A baby's first proper social smile generally occurs at about four to six weeks, though it may be seen earlier and dismissed as wind. Psychologist Steve Biddulph says that, as newborns, boys tend to make less eye contact and to smile less than female babies. This means we have to be extra sure to interact and chatter with them, so that they grow to be toddlers who can talk as well as their girl peers.

 http://www.youtube.com/watch?v=gM8d4nSe-MU