Is monogamy part of human nature? This question is always controversially discussed. A study has now determined for a variety of modern and historical human societies worldwide what proportion of siblings have both parents in common and how many half-siblings there are. Accordingly, around two thirds of human offspring are full siblings. Compared to 34 different species of mammals, humans are clearly among the monogamous species. Most other primates, on the other hand, rank low on the “monogamy rankings”.
In most societies today, monogamous couple relationships are considered the norm. But does this way of life really correspond to human nature? This question is always controversially discussed in research. Proponents of the monogamy thesis argue that it is precisely this form of coupledom that laid the foundation for the evolutionarily important strong cooperation in human communities. Opponents, on the other hand, argue that today’s form of monogamy is a fairly new social construct, while 85 percent of pre-industrial societies allowed polygynous marriages, in which a man had multiple wives.
Half or full siblings?
Evolutionary anthropologist Mark Dyble from the University of Cambridge in Great Britain has now developed a new measure to classify how monogamously different people and animals reproduce: He recorded the proportion of full siblings and half-siblings for more than a hundred different modern and historical human societies as well as 34 mammal species. For the human dataset, he incorporated both genetic data from archaeological sites dating back to the Neolithic period and ethnographic data from 94 human societies around the world, including the Hadza, a hunter-gatherer people in Tanzania, and the Toraja, a rice farming people in Indonesia.
There was considerable diversity: “The proportion of full siblings compared to half siblings varied greatly within the human sample,” reports Dyble. “In an early Neolithic site in Great Britain, only 26 percent of the offspring had both parents in common; in other populations, including a Neolithic site from northern France, the proportion of full siblings was up to 100 percent.” On average, about two-thirds of human offspring were full siblings.
Comparison with our closest relatives
Despite the large variations between different human societies, according to Dyble, humans are clearly in the range of monogamous animal species with these values. “There is enormous cross-cultural diversity in people’s mating and marriage habits, but even the extremes of the spectrum are still beyond what we observe in most non-monogamous species,” he explains. For comparison: among our closest relatives, the chimpanzees, only four percent of the offspring have both parents in common. Most other primates, including gorillas, baboons and macaques, also rank low in Dyble’s “monogamy ranking.” In crested macaques, the proportion of full siblings is only 0.8 percent.
Of the species examined, only the white-handed gibbons resemble humans. The proportion of full siblings is 63.5 percent and, apart from humans, they are the only species in the top ranks of monogamy that usually only gives birth to a single offspring per pregnancy. The mustache tamarin, on the other hand, which, with 78 percent of full siblings, is the only primate species ahead of humans in the monogamy ranking, usually gives birth to twins or triplets.
Evolution of monogamy
“Based on the mating patterns of our closest living relatives, such as chimpanzees and gorillas, human monogamy likely evolved from non-monogamous group living, a transition that is highly unusual among mammals,” says Dyble. However, the researcher also points out that in humans, monogamy in reproduction does not necessarily mean that there are no other sexual contacts. “In humans, contraceptive methods and cultural practices break the connection between mating and reproduction,” he explains. Infidelities that do not lead to offspring are left out of his study.
Nevertheless, Dyble sees his results as an indication that a predominantly monogamous lifestyle is typical of human mating behavior – albeit with exceptions. “Humans have developed a range of partnership forms that create the conditions for a mix of full and half siblings with strong parental investment, from serial monogamy – that is, a sequence of several monogamous partnerships one after the other – to stable polygamy.”
Source: Mark Dyble (University of Cambridge, UK) et al., Proceedings of the Royal Society B Biological Sciences, doi: 10.1098/rspb.2025.2163