Then she attended a meeting of Williams families that included infants and toddlers. “That was about a year into my research project,” she says. “The room was full of little ones — babies, toddlers who weren’t speaking yet. And when I came in the room all the young children old enough to walk ran to the door to greet me. No clinging to Mom; they just broke away. And when I would talk to mothers holding infants — literally babes in arms — some of these babies would almost dive out of their mothers’ arms to meet me.

“I knew then I was wrong. The language wasn’t driving the sociability. If anything, it was the other way around.”

Developmental psychologists sometimes call the social urge the “drive to affiliate.” It seemed clear early on that the Williams deletion, which was definitively identified in the mid-1990s, either strengthened this drive or left it unfettered. But how do missing genes steer behavior toward gregariousness and engagement? How can a deletion heighten a trait rather than diminish it?

I got a hint when I met Nicki Hornbaker, who is 19, at Bellugi’s office in La Jolla. Nicki, whose Williams was diagnosed when she was 2, has been participating as a subject in Bellugi’s research for 15 years. She and her mother, Verna, drove down from Fresno that day to continue testing and to talk with me about living with Williams syndrome. Like most people with Williams, Nicki loves to talk but has trouble getting past a cocktail-party-level chatter. Nicki, however, has fashioned at least a partial solution.

“Ever since she was tiny,” Verna Hornbaker told me, “Nicki has always especially loved to talk to men. And in the last few years, by chance, she figured out how to do it. She reads the sports section in the paper, and she watches baseball and football on TV, and she has learned enough about this stuff that she can talk to any man about what the 49ers or the Giants are up to. My husband gets annoyed when I say this, but I don’t mean it badly: men typically have that superficial kind of conversation, you know — weather and sports. And Nicki can do it. She knows what team won last night and where the standings are. It’s only so deep. But she can do it. And she can talk a good long while with most men about it.”

In the view of two of Bellugi’s frequent collaborators, Albert Galaburda, a Harvard Medical School professor of neurology and neuroscience, and Allan Reiss, a neuroscientist at the Stanford School of Medicine, Nicki’s learned facility at sports talk illustrates a central lesson of Williams and, for that matter, modern genetics: genes (or their absence) do not hard-wire people for certain behaviors. There is no gene for understanding calculus. But genes do shape behavior and personality, and they do so by creating brain structures and functions that favor certain abilities and appetites more than others.

Reiss and Galaburda’s imaging and autopsy work on Williamses’ brains, for instance, has shown distinct imbalances in structure and synaptic connectivity. This work has led Galaburda to suspect that some of the genes missing in the Williams deletion are “patterning genes,” which direct embryonic development and which in this case dictate brain formation. Work in lab animals has shown that at least one patterning gene choreographs the developmental balance between the brain’s dorsal areas (along the back and the top of the brain) and ventral areas (at the front and bottom). The dorsal areas play a strong role in vision and space and help us recognize other peoples’ intentions; ventral areas figure heavily in language, processing sounds, facial recognition, emotion, music enjoyment and social drive. In an embryo’s first weeks, Galaburda says, patterning genes normally moderate “a sort of turf war going on between these two areas,” with each trying to expand. The results help determine our relative strengths in these areas. We see them in our S.A.T. scores, for example: few of us score the same in math (which draws mostly on dorsal areas) as in language (ventral), and the discrepancy varies widely. The turf war is rarely a draw.