Westermarck proposed that humans have an incest avoidance instinct, triggered by frequent intimate contact with family members during the first several years of life. Westermarck reasons that (1) familial incest will tend to produce less fit offspring, (2) those humans without instinctive incest avoidance would hence have tended to die off and those with the avoidance instinct would have produced more viable offspring, and hence (3) familial incest would be, as indeed it is, universally and instinctively avoided (the desire simply does arise given early continuous intimate contact; the “potty principle” as some psychologists have succinctly termed it). Victorian Westermarck claimed this as a human adaptation. Evolutionary psychologists have generalized these claims to Pleistocene humans and their ancestors, to primates, and indeed to animals generally. Yet there is surprisingly little evidence for these claims of universal instinctive avoidance. Considerable inbreeding appears among large, territorial primates and may have been so with early humans and with their ancestors. While there is little reliable non-anecdotal evidence about incestuous behavior or the lack of it among humans, what little there is does not fit well with the Westermarck thesis.