Inclusive fitness theory predicts that organisms will tend to help close kin more than less related individuals. In a variety of birds and mammals, relatives are recognized by comparing their phenotype to an internal representation or template, which might be learned through either repeated exposure to family members or self-inspection. Mirrors are ubiquitous now, but were absent during our evolutionary history; hence it is hard to predict, and empirically unknown, whether human kin recognition is family- or self-referential. Here we put this issue to the strongest possible test by comparing nepotistic behaviour towards self- versus co-twin-resemblant individuals. Seventy monozygotic and dizygotic twins were shown same-sex faces, covertly manipulated to resemble either themselves or their co-twin, and indicated which individual they would prefer in two prosocial contexts. Self-resemblant faces were significantly preferred to twin-resemblant faces, showing that visual information about the self supersedes that about close family members in the kin-recognition template. Because, under conditions of paternal uncertainty, a reliable family-referent template could be based only on one's mother and maternal relatives, a unique advantage of self-referent phenotype matching is the possibility of (consciously or unconsciously) identifying one's father and paternal relatives as kin.
Keywords: kin recognition, nepotistic behaviour, facial resemblance, phenotype matching, inclusive fitness