Are humans naturally monogamous?

This BBC podcast is supported by ads outside the UK. Hello, it's Ray Winston. I'm here to tell you about my podcast on BBC Radio 4, history's toughest heroes. I've got stories about the pioneers, the rebels, the outcasts who define tough. And that was the first time anybody ever ran a car up that fast with no tires on. It almost feels like your eyeballs are going to come out of your head. Tough enough for you? Subscribe to History's Toughest Heroes wherever you get your podcast.

And I now live in London. I've lived in London since 2010. This is CrowdScience from the BBC World Service. I'm Caroline Steele and this is listener Alina. I recently met someone we've been going out for five months and they're poly. They've got one other partner and they kind of go on dates and see other people.

They've been poly their entire life. So that's kind of a new world I'm exploring now. Poly is short for polyamorous, meaning having or being open to having multiple partners instead of just one, a.k.a. monogamy. This is the show that answers your science questions. And this episode, our question comes from Alina.

So I just wanted to know, like, why have we settled as a society on monogamy? Is it more beneficial from an evolutionary perspective? Why is polyamory sort of taboo? And has it always been this way? So you asked this question, was it four years ago? Four years ago. Well, thank you for replying when we got back in touch. So I imagine quite a lot has changed for you in...

Yeah, it's been quite a journey. I mean, I can barely remember four years ago. I think it was at the time when I asked the question, I was in a relationship with someone and we decided to kind of open up our relationship.

But that didn't really work out. We ended up breaking up. And since then, I have, I guess, kind of remained interested in alternative... relationship arrangements because i'd like to keep an open mind about what it means to create a successful relationship so you mentioned you have a partner that's right yeah he happens to be outside That's correct. Shall we bring him in? Absolutely. Hello. Come on in.

Grab a chair. We've been chatting in our CrowdScience recording studio, and I'm interested to hear what Alina's partner thinks of her question. OK, so first question. How did you guys meet at work? We knew each other and we occasionally would exchange an email. Then one night we ended up chatting for many, many hours and then we went for lunch and then it all happened from there. What made you guys interested in each other initially?

Yeah, well, actually, I think one of the things we talked about very early on was the fact that you have multiple partners. And I was like, that's interesting. But also just... common interests sense of humor i think is super important we are silly a lot and so how early on did the conversation about you being non-monogamous come up

We talked about relationships beforehand. And for me, it's just who I am and what I do. So you kind of knew, right? That's the deal with me. So you currently have one other partner? Yes. It's an interesting one because I'm open to all kinds of relationship styles. So I have lots of incredibly important people in my life. They're in my life in lots of ways that other people would consider a partner. So it's not so firmly defined.

Alina, would you like to have another partner as well? I'm open to the idea, I guess. But for me, it takes me a long time to meet people and to... become interested in them in that way so I am dating but I am not seeing anyone else at the moment. It was a privilege to meet Alina and her partner and be given a window into their happy relationship. We'll come back to them in a bit. But first, it's time to try and answer Alina's question.

Why have humans, on the whole, settled for monogamy? Now it's worth saying that not all humans have chosen monogamy. There are different types of non-monogamous relationships all over the world, including same-sex partnerships. But most of the research in this area has primarily looked at opposite sex relationships. So that's going to be our focus in this episode.

One way to find clues about why we've evolved the way we have is to turn to our closest primate relatives. Could their reproductive strategies reveal anything about monogamy in humans? So, we've just come to the gorilla kingdom, I think it's called, and we're standing in front of the gorillas as they slowly move around. their enclosure. We visited London Zoo with Kit Opie, an evolutionary biologist from the University of Bristol. He studies mating systems in primates and early humans.

We can see the back of the silverback, and he is massive. So is that the head gorilla? It's the male gorilla. It's the male. Okay, so the rest are females. I don't know anything about gorillas. The rest are females. So he's about twice the size. of the females. Oh my gosh, okay, and a baby has just appeared in front of us. And the interesting thing is that the baby will be his.

OK, because there's only one male in this group. Only one male and a number of females. It's quite distracting having the conversation in front of the cutest baby gorilla ever, who is now sat on the floor and is sorting through... The wood chips, oh my goodness me, adorable. So are gorillas monogamous? No, they're what we would call polygynous. So that is a single male and multiple females. So the offspring within the group are all fathered by the male.

mothered by the different females within the group and why do you think it's a good reproductive strategy it's not OK, so why do they do it if it doesn't work? That's a very good question. It's not entirely clear. What we find is gorillas are threatened with extinction because infanticide is the real big problem.

