evolutionary psychological approach to the study of sibling influences

Research in the psychological sciences has long focused on the impact families have on later development. One feature of families is the birth order in which children from one family develop. Birth order serves as a proxy for developmentally relevant criteria including differences in age, size, and status between siblings. Following from theories derived from evolutionary psychology, differences in perceived favoritism, personality dimensions, and sexual strategy are examined. As an initial examination of the influences siblings can have beyond childhood, responses to the upset over the infidelities of in-laws are examined. Results from this series of studies suggests (1) birth order does not covary with personality but that distinctions among the relatedness between siblings can provide fruitful avenues of future research, (2) features of sexual strategy do covary with birth order, particularly variables related to projected sexual strategy, (3) perceptions of parental favoritism covary with the birth order of the participant, and (4) men and women do not differ in their upset over the infidelities of brothers-in-law and sisters-in-law but that the mechanisms generating upset over a sibling-in-law's infidelity may be sensitive to the age of the sibling and thus, the birth orders of the participant and sibling.