Much clinical and ethnographic evidence suggests that humans, like many other organisms, are selected to avoid close inbreeding because of the fitness costs of inbreeding depression. The proximate mechanism of human inbreeding avoidance seems to be precultural, and to involve the interaction of genetic predispositions and environmental conditions. As first suggested by E. Westermarck, and supported by evidence from Israeli kibbutzim, Chinese sim-pua marriage, and much convergent ethnographic and clinical evidence, humans negatively imprint on intimate associates during a critical period (...) of early childhood.There is also much evidence that, like other social animals, humans do not seek to maximize outbreeding, but rather to maintain an optimal balance between outbreeding and inbreeding.Closeinbreeding reduces fitness through inbreeding depression, butsomeinbreeding brings the benefits of nepotism. For simple, stateless, horticultural societies, the optimal balance seems to be achieved by a combination of precultural inbreeding avoidance of relatives with anr≤·25 and cultural rules of preferential marriage with kin withr≥·25. Adjustment of the coefficient of inbreeding to other ecological settings seems to be largely cultural. An interactive model of “culture in nature” is presented, in which culture is seen as coevolving with genes to produce the maxiniization of individual inclusive fitness. (shrink)

Two areas of research for testing Nell's theory are suggested. One is cruelty's seemingly negative correlation with age, which would confirm its linkage with testosterone, sex, and dominance. The other is the special field of leisure activity called thanatourism, that is, the transformation of loci of human horror into tourist attractions.

Nothing is gained by opposing and or by proclaiming the supremacy of one over the other. Instead, we should develop a unitary model of gene-culture coevolution, allowing for the complex interaction of both, and varying importance of each, all within our double, species-specific, adaptive, evolutionary track.