A really interesting, but unfortunately unsurprising, study indicates that the same hormone that helps us bond with others, also makes us preemptively aggressive towards those outside that group with which we bond:

Our findings show that oxytocin, a neuropeptide functioning as both a neurotransmitter and hormone, plays a critical role in driving in-group love and defensive (but not offensive) aggression toward out-groups. Perhaps offensive forms of out-group hate have their biological roots elsewhere, or perhaps these tendencies are primarily grounded in perceived in-group love and protectionism in competing out-groups. After all, if competing out-groups become strong and powerful, they become a threat to the in-group, and this in and of itself not only motivates in-group members to display in-group love but also motivates protectionism and preemptive strike. As shown here, this “tend and defend” form of parochial altruism is precisely what oxytocin modulates.

Via Science and Religion Today, and thanks to Jessica.

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