Adaptive variation refers to the lock-and-key type relationship between an organism and its ecological niche. This includes a patterned relationship between human behavior and societal variation. As societies become more complex, for example, our capacity to process a great deal of information quickly increases (the Flynn effect). Such adaptive processes illuminate societal variation because there is a reliable pattern in comparative research whereby behavioral differences are predicted by ecological and demographic differences. This pattern cannot be explained in terms of genetic adaptation but reflects changes mostly occurring in the lifetime of the individual. It involves an evolved capacity of humans (and other species) to adapt their behavior developmentally to local challenges. The resulting phenotypes are ecologically predictable (or "adaptive") but may not maximize reproduction (e.g., low fertility in modern urban cities). This innovative theoretical approach is scientifically valid and heuristically useful based on comparisons across species and between societies.