Recent studies on the effect of genetic relatedness on helping behavior have demonstrated that

Siblings, whose relationship can endure longer than any other human tie (Mancini & Maxwell, 1990), play an integral role in each other’s social, cognitive, and psychosocial development (Brody, 1998). Some siblings sacrifice their own interests to come to one another’s aid, but sibling relationships can also be characterized by strife, conflict, and aggression. In spite of wide variation in sibling relationship quality, little research has explored the factors governing this variation. An evolutionary psychological perspective may help us understand individual differences in sibling relationship quality.

Altruistic behavior toward kin can evolve when the benefit to the recipient, multiplied by the recipient’s genetic relatedness to the actor, exceeds the costs to the actor (Hamilton, 1964). The ability to recognize and discriminate among kin of differing degrees of relatedness enables individuals to adaptively modify their sexual (DeBruine, 2004a, Fessler and Navarrete, 2004, Lieberman et al., 2003, Lieberman et al., 2007), parental (Alvergne et al., 2009, Alvergne et al., 2010, Burch and Gallup, 2000, Platek et al., 2002), and social behaviors (DeBruine, 2002, DeBruine, 2004b).

Recent research suggests that kin recognition influences sibling relationship quality. Lieberman et al., 2003, Lieberman et al., 2007 identified two cues that predict altruistic behavior between siblings: maternal perinatal association (MPA) – the observation of a neonate nursing from one’s own mother, and coresidence – the duration of sibling cohabitation. For siblings, maternal association cues such as MPA and coresidence would have represented reliable indicators of shared maternal ancestry because ancestral women were certain of the maternity of their offspring. The adaptive problem of identifying full siblings, however, would have required recognizing siblings who shared the same mother and biological father.

Recognizing siblings of common paternal ancestry would have required recognition cues other than MPA and coresidence because MPA and coresidence would not have reliably indicated sharing paternal genes. Anthropological data from traditional societies indicate that women commonly have children with multiple men through extramarital affairs or serial marriages (Hill & Hurtado, 1996). As a consequence, successive children of the same woman may have been just as likely to be maternal half siblings as full siblings in ancestral conditions (Daly, Salmon, & Wilson, 1997). Distinguishing between full and maternal half siblings would thus have been a recurrent selection pressure during human evolution. Full siblings are twice as likely as half siblings to share specific genes, a difference in genetic relatedness equal to the total relatedness between grandparents and grandchildren (Michalski & Shackelford, 2005). Individuals able to discriminate between their full and half siblings, compared to individuals unable to make this distinction, could have preferentially directed altruistic acts toward individuals who were twice as likely to share specific genes with them, giving them a selective advantage in the propagation of their own genes into future generations (Hamilton, 1964). Due to paternity uncertainty and cuckoldry, however, paternal association cues between one’s putative father and one’s putative sibling would have been fallible indicators of shared paternal ancestry. Other cues to relatedness should thus be involved in the recognition of siblings of common paternal ancestry.

Participants

Participants were 85 undergraduates (20 men, 65 women; mean age 19.0 ± 1.8 yr) enrolled in an introductory psychology course at The University of Texas. To be eligible, participants were required to have at least one putative full biological sibling. Participants reported having from 1 to 3 full siblings (mean 1.29 ± 0.63). These siblings (112 total; 46 men, 66 women) ranged in age from 1 to 29 (mean age 19.1 ± 5.0 yr). A separate sample of 56 undergraduates enrolled in an introductory psychology

Statistical analysis

To prevent extreme scores on single items and between-item differences in variance from skewing scores for the closeness, altruism, and conflict scales, composite scores were generated by standardizing each scale’s respective items and computing the mean of these values. All three scales showed good reliability (all αs > .81). High inter-rater reliability was confirmed for the third party raters of resemblance (ICC > .90).

Testing study hypotheses required hierarchical linear modeling (HLM), as the

Discussion

Within families, siblings of greater resemblance reported greater emotional closeness and altruism than siblings of lesser resemblance. Individuals were more likely to view their siblings of greater resemblance as valuable social partners, share private information with them, and experience greater feelings of closeness, a proximate motivator of helping behaviors predicted by actual genetic relatedness (Korchmaros & Kenny, 2001). Individuals more frequently sacrificed their own time and

Acknowledgments

The author wishes to thank David Buss, Jaime Confer, Daniel Conroy-Beam, Judith Easton, Cari Goetz, Carin Perilloux, and Laith Al-Shawaf for their feedback on previous drafts of this paper. The author also extends special thanks to Greg Hixon for statistical advice and to Allegra Kaough for assistance in preparing this manuscript.

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