Cross-cultural patterns and the search for evolved psychological mechanisms.

Darwin's principle of evolution by natural selection provides a theoretical basis for functional analyses of behaviour. This approach is complementary to traditional psychology: ideas about what behaviour was designed to do suggest how it might be organized. The cross-cultural record, because it focuses on the broadest characterization of human behaviour, can guide our search for psychological adaptations. Both universals (behavioural traits that are invariantly expressed despite cultural diversity) and conditional universals (behaviours that vary predictably with some environmental parameter) deserve attention from evolutionary psychologists. These behaviours are not themselves adaptations but are markers for underlying psychological traits that have been favoured by selection because they produce adaptive behavioural output. One universal (reciprocity) and one conditional universal (matrilineal investment bias) are used to exemplify how behavioural universals can guide the search for psychological adaptations.