Humans rank surprisingly high on the monogamy scale

How do human relationships compare with those of other mammals? A new study from the University of Cambridge offers fresh insight by ranking species based on monogamy levels.

The team used a simple question to explore a complex topic. How often do siblings share both parents, and what does that reveal about mating behavior?

A close look at monogamy

Dr. Mark Dyble from Cambridge’s Department of Archaeology suggests a new method to understand how often animals, including humans, form long term pairs.

Earlier studies depended on fossils, old marriage records, or notes from anthropologists. These sources help us guess how people lived, but they do not always give a complete picture.

Animal studies often use direct watching. Researchers observe who mates with whom in the wild. They also use paternity tests to see which male fathered each baby. This gives useful information, but it takes many years of fieldwork.

Dyble uses a different idea. He looks at genetic studies that already exist. These studies show how many children in a group share both parents and how many share only one.

Full siblings share a mother and a father. Half siblings share only a mother or only a father.

How the method works

If a species has many full siblings, it means the parents stayed together for longer and raised several children. This suggests more monogamous behavior.

If a species has many half siblings, it means the parents did not stay together. This suggests polygamous or more open mating patterns.

Dyble created a model that compares sibling numbers with known mating systems. He then uses this comparison to estimate how monogamous each species is.

This method provides a simple measure of monogamy using real genetic data rather than guesses from behavior or history.

Humans on the monogamy scale

While the method is not perfect, Dyble noted that it provides a more direct and concrete way to gauge patterns of monogamy than many previous methods when looking at a spectrum of species, and human societies over thousands of years.    

When comparing species, Dyble found something surprising. Humans rank much higher on the monogamy scale than many people expected.

“There is a premier league of monogamy, in which humans sit comfortably, while the vast majority of other mammals take a far more promiscuous approach to mating,” said Dyble.

Humans choose monogamy

Humans have many children who share both parents. About 66 percent of siblings are full siblings.

This number places humans in the upper group of species that usually form long term pairs. It shows that humans tend to stay with one partner long enough to raise several children together.

“The finding that human rates of full siblings overlap with the range seen in socially monogamous mammals lends further weight to the view that monogamy is the dominant mating pattern for our species,” Dyble noted.

Human diversity and ancient clues

The question of human monogamy has deep roots. Many scholars argue that monogamy supported cooperation and helped humans grow socially. Yet cultural practices vary widely.

Most pre industrial societies permitted polygynous marriage. This allowed one man to marry several women. People also practiced serial partnerships or stable polygamy as part of local norms.

To explore human variation, Dyble used archaeological genetic data from Europe and Anatolia. He combined it with information from 94 societies around the world. These included the Hadza of Tanzania and the Toraja of Indonesia.

Despite wide differences, human groups still tend to produce more full siblings than most non-monogamous mammals.

“There is a huge amount of cross cultural diversity in human mating and marriage practices, but even the extremes of the spectrum still sit above what we see in most non monogamous species,” said Dyble.

Comparing monogamy among species

The study also offers surprising comparisons with other species. Meerkats show a full sibling rate of 60 percent. Beavers reach 73 percent, slightly above humans.

These species engage in cooperative care of young and flexible pair bonds. The white-handed gibbon reaches 63.5 percent, sitting closest to humans among the species studied.

The moustached tamarin ranks highest among non-human primates at nearly 78 percent. These monkeys often raise twins or triplets with strong parental involvement.

Other primates fall far down the scale. Mountain gorillas reach six percent, while chimpanzees reach four percent. This matches the rate seen in dolphins.

Japanese macaques sit at 2.3 percent, and rhesus macaques reach only one percent.

Unusual evolutionary paths

The numbers highlight an uncommon evolutionary shift.

“Based on the mating patterns of our closest living relatives, such as chimpanzees and gorillas, human monogamy probably evolved from non monogamous group living, a transition that is highly unusual among mammals,” said Dyble.

Some canids exhibit similar changes. Wolves and foxes now show elements of social monogamy even though their ancestors lived in polygynous groups.

The grey wolf and red fox hold almost half full siblings in their family groups. African canids score even higher. The Ethiopian wolf reaches 76.5 percent. The African Wild dog reaches 85 percent.

The California deermouse, which forms lifelong pairs, sits at the top of the monogamy list with a 100 percent rating. The Soay sheep sits at the bottom with 0.6 percent because each ewe mates with several rams.

What makes humans different

Dyble says many animals that mate for the long term follow strict family rules. In some species, only one male and one female have babies. In others, only one female in the group is allowed to breed.

Humans do not follow these rules. People live in bigger communities where many women can have children at the same time. This is unusual in the animal world.

Only a few animals show something similar. One example is the Patagonian mara. It forms long-lasting pairs but lives in large shared burrows with many other pairs.

Monogamy and reproduction

Dyble explains that having children with one partner is not the same as only having sex with one partner.

In humans, things like birth control and cultural rules change how families are formed. This makes human relationships more flexible than those of most animals.

“Humans have a range of partnerships that create conditions for a mix of full and half siblings with strong parental investment, from serial monogamy to stable polygamy.”

The study places humans in the bigger picture of mammal behavior. It shows that humans generally lean toward monogamy, even though we allow many relationship styles.

The research also shows that both biology and culture shape how humans build families.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences and bioRxiv.

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