Plants and animals exploit the work of others any way they can. Same thing going on in humans.
Culture also plays a part. Teaching immediate gratification (make the future pay the costs), being envious of others. Teach women to seek high resource men, teach men to seek high resources to get high demand women.
Also the total amount of resources available affects whether people would rather (or even can) go on there own (be selfish) or work together out of necessity (and cooperate).
RE: Idealism... and What Are People THINKING??? Is Cheating and Stealing Inherently Human?