Growing up together as kids suppresses sexual attraction later, even if unrelated

Finnish sociologist Edward Westermarck proposed in 1891 that close physical proximity during the first few years of life suppresses later sexual attraction, an evolved mechanism to prevent inbreeding. Israeli kibbutz children raised together rarely married each other as adults, and Taiwanese 'minor marriages,' where a bride was raised from infancy alongside her future husband, had markedly higher divorce and infidelity rates than normal arranged marriages.

— Edward Westermarck, The History of Human Marriage — 1891; kibbutz data from Joseph Shepher, 1971; Taiwanese minor-marriage data from Arthur Wolf