Altruism between romantic partners: biological offspring as a genetic bridge between altruist and recipient.

When the cost of altruism is low, individuals are more likely to help non-kin (i.e., friends and romantic partners) than kin. This trend is thought to reflect the fact that people tend to be emotionally closer with friends and romantic partners than kin. However, as the cost of altruism increases, altruistic preference shifts to kin. The present study highlights this phenomenon by examining altruism between siblings, romantic partners, romantic partners who have biological children together, and romantic partners who have adopted children together. Participants (n = 203) completed a questionnaire about altruism in low-, medium-, and high-cost situations. Participants gave more low-cost help to their romantic partners (regardless of whether they had a child together) than their siblings. More medium-cost help was given to romantic partners who had a child (biological and adopted) than siblings and romantic partners without children. In the high-cost condition, the estimated altruistic tendencies were stronger toward siblings and romantic partners who have a biological child than toward romantic partners with no children and partners with adopted children. Participants also believed they were more altruistic than their siblings and romantic partners.