Infanticide is a pretty gruesome aspect of gorilla life. It's when a male gorilla kills baby gorillas that he's not related to, so that their mother will become fertile more quickly and he can then mate with them. probably not an evolutionary strategy we'd want to emulate. But with other apes more closely related to humans, like chimps and bonobos, the females have come up with a cunning plan to stop the murderous males.

This male strategy for infanticide, I mean, obviously the females are not going to take this lying down. They're going to come up with counter-strategies. The first is to confuse paternity. And that is exactly what chimpanzees and bonobos do. So the females will mate with all the males within the group. So they all have the idea that the offspring... Maybe it's theirs. There might be theirs, but might not be. But enough that they will leave it alone. They won't cause it any harm. The downside...

is that they won't do anything to help out. And chimpanzee males basically sit around, feet up, drinking beer, watching TV. Oh, the equivalent. human social structures are more similar to those of chimps and bonobos than infanticide practicing gorillas.

So do human females also have an ancient evolutionary drive to have sex with as many males as possible to confuse paternity? The difference is that the paternity confusion strategy no longer worked during our evolution. So the research that I work on, reconstructing... ancestral. mating structures, it's probably the case that the ancestor for chimpanzees, bonobos and us, about six million years ago, would have had a similar mating strategy to chimpanzees and bonobos.

female groups, where the females would mate with all the males there to confuse their paternity, and that would have worked fine. The difficulty came about about two million years ago, and the reason for it was climate change. So in sub-Saharan Africa, where our ancestors lived, there was a drying and large parts became savannah. And at that point...

Early humans needed to be in large groups in order to protect themselves from huge numbers of predators. Lions, leopards, crocodiles, eagles and so on. Brains got larger. in order to deal with these large, complex groups, and therefore the lactation period had to extend, and even that wasn't enough. And also it was very difficult for the females to mate with all the males within the group because there were rather a lot of them.

and what the females needed was to have a male to work with them. They weren't able to confuse paternity enough because there were too many males, and also they needed the help. from one of those males to bring up their offspring. And so they switched to monogamy. Would you say monogamy has evolved to be the best strategy for humans? It's the only strategy for humans. We see non-monogamous people. Yes, so in modern humans, yes, we start to see...

But during our evolution, there really wasn't another alternative because the large brain needed a lot of support. It needed the dad to be involved in that.

So according to Kit's research, early humans did evolve to be monogamous. But even humans that choose monogamy today often struggle to be faithful to one partner. Why? I think this is true in... most monogamous species yes there are species that stick with a single partner through the whole of their life and they don't cheat but they're quite rare

Our closest relatives that are monogamous are the Gibbons. What I think... makes a difference for us compared to say the gibbons is that gibbons are separated off from the other pairs and it's it's probably easier

for both the male and the female to kind of police who's coming into their little patch of the rainforest and who isn't. When you're in a large multi-male, multi-female group, like... humans are it's much more difficult to police that both for a male and a female to see whether their partner is cheating or they're not and perhaps that's why it

expresses itself more but i think we're we're not doing so bad we're not doing so bad no we do similar to most other monogamous species i'd say So, according to Kit, monogamy developed in humans as the best strategy to raise children, even though it comes with cheating.

But whatever happened way back in our evolutionary past, support from modern societies means women don't always need to depend on a man to raise a child. And we see a huge range of successful child raising strategies around the world. That's what we're looking at next. Hello, it's Ray Winstone. I'm here to tell you about my podcast on BBC Radio 4, history's toughest heroes.

I've got stories about the pioneers, the rebels, the outcasts who define tough. And that was the first time anybody ever ran a car up that fast with no tires on. It almost feels like your eyeballs are going to come out of your head. Tough enough for you? Subscribe to history's toughest heroes wherever you get your podcast. You're listening to CrowdScience from the BBC World Service.

I'm Caroline Steele, and we're answering a question from listener Alina. Why do most humans choose monogamy? So far, we've heard about apes mating strategies, and how monogamy developed in early human history.

But nowadays, patterns of monogamy or non-monogamy are very different from culture to culture. I'm Katie Starkweather. I'm an assistant professor of biological anthropology at the University of Illinois in Chicago. You've studied non-monogamy all over the world. Which countries have you looked at?

The first type of research that I did was basically just going through lots and lots of historical and ethnographic documents looking for examples of polyandrous marriage or polyandrous relationships you know one woman has multiple husbands or multiple recognized boyfriends at the same time and the reason being was that there's this idea among anthropologists and social scientists more broadly, that polyandry is extremely rare.

across cultures would have been extremely rare throughout human evolutionary history. And as some have said, it really just doesn't even warrant understanding from an evolutionary standpoint. Interesting. Do you agree with that? No, I don't agree. The purpose of this research was to look for lots of different examples. And I found, I think it was 53, and then lots more have been brought to my attention since then. And they showed up on...

Almost every continent, several different countries in Africa. South America, North America, there are lots of cases. Asia is really where some of the more like classical examples lie. Nepal and Tibet are two of the main areas where polyandry in the past was super.

formal like it was the preferred form of marriage among some you know wealthy people in some communities um so why do you think it is that we've had this idea that men sometimes have multiple partners but women so rarely do that it's hardly worth looking into so sort of from like a statistical standpoint it's much less common

for one woman to have multiple husbands across cultures as far as we know than it is for one man to have multiple wives. Why do you think it is that it's more common for men to have multiple partners than women? That's a great question. I think there are lots of different answers to that. One of them is that from an evolutionary perspective.

If we're thinking about the number of kids that a person can have, men are not restricted by really much of anything about their own biology. Really, they're only restricted in terms of how many... partners they can have children with. Whereas women are really... really restricted, right? Like it takes us nine months to gestate and then lactate. And so we're talking about, you know, one child typically for one woman every two to three years, if you're sort of.

having kids just sort of back to back. Okay. So sort of from a practical point of view, it takes a long time for a woman to have one kid, whereas a man theoretically could be having hundreds in the time it takes one woman to... No, they don't. There are very, very few examples of that actually happening, no matter what, you know, the manosphere wants to happen. But from a practical standpoint, that's right. Economically, there are also lots and lots of deeply ingrained economic reasons why men

in particular contexts, are sort of better able to maintain marriages with multiple people at the same time. And one reason for that is that in lots of contexts, the women... in the communities actually do the majority of the work. And so the husband doesn't have to provide food for the entire family. The women are doing that.

The form non-monogamy takes varies, but it is something we see repeatedly across the globe. So even though there are good evolutionary reasons for monogamy, like support with childcare, non-monogamy must also have its advantages. There are genetic advantages in environments where people get sick a lot and they die from those illnesses and things like that. You might be really doing well for yourself if you have multiple kids who have slightly different.

genetic makeup. They might be a slightly better fit with their current environment. Usually what anthropologists see is that the benefits are almost always economic. Women can benefit economically from having multiple partners, something that popped up in lots of different cultural contexts around the world. If you're living in an environment where the men are responsible for providing a large portion.

of the resources. If your primary husband dies, or he has to be gone from home for very long periods of time, like was the case in some Native North American groups, it really behooved you to have essentially a backup plan. That makes sense. You're sort of, I guess, not putting all your eggs in one basket, sort of metaphorically and literally speaking. Yes. But not all humans are.

non-monogamous so I guess there must be some disadvantages as well right yeah it's really really hard to maintain multiple spouses whether you're a man or a woman economically it's hard emotionally it's hard there are Lots of examples of co-wife situations in African pastoralist communities and agricultural communities where there's...

a lot of jealousy between co-wives. Now, this isn't always the case, certainly, but it's a really delicate balance and it's not super easy to maintain, right? Sometimes maintaining one relationship is... Like with one person is hard enough. So, you know, trying to manage multiple relationships at the same time is really hard. That's the primary reason I think why monogamy.

is still the most statistically common form of marriage. Most people around the world, for most of their lives, are living in monogamist relationships.

For both men and women, non-monogamy can offer economic advantages, more hands to share responsibilities and more sources of support. But it can also bring something else, more opportunities for jealousy. So how do people in non-monogamous relationships deal with this? Our listeners Alina and her partner are in a polyamorous or poly relationship. So I asked how jealousy plays out for them.

I understand it and I recognise the feeling, right? Like I'm very familiar with jealousy. It can be a very difficult emotion and very strong and very kind of overpowering. For me personally, a lot of it can come from feeling like they're not being honest with you. And as soon as I know that they're honest...

That helps with the feeling of jealousy. That makes sense. Yeah, sorry, it's not an easy answer, I don't think. No, no, it's not easy, but that makes sense. I think the expectation is there that you will feel jealous and so you're more enabled to talk about it. The other challenge with any form of non-monogamy is the amount of emotional work you do. Not only are you encouraged to talk about things or it's essential, you do it with multiple people.

You might have two, three, four partners. You're doing two, three, four times the amount of emotional work. I would say jealousy is actually not the bigger issue. I would say the amount of time and effort it takes just to maintain healthy relationships. can be a bit overwhelming. Definitely it forces you to have conversations that you otherwise might not. You just need to talk about what's going on because there aren't any predefined rules. So it's made for a pretty great relationship.

The extra emotional work and navigating jealousy are perhaps two reasons why many modern humans choose monogamy. But these feelings we humans contend with in the making of relationships, like love, jealousy and safety, all come from somewhere. our brains. And unexpectedly, one way to get an insight into that is to study prairie voles.

They're these cute little rodents. They're a little bigger than a mouse, kind of the size of a hamster. And they're found in the prairies of North America. And we study them because they're monogamous. This is Sarah Blumenthal. a neuroscience PhD student at Emory University in the USA. And so we can learn a lot about bonding and relationships from these animals. So you say they're monogamous. Does that mean they find one mate for life?

Not necessarily for life. A lot of them are monogamous, but sometimes they choose to find a new mate. But we call them enduring pair bonds because they last over a long period of time. So are they kind of like humans or are they more monogamous than humans, would you say?

I think that they're very similar to humans in the way that their mating styles change in different contexts. So some of them will be more monogamous, but if there's environmental pressures, then it may be more advantageous to adopt a non-monogamous mating style.

they will sort of shift into that. And so same with humans, you know, many cultures and humans around the world opt into monogamy, but there are certainly situations where that's not the case or where people change their mind about that relationship.

Have you been able to look at what's going on inside the brain of prairie voles when they form these pair bonds? Our lab has done a lot of work recording from their brains while they're meeting their partner for the first time and then also sort of looking at their behaviour.

during the first few hours of that bond forming, and then later on when they've had that bond and it's really, really existing there. And what we've found is that there's a neurochemical in the brain called oxytocin that is really important, we think, for this bonding. Neurochemicals are the hormones or other chemicals that are released in our brains and drive our behaviour. Oxytocin is one of those neurochemicals.

It's often referred to as the cuddle hormone because it's released during physical touch or positive social interactions. So if you don't have proper oxytocin signaling, if we experimentally mess up that signaling in some way, they're not able to form this bond. They spend less time around the partner. If they do form the bond, it takes them much longer.

they seem to have less preference for their partner going forwards. And do we see the same oxytocin process in human brains as well? Yeah. So when you take an individual and you... show them pictures of their partner, certain areas of the brain light up. And if you give somebody oxytocin, you see more activation in the brain regions that are involved with reward. So it seems that oxytocin is sort of enhancing the rewarding properties of seeing...

Is it fair to say that oxytocin is the hormone that drives monogamy? Have you been able to see an absence of it in non-monogamous animals? Yeah, so something that's really cool that our lab found and really got us interested in prairie bulls was the finding that there's less oxytocin receptors in these reward regions of the brain.

in non-monogamous voles. So there are certain species of voles like meadow voles and montane voles they're not monogamous and they have way fewer receptors for oxytocin in this brain region called the nucleus accumbens interestingly non-monogamous mice and rats also don't have many oxytocin receptors in this brain region, but humans do.

We have a lot of oxytocin signaling in this reward region. And so that may be part of the reason that we're able to form these monogamous bonds with other individuals. Do we only see this oxytocin release in monogamous pair bonds? Or could it be that you get this oxytocin release, you know, five times over if you have five different partners? Yeah, so the oxytocin is being released pretty much in any social situation. So you and I talking right now.

our brains are releasing oxytocin. I'm not necessarily bonding with you, although you seem lovely. And that's because there's a lot more going on, right? So oxytocin is just sort of priming your brain. And so it puts the animal in a state of being socially receptive. said if they form other pair bonds oxytocin is certainly going to be involved in that. But Sarah thinks another neurochemical might hold the answer.

What really may be causing these differences is dopamine, actually. So dopamine in the reward regions of the brain is... really important for driving this want of something, right? It's driving the rewarding properties of food, of a partner, of sex. And we find that there's differences in the way that dopamine signals when an animal

is forming a bond versus when they have formed a bond. So when they're forming a bond, there's more signaling of dopamine that's indicative of being receptive and open to other animals. But once that bond is formed, we see changes in the way that dopamine signals in that region that's more reflective of being actually aggressive towards new partners in lieu of spending more time with your partner. So it may be more dopamine and reward related than necessarily oxytocin.

in that situation. The way prairie voles' brains process