The cooperation paradox: evolutionary theory predicts that natural selection should favor self-interested behavior, since cooperating with non-relatives at personal cost should reduce fitness. Yet humans routinely help strangers, follow rules when no one is watching, donate to distant charities, and sacrifice personal benefit for abstract principles. The prisoner's dilemma formalizes the problem: two parties cooperating beats both defecting, but each individually benefits from defecting regardless of what the other does. So why doesn't defection dominate?

Human cooperation is evolutionarily unique in scale. Other great apes cooperate within small bands of close kin and familiar individuals. Humans routinely cooperate with millions of strangers across anonymous markets, democratic institutions, and global supply chains. This difference is not explained by our genes alone — our closest relatives, chimpanzees, share 99% of our DNA but do not build cities. The difference lies in our uniquely human capacity for shared intentionality: the ability to collaborate toward shared goals with others who are not close relatives.

Understanding cooperation matters practically. When cooperation breaks down — between nations, corporations, political parties, or neighbors — the results can be catastrophic. Climate change, pandemic preparedness, nuclear non-proliferation, and global financial stability all require unprecedented cooperation among actors with different interests. The same collective action problems that prevented cooperation among small groups in ancestral environments now operate at planetary scale. What worked then may not work now, and understanding the mechanisms of cooperation is the first step toward designing better institutional solutions.