Economics, as it is tought these days, attempts to model the behavior of consumers, their confidence, their misery etc, in its construction of the larger models of whole economies. You can get a Nobel prize if you can figure how how it all works. The lesson for those not pursuing a PhD in the dismal science is that all those smart folks just accept that the whole only functions because its tiniest parts function, the macro can only arise from the micro.
But long before those insights were cast in equations, we all had an economics lesson in a proverbial form: "Bad money drives out good". What we need to see is that little truism held because a wider one is with few exceptions:
Selfish behaviour drives out altruism and casts harsh light on our inherently total mutuality so that its negative side is seen and its positve obliterated in shadow.
You can probably think of an example you have seen: the most selfish examples that people get away with set the standard. Ken Lay or Bernie Ebbers surely didn't think they were inventing fraud but rather just doing a better job of it than any number of predecesors who will never see jails or judges. So, this is the force that sets our individual models of how the world works and how we shall survive onto rails, destined for some genteel form of "dog eat dog". Don't you wonder at the relief you feel, the way you are so heartened by the rare example of selflessness that your eyes moisten and for a moment you have no misgivings for our species? We are too adaptable by nature: we are capable of believing anything from "I am just one more wolf like all ther others" to "We're all in this together and I hurt noone without hurting myself". Which model we hold depends on experience more than on teachings. And the commonly held model gives rise to the most common experience.