The population regulation one.

“The chief goal of all other species is to turn food into offspring. More food means more offspring. It is this biological logic that underlies the perennial fears of human overpopulation. Most creatures live in environments that correspond to open access commons. Recent fertility trends strongly suggest that the simple biological model of human breeding is wrong, or at least, is wrong when the institutions that support economic freedom and the rule of law, e.g., markets, price stability, honest bureaucracies, security of private property, and the fair enforcement of contracts, are well-developed. Economic freedom and the rule of law are the equivalent of enclosing the open access breeding commons, causing parents to bear more and more of the costs of rearing children. In other words, economic freedom actually generates an invisible hand of population control.”