Why do people cooperate?

The greatest challenge for all societies is to ensure and protect collective welfare. This challenge arises from the fact that the (long-term) interests of the collective do not necessarily coincide with the (short-term) personal interests of the people belonging to that collective. people have to decide whether or not they cooperate by furthering the collective interest at the expense of their self-interest, or free-ride on others' cooperative efforts without contributing themselves. My research is aimed at answering the question of when and why people are willing to cooperate in such social dilemma situations.