Lower levels of oxytocin may make people shy, according to Lawsun Wilsun, a professor of psychiatry and family medicine at the University of Cinncinnati. He mentions unnamed studies showing that the brains of normal people who were given oxytocin were less fearful when threatening people came around.
Of course, shyness goes along with difficulty in bonding, another effect of a thwarted oxytocin response.
Wilsun writes in the Cinncinnati Enquirer,
These studies raise the possibility that people with social phobia or paranoia lack sufficient oxytocin to develop trust. Soon we may have new treatments that regulate and restore oxytocin.