Trust and Free Will

Why do we mistrust strangers?  Is it a rational and logical choice based on self-preservation (i.e. "I don't know that man, therefore he may be dangerous, so I'd better avoid him")?  In that case, why do children with Williams syndrome approach strangers without fear?  Are they making a different rational choice?

A recent discovery has found that a simple chemical signal (oxytocin, given as a nasal spray) can turn a group of college students into a bunch of gullible, credulous suckers - willing to trust others more readily.

Therefore, any gene variant which regulates the amount of oxytocin in the brain may also regulate the amount of trust we are willing to have in others.  It's hardly a rational choice.  With the right chemicals, we could all have Williams syndrome.