Birth order supposedly influences individuals’ cooperative attitudes: firstborns are more family-oriented and favor their kin, while laterborns are more likely to turn to non-kin. However little direct experimental evidence exists concerning costly resource sharing between full siblings. The present study investigates sharing decisions with respect to a monetary resource by full sisters when either: (i) interacting with an unknown individual or (ii) their sister. A total of 112 sisters from 56 different families participated in an economic experiment regarding the distribution of an actual monetary reward. In line with kin selection theory, the results showed that participants favor their sister over strangers. Additionally, firstborns invested more costly resources in their sister (than laterborns) while they knew that the latter did not.

据推测，出生顺序会影响个人的合作态度：长子更倾向于家庭，偏爱亲戚，而后世的人更倾向于非亲戚。然而，几乎没有直接的实验证据表明全兄弟姐妹之间的资源共享成本高昂。本研究调查全姐妹在以下情况之一时在金钱资源方面的共享决策：（i）与陌生人互动或（ii）他们的姐妹。来自56个不同家庭的112个姐妹参加了关于实际金钱奖励分配的经济实验。根据亲属选择理论，结果表明参与者比陌生人更喜欢自己的妹妹。另外，长子在姐姐上投资的资源要比后辈多，而后辈却不知道。