First, from PLos One :
Oxytocin Increases Generosity in Humans
In this study, participants were infused with 40 IU oxytocin (OT) or placebo and engaged in a blinded, one-shot decision on how to split a sum of money with a stranger that could be rejected. Those on OT were 80% more generous than those given a placebo. OT had no effect on a unilateral monetary transfer task dissociating generosity from altruism. OT and altruism together predicted almost half the interpersonal variation in generosity. Notably, OT had twofold larger impact on generosity compared to altruism.
Then there's The Neurobiology of Trust(PDF) :
To summarize our findings, peripheral OT (Oxytocin) responds to the receipt of a social signal of trust and is statistically related to trustworthy behavior. When the social signal of trust is removed, so are the OT response and the high degree of trustworthiness.Our behaviour is a little more influenced by neurotransmitters than we'd like to believe. But the level of the neurotransmitters themselves are dictated by our environment, so it's a mechanism rather than a first cause.