The architecture of human kin detection

Evolved mechanisms for assessing genetic relatedness have been found in many species, but their existence in humans has been a matter of controversy. Here we report three converging lines of

evidence, drawn from siblings, that support the hypothesis that kin detection mechanisms exist in humans. These operate by computing, for each familiar individual, a unitary regulatory

variable (the kinship index) that corresponds to a pairwise estimate of genetic relatedness between self and other. The cues that the system uses were identified by quantitatively matching

individual exposure to potential cues of relatedness to variation in three outputs relevant to the system’s evolved functions: sibling altruism, aversion to personally engaging in sibling

incest, and moral opposition to third party sibling incest. As predicted, the kin detection system uses two distinct, ancestrally valid cues to compute relatedness: the familiar other’s

perinatal association with the individual’s biological mother, and duration of sibling coresidence.

The authors thank P. Boyer, D. Fessler, S. Gangestad, P. Pocker, H. Waldow, G. Williams, D. Williams, UCSB Academic Senate and the providers of the NSF Presidential Young Investigator Award

(J.T.), and NIH Director’s Pioneer Award (L.C.).

Center for Evolutionary Psychology, University of California Santa Barbara, Santa Barbara, California 93106, USA,

Department of Psychology, University of Hawaii, Honolulu, Hawaii 96822, USA,

Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

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Inbreeding is a bad thing, and many animals have evolved mechanisms for assessing genetic relatedness in order to avoid it. Whether such an in-built mechanism exists in humans has been

controversial, but a new study of sibling behaviour finds evidence to support the idea. The study suggests that we use two distinct cues to compute a 'kinship index'. If a potential sibling

is younger, we watch to see how much time he or she spends with our mother. If the sibling is older, we assess how much time we ourselves have spent with that person.