Neither monogamous nor polygamous: it depends. Here’s what science really says about our capacity for commitment.
Monogamy is often presented as the “natural” form of human romantic relationships, but science shows that the reality is much more complex. The human species has a strong capacity to create intimate bonds, but this does not mean that it is biologically programmed for rigid and permanent sexual exclusivity.
The question that has intrigued anthropologists, biologists and evolutionary psychologists for decades is simple: if sexual monogamy were so natural for humans, why does it seem to require so much effort, so many social norms and so much cultural vigilance to maintain?
Human history offers many examples of non-monogamous forms of affective and sexual organization, from polygamy to infidelity, including casual relationships and contemporary models such as polyamory. A study from the 1960s, recently cited in , concluded that 87% of pre-industrial human societies allowed some form of polygyny — that is, the possibility of a man having several women at the same time. But this cultural authorization did not mean that most people lived in relationships of this type. Even in these societies, monogamy was, in practice, the most common experience for a large part of the population.
Among the mammalsmonogamy is rare. Less than 10% of species form long-term stable pairs. Among primates, the percentage is higher, around 25%, but our closest evolutionary relatives, chimpanzees and bonobos, are markedly promiscuous. Males and females often mate with multiple members of the group.
What explains the evolution of monogamy
One of the most influential theories to explain the evolution of human monogamy suggests that it emerged as a defense strategy against male infanticide. In a study published in 2013, researchers analyzed 230 species of primates and concluded that, in promiscuous species, males sometimes killed other males’ young to make females available to mate again. In contexts where this risk was high, males that remained close to females and offspring increased the offspring’s chances of survival. The formation of more stable pairs could therefore have been favored by evolution.
Other species followed different paths. Female chimpanzees, for example, tend to mate with several males during the fertile period, making the paternity of their offspring uncertain and reducing the risk of infanticide. Bonobos, on the other hand, often use sex as a social tool: to form alliances, ease tensions, and resolve conflicts.
Other explanations for human monogamy point to importance of paternal care, protection, food sharing and geographic distribution of females. Under certain conditions, for a male, investing in a mate and offspring could be more advantageous than continually seeking new mates. Social monogamy would therefore be linked to increased survival of offspring.
The types of monogamy
But there is a fundamental distinction: forming a pair is not the same as being sexually monogamous.
Biologists distinguish three types of monogamy: social, sexual and reproductive. Social monogamy involves living in partnership, cooperating and sharing resources. Sexual monogamy implies sexual exclusivity. Reproductive monogamy means having offspring only with the same partner. In the animal world, these categories may overlap, but they do not necessarily coincide.
In humans, romantic and social bonds are strong. According to bioanthropologist Agustín Fuentes, from Princeton University, we tend to form a very intense connection, sexual or social, sometimes with the same person, sometimes with different people. These ties would have been essential for the development of cooperation, compassion and broad networks of care, including the shared creation of children.
Neuroscience also shows that couple bonding has powerful biological underpinnings. Studies with field mice, a species known for forming monogamous pairs, show that substances such as oxytocin and vasopressin play a central role in creating bonds. After mating, the female brain is flooded with molecules associated with pleasure, reward and attachment, favoring the formation of stable pairs.
Even so, Measuring human monogamy is difficult. Data on sexual behavior is unreliable: men and women tend to report the number of partners differently, and many early ethnographic studies were collected in contexts marked by cultural biases.
Anthropologist Mark Dyble, from the University of Cambridge, tried to get around this problem by observing not the declared behavior, but its results. Using genetic and ethnographic data from more than 100 human societies, he compared the proportion of full siblings — with the same mother and father — with that of half siblings. The result: about 66% of human siblings were full siblings. This places humans among the socially monogamous mammals, although far from absolute exclusivity.
The conclusion is paradoxical only at first glance. If we were evaluated like any other species, humans would probably be described as monogamous. But we are a species with large margin of variation. We have the capacity for deep bonds, but also to break, renegotiate or multiply them.
Culture adds another layer. Religion, civil laws, inheritance, economic inequality, family norms, and social expectations have strongly shaped how societies define and enforce monogamy.
Ultimately, science neither confirms the idea that humans are naturally promiscuous nor that they are rigidly monogamous. The most honest answer is: it depends. We are capable of commitment, attachment and lasting cooperation. But we are also a species that creates rules about love and sex — and often bends them.
