Why don’t people want sex with their sister? Dr Debra Lieberman explains the evolved psychology behind inbreeding avoidance in plain terms. This condensed version trims a 1-hour Modern Wisdom episode down to 3 minutes while preserving the core insights. You’ll learn how kin-detection cues — maternal investment signals (like breastfeeding) and co-residence duration (the Westermarck effect) — build a reliable incest aversion, why these systems can fail in modern contexts (sperm banks, dispersed half-siblings), and how shared genetics can paradoxically increase attraction. Chris Williamson and Dr Lieberman also connect these findings to moral psychology, sex differences in disgust, and an unexpected theory of crying as a social signal. Keywords: evolutionary biology, kin detection, inbreeding avoidance, moral psychology, neuroscience. Listen now to get the key ideas in minutes.