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, 445 (7129), 727-31

The Architecture of Human Kin Detection


The Architecture of Human Kin Detection

Debra Lieberman et al. Nature.


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.


Figure 1
Figure 1. Proposed model of the computational architecture of sibling detection
Cues to kinship are registered by cue monitoring circuits, which deliver their outputs to a kinship estimator. The kinship estimator uses these cues to compute the magnitude of a regulatory variable—a kinship index—for each individual, i, who is a potential sibling. The kinship index feeds into programs that regulate sibling altruism and sexual aversion.
Figure 2
Figure 2. Converging evidence indicates that the same computational variable, the kinship index, regulates disparate kin-relevant behaviours
The x-axis divides subjects into two groups—those who observed their mothers caring for their sibling as a neonate (MPA cue present) and those who did not (MPA cue absent). The y-axis shows the size of the correlation between coresidence duration and each dependent measure. Duration of coresidence predicts, with similar effect sizes, altruism and sexual aversions only when the cue of maternal perinatal association (MPA) is absent, as it is when younger siblings are detecting older ones. When the MPA cue is present, coresidence duration fails to predict sibling directed behaviours. This pattern appears for all measures: behavioural altruism, dispositional altruism, sexual disgust and moral judgments of sibling incest. Adaptive regulation of two distinct motivational output systems by the same pattern of inputs implicates a common underlying regulatory variable (see also Supplementary Information section 7).
Figure 3
Figure 3. When MPA and coresidence duration cues are both available, the kin detection system defaults to MPA, the more reliable cue
a, b, The only individuals for whom these cues could be jointly available are olders detecting younger siblings; each bar on the graph shows the size of the correlation between a cue and an outcome measure for this group. For olders responding to youngers, exposure to the MPA cue predicts both altruism (a) and moral opposition to sibling incest (b), and with the same effect size (black bar, first pair, each panel). The MPA cue continues to predict these disparate measures even after the effects of coresidence duration are statistically removed (black bar, second pair, each panel). In contrast, coresidence duration ceases to predict either altruism or moral opposition to sibling incest once the effects of MPA are removed (grey bar, second pair, each panel). ***P < 0.001, **P < 0.01, *P = 0.05, +P < 0.10.

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