Humans can detect individuals that are blood-related kin (mothers/fathers and children but not husbands and wives) from olfaction. Mothers can identify by body odor their biological children but not their stepchildren. Preadolescent children can olfactorily detect their full siblings but not half-siblings or step siblings and this might explain incest avoidance and the Westermarck effect. Functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, the insula, and the dorsomedial prefrontal cortex but not the primary or secondary olfactory cortices, or the related piriform cortex ororbitofrontal cortex.