Williams syndrome nytimes article
http://www.nytimes.com/2007/07/08/magazine/08sociability-t.html
People with Williams syndrome, a genetic disorder, have “trouble with space and numbers” but “a love of company and conversation combined, often awkwardly, with a poor understanding of social dynamics and a lack of social inhibition”. Elsewhere, I believe that Williams syndrome patients have been described as feeling as though everyone is their friend.
The genetic cause of Williams syndrome is known.
The article also goes into the theory that human intelligence and language evolved to deal with large social groups, especially such complexities as deception, and posits that Williams syndrome allows us to dissociate some aspects of social intelligence from others, and from other aspects of intelligence.