Monogamy still trumps polygamy in our species. We stand out in the animal kingdom for living with several members of the opposite sex and, even so, maintaining sufficient couple bonds to generate a high proportion of siblings from the same father and mother.
Humans are more monogamous than many realize. A new study surprised many people this week by placing many species of animals as “freer” than us in this regard.
The idea that the human species is “programmed” for fidelity coexists with a social reality marked by infidelity and divorce, leaving the question open: to what extent is monogamy biology, culture, or a mixture of the two?
To overcome the limitations of often biased surveys and self-reporting, Mark Dyblean evolutionary anthropologist at the University of Cambridge, used an indicator based on genetics. Instead of asking who was with whom, he calculated a “loyalty index” based on the proportion between full siblings (same father and mother) and half-siblings (only one parent in common). The logic explains: in highly promiscuous systems, full siblings tend to be rare, because females change partners between births; In monogamous systems, on the contrary, most offspring share both parents.
Dyble gathered data from 103 human populations and compared them to 34 mammal species. The human sample included current populations with diverse lifestyles and also ancient DNA, originating from archaeological contexts.
The results place humans in a intermediate position between two references from the animal kingdom: meerkats and African wild dogs.
According to the analysis, the global average of full siblings in humans is 66%, a value close to what was observed in socially monogamous mammals (70.6%) and well above the average recorded in non-monogamous mammals (8.6%). For comparison, meerkats have around 59% full siblings, while African wild dogs reach 85%.
The difference in relation to our closest evolutionary relatives is particularly significant. In chimpanzees, the non-monogamous “baseline” reference, only 4.1% of sibling relationships are full siblings, reflecting a promiscuous mating system in groups with multiple males and multiple females.
Although many human societies admit marriage, the author emphasizes that, even in these contexts, the majority of marriages are monogamous. Still, the study highlights that, unlike many monogamous mammals, which live in isolated pairs or small family units and avoid contact with potential rivals, humans organize themselves into large social groups, with multiple men and women living closely together, and yet maintain enough couple bonds to generate a high proportion of full siblings.
The research has implications for debates about the origin and social function of monogamy. A greater number of full siblings can favor cooperation between siblings and create incentives for support in raising children within the same household, reinforcing collaboration networks. At the same time, human monogamy is not “pure”: divorce and remarriage (serial monogamy) introduce half-siblings, and there is also extra-marital reproduction, although often estimated at less than 5%.
Old data further shows great variation between communities. In an early Neolithic tomb in Britain, the full sibling rate was 26%, while in a Neolithic cemetery in France it reached 100%. For Dyble, this diversity suggests that human monogamy could function as a important social “queue”but with variable expression over time and possibly shaped by both cultural and biological factors.
