Why do some three-year-olds share more than others? To find out, psychologist Arial Knafo deploys what we can call Bamba test. At Knafo’s large, playroom-like lab at Jerusalem University, a toddler spends an hour or so drawing, playing games, and making dolls with a friendly research assistant before the assistant announces snack time and brings out two packages of Bambas — peanut-butter-flavored corn puffs much coveted in Israel. The child’s pack, like every normal pack, holds 24 of the little treats. But when the researcher opens her pack, she laments, “Mine has only three!” Which it does, for the research assistant has earlier removed the rest so as to set up this test of social perception and generosity: Will this three-year-old across from share without being asked.
Most do not. “Self-initiated sharing is a difficult task,” says Knafo. “You have to detect the need, then decide to do it.”
A few 3-year-olds, however, will offer their Bambas. And in this study, the toddlers most likely to share happened to carry a gene variant generally tied to antisocial behaviour. A pile of previous studies had examined this variant — the “7R” (or long-repeat) version of DRD4, a gene that affects levels of dopamine, a key neurotransmitter — and found showed that it put people at extra risk of attention and conduct problems if they had harsh or emotionally distant parents. These studies gave this 7R variant a bad rap. It got dubbed the ADHD gene, the bully gene, the brat gene, the drinking gene, the slut gene. Now Knafo was effectively calling it the Bamba-sharing gene."