzle new parents and guilt-trip mothers, such as whether the child will be damaged if sent to day care while the mother works outside the home, are essentially meaningless, Ms. Harris suggested, because being in day care or having the mother at home is less important than the child’s genes and social group.

Her central insight, which she said came to her while reading a psychology paper in 1994, was that adolescents are not trying to be like adults, they are trying to be like other adolescents.

“If teenagers wanted to be like adults they wouldn’t be shoplifting nail polish from drugstores or hanging off overpasses to spray I LOVE YOU LIƨA on the arch,” she wrote. “If they really aspired to ‘mature status’ they would be doing boring adult things like sorting the laundry and figuring out their income taxes. Teenagers aren’t trying to be like adults: they are trying to distinguish themselves from adults!”

It was her work as a textbook writer that gave her the broad perspective across disciplines to develop her thesis. Because she was not affiliated with a university, she was in no position to undertake large studies herself. But she was deeply familiar with the literature.

“And she was so damn smart,” Steven Pinker, the Harvard psychologist and author who has championed Ms. Harris’s work, said in a phone interview. “Her gift was she could understand the technical aspects of behavioral genetics and at the same time was a psychologist and a very sharp observer of human behavior.”