I wonder if results would be different if cooperation would result in a <i>loss</i> (as opposed to merely smaller reward).<p>Psychologically, it's much easier to forgo additional reward than to give up what you <i>already</i> have.<p>People have tendency to strongly prefer avoiding losses to acquiring gains:<p><a href="http://en.wikipedia.org/wiki/Loss_aversion" rel="nofollow">http://en.wikipedia.org/wiki/Loss_aversion</a>