People with Williams syndrome have a genetic deletion that has a wide range of effects, including narrowed blood vessels, developmental delays and -- oddly -- love of music and extreme friendliness.

The extreme friendliness of this small population -- about 1 in 8,000 U.S. babies is thought to have the genetic anomaly -- has always interested personality psychologists and neuroscientists. The remarkable regularity of Williams kids' trusting personalities and social fearlessness has prompted many researchers to wonder how the known deletion of 25 genes in the massively complex human genome could relate to the uniformly predictable appearance of a personality trait -- extraversion -- that helps make us who we are.

In a study reported in the journal Current Biology and published Monday, German and French researchers report on a heretofore undiscovered behavior pattern that also unites people with Williams syndrome: They do not seem inclined to stereotype those of minority ethnic or racial groups in the insidious and generally unacknowledged ways that most of the rest of us do. The researchers took a group of 20 children with Williams syndrome and a matched-group of normal-development children and had the subjects look at a series of people -- some with brown skin, others white -- engaged in a range of daily activities. The kids were asked to describe the pictured person and to characterize that person using one of a range of positive and negative prompts.

While the normal-development kids followed long-established patterns of viewing the pictured person with the skin color different from their own in a less favorable light, children with Williams syndrome were far less likely to make any such distinctions.

The researchers relate the absence of the impulse to racially stereotype to the hallmark personality trait of those with Williams syndrome: the same lack of social fear or wariness that makes individuals belonging to this special population sometimes unnervingly friendly to strangers. And the researchers suggest that this says something interesting about racial stereotyping: that it is based in fear and is a primitive impulse that helps us hold those who are identifiably different from us -- the "outgroup" -- at arm's length.

Interestingly, the Williams syndrome kids -- half male, half female -- did engage in gender stereotyping.

To author Andreas Meyer-Lindenberg of the University of Heidelberg's Central Institute of Mental Health in Germany, this suggests that gender stereotyping springs from a different source than racial stereotyping. Unlike even the most subtle forms of racism, gender stereotyping serves no deep-seated fear impulse, Meyer-Lindenberg suggests. Rather, the impulse to make assumptions on the basis of sex is learned behavior -- "social imitative learning and over-generalization" -- not fear. (For if it were fear, it too would have been extinguished in Williams syndrome.)

It's at moments like this that neuroscience has the potential to change -- or at least explain -- the things we do. Past studies of kids with Williams syndrome have shown that those who are most socially fearless have an amygdala -- the primitive nugget deep in the brain from which the sensation of fear springs -- that doesn't function properly. For the rest of us, the amygdala does perform normally. And the primitive fear it pumps out at the sight of a member of an "outgroup" representative can be extinguished only if our prefrontal cortex -- the seat of reason -- steps in and overrides it.

It makes you wonder who is the most "evolved" -- someone with normal development or the one with the genetic deletion?

Learn more about Williams syndrome here.

--Melissa Healy