The study integrates inclusive fitness theory with life history theory to explain the developmental patterns of kin altruism. Respondents answered a questionnaire in which the 10 life-saving and 10 major-favor scenarios were randomly presented. For each scenario, respondents chose 1 from a triad set of kin (e.g., grandfather, father, and son) to help. We found that genetic relatedness could serve as a relatively stable determinant of the major-favor choice, whereas reproductive value played a critical role in life-saving situations across the life span, as shown by several findings: (a) respondents tended to shift to save the offspring with greater reproductive value; (b) as respondents aged, they were increasingly likely to save offspring over parent, spouse, or sibling; and (c) respondents began to save offspring over each of these kin from a particular life period on. Unlike the robust age effect, female individuals and married parents tended to help only offspring over some kin under some situations, and the filial attitude did not predict the choice between parents and offspring. Our findings suggest that the developmental patterns of kin altruism, together with their underlying evolutionary mechanisms, may form the working basis of the kin system universally.