Human reproductive patterns produced sibling distributions that clustered closely with socially monogamous mammals rather than polygynous species, according to research published in Proceedings of the Royal Society B.
Mark Dyble, of the Department of Archaeology at the University of Cambridge, analyzed sibling composition across 103 human populations comprising 197,658 sibling dyads and compared these data with 34 non-human mammal species totaling 61,163 sibling dyads. He measured proportions of full siblings vs half-siblings as indicators of mating system structure. Across the human sample, 66% of siblings were full siblings. This rate closely matched those observed in socially monogamous non-human mammals (mean, 71%; range, 45%–100%; n = 10) and consistently exceeded those in non-monogamous mammals (mean, 9%; range, 0%–22%; n = 24).
"While cross-culturally variable, rates of full siblings in humans cluster closely with rates seen among socially monogamous mammals and fall consistently above the range seen in non-monogamous mammals," Dyble wrote.
Human mating patterns produced sibling distributions similar to socially monogamous species such as meerkats (Suricata suricatta, 59% full siblings) and African wild dogs (Lycaon pictus, 85% full siblings) rather than nonmonogamous species such as chimpanzees (Pan troglodytes, 4% full siblings).
The human data set incorporated ancient DNA from 9 archaeological sites spanning the Neolithic through Bronze Age periods and genealogical data from 94 preindustrial societies with diverse subsistence strategies. Archaeological samples covered approximately 8,000 years and included Avar-period cemeteries and burial grounds in Central Europe, a Neolithic settlement in Anatolia, and sites in Great Britain and the Southern Urals. Full-sibling proportions across these contexts ranged from 26% to 100%.
Computational modeling revealed a nonlinear relationship between deviation from monogamy and sibling composition. Dyble constructed a model using the parameter e to represent deviation from exclusive monogamy, where e = 0 indicated exclusive monogamy and e = 1 represented random mating. The model simulated populations in which females produced offspring with varying degrees of partner exclusivity and demonstrated how even small increases in extra-pair reproduction disproportionately reduced full-sibling proportions.
At e = 0.25, approximately 40% of sibling dyads remained full siblings; at e = 0.5, only 15% were full siblings. This nonlinear pattern may help explain why cooperative breeding is rare even among socially monogamous species—extra-pair paternity exceeding 20% in socially monogamous birds may sufficiently reduce relatedness to impede transitions to cooperative breeding, potentially explaining why only about 9% of birds are cooperative breeders despite roughly 90% being socially monogamous.
Extrapolating from observed full-sibling rates, estimated deviation from monogamy across human populations averaged 12% (range, 0%–35%), compared with approximately 10% in monogamous nonhuman mammals and 68% in nonmonogamous species.
Dyble also measured reproductive monogamy directly as the proportion of reproductive individuals who produced offspring with only one partner who likewise reproduced exclusively with them. Analysis of 16 ethnographic data sets showed that 63% of individuals met this criterion—a rate exceeding that of monogamous nonhuman mammals (39%; n = 5) and far surpassing nonmonogamous species (7%; n = 13).
Human populations displayed predominantly paternal rather than maternal half-siblings, indicating higher male reproductive skew consistent with greater cross-cultural permissiveness of polygynous versus polyandrous marriages. Dyble noted that human polygynous marriages differ from polygynous mating systems in most mammals—while animal polygyny often features rapid turnover in male dominance and produces few full siblings, human polygynous marriages can remain stable for many reproductive years, generating both full siblings and paternal half-siblings while still supporting substantial paternal investment.
Consistency between genetic archaeological data (mean full siblings, 65%) and genealogical ethnographic data (mean, 66%) suggested minimal impact from misattributed paternity, which is typically estimated below 5% in human populations.
Dyble emphasized a key distinction: the study measured reproductive monogamy—who successfully has children with whom—rather than mating behavior itself. Birth control practices, including modern contraception and traditional fertility regulation methods, mean reproductive outcomes may diverge from mating behavior. Serial monogamy can also generate half-siblings while still providing paternity certainty often considered important in the evolution of human paternal investment.
"Humans also differ from most socially monogamous species in being monotocous," he noted, explaining that humans typically produce single offspring per pregnancy rather than litters. Among monogamous mammals in the sample, only white-handed gibbons shared this trait.
Humans further diverge from most socially monogamous mammals by living in stable multi-male, multi-female groups containing multiple breeding pairs. Only the Patagonian mara (Dolichotis patagonum) has been suggested to display comparable social organization among mammals. This combination of traits—reproductive monogamy, single births, and plural breeding in mixed-sex groups—makes humans nearly unique and suggests that monogamy evolved under different selective pressures in humans than in other species, potentially related to the energetic demands of large brains and slow growth.
"Given that all other African great apes live in groups and have either polygynous or polygynandrous mating systems, it is probable that human monogamy evolved from a nonmonogamous group-living state, a transition that is highly unusual among mammals," Dyble wrote.
The analysis included populations engaged in a wide range of subsistence strategies, including horticulture, hunting and gathering, agriculture, and pastoralism. Although the ethnographic sample overrepresented populations from the Americas (52 of 94 societies, 55%), full-sibling proportions were comparable between American societies (66%) and those from other regions (also 66%).
Dyble received no funding for this work and declared no competing interests. Data and analytical code are available through GitHub and archived in Zenodo.