Westermarck Effect

The Westermarck effect, or reverse sexual imprinting, is the hypothesis that people who live together in early childhood become desensitized to sexual attraction toward one another. Proposed by Edvard Westermarck in 1891 to explain the incest taboo, it posits a critical early-life window - generally birth through about age six - during which prolonged close domestic association triggers an aversion to later sexual or marital pairing among those peers.

Empirical evidence cited includes communal Israeli kibbutz data in which nearly 3,000 marriages produced only 14 unions of children from the same peer group, none of whom had been reared together in the first six years; the high failure rate of Chinese shim-pua marriages where a girl was adopted as a future wife in infancy; and studies showing lower success or appeal for cousin marriages when cousins were raised in sibling-like conditions (Lebanon, Morocco) or when age gaps were small (Pakistan). Reports that siblings separated long-term in childhood are more likely to engage sexually support the mechanism. A later reanalysis of kibbutz cases found instances of attraction not acted on and concluded that innate aversion operates alongside, and often requires reinforcement from, social norms and pressures.