Abstract: This paper reports experimental and field studies of risky investment. Evolution by natural selection should have equipped humans with a kith-and-kin rationality for investment decisions while sexual selection entailed a higher expectation for investments from a man than from a woman. In Study 1, participants estimated how a typical man or woman in the same age cohort would allocate lottery money to self and other possible recipients. The results showed that (1) the degree of kith-and-kin relatedness largely determined the amount of investment. (2) An imagined typical man was more generous than an imagined woman. However, male participants were more selfish than female participants. (3) Women were more accurate in estimating the other sex’s investment distribution than men. (4) Women invested in more recipients, thus had a larger scope of social sharing. Study 2 tested the evolutionary psychological hypothesis that differential parental investment in sons and daughters would be affected by the family’s relative rather than absolute wealth. Using breastfeeding and interbirth interval as the measurements of parental investment, the results showed that (1) real household income affected overall amount of parental investment irrespective of the sex of a child; and (2) relative wealth perceived by parents against their neighborhood households affected differential investment in sons versus daughters. Parental investment in sons is viewed as a riskier prospect than investment in daughters since men have a universally higher variance in wealth and reproduction than women. The two studies suggest that human decision rationality is bounded by social relations and adapted to relative wealth conditions

Key words: evolutionary psychology, rationality, risky decisions, monetary allocation, parental investment, sex differences