Abstract: In late 2013, the Japanese Supreme Court voided inheritance rules giving nonmarital children half the shares of their marital half-siblings. To punish children for the sins of their parents, it explained, violated the equal protection clause of the Constitution. Like the stigma that most traditional societies attached to illegitimacy, the inheritance rules had reflected a simple selection bias: the societies that survive are those where more children live to reproductive age; in harsh environments (the norm until a few centuries ago) whether children survived turned on the level of investment adults made in them; men tend not to invest in children whose paternity they do not know; hence, non-marital children had been substantially less likely to survive; but the stigma attached to illegitimacy and the accompanying legal disabilities had helped minimize the number of such children by channeling sex into stable dyadic relationships. The pre-2013 inheritance rule had promoted that relational stability by helping women hold men to their promises. In order to induce women to marry them, men routinely promise to invest in the children they bear together. The earlier rule had assured women that if their husbands breached those promises in life, they could at least trust the law to favor their children in his death. After 2013, the courts could no longer offer even that assurance.