[dri_ft]

But there are also obvious evolutionary pressures towards self-interest, even when it means breaking promises/trustworthiness. (Remember, selection happens at the level of individuals, not of whole groups and societies!) The balance between the two pressures could be different in different environments. In other words, it's quite possible that one environment could select more for trustworthiness, another more for untrustworthy self-interest.

[monocasa]

Not being a dick and working together without everyone constantly on edge that everyone else is going to screw them the first chance they get is a totally valid strategy, even from a self-interest standpoint.

Mutual Aid: A Factor of Evolution

http://www.gutenberg.org/ebooks/4341