Why are human societies so psychologically diverse? The discipline of behavioral ecology is rich in both theory and data on how environments shape non-human animal behavior. However, behavioral ecological thinking has not received much attention in the study of human cultural psychological variation. I propose that ecological relatedness—how genetically related individuals are to others in their proximate environment—is one aspect of the environment that shapes human psychology. I present three studies here that examine the influence of ecological relatedness on multiple aspects of psychology. In the first study, I find that higher levels of ecological relatedness at the nation level is associated with a greater willingness to put oneself at risk for others, greater localized trust, and a stronger sense of belonging to one’s community. In the second and third studies, using experimental manipulations of perceived ecological relatedness, I examine the effects of ecological relatedness on helping behavior across situations, monetary sharing on a dictator game, interpersonal judgments, and alloparenting behaviors. I find that individuals led to perceive higher ecological relatedness became more sensitive to need in potential helping situations. The implications of ecological relatedness for thinking about psychological variation across groups are discussed.

The current research seeks to examine whether individuals display or downplay intelligence in various mating contexts. I hypothesized that both men and women should display fluid intelligence when attempting to attract a potential long-term partner, and that only men should display fluid intelligence when attempting to attract a potential short-term partner. Contrary to predictions, I find that men perform worse at a fluid intelligence test when motivated to attract a long-term partner. With respect to crystallized intelligence, I predicted that both men and women should display crystallized intelligence when attempting to attract a potential long-term partner, but women should downplay crystallized intelligence when attempting to attract a potential short-term partner. However, there were no effects of mating contexts on displays of crystallized intelligence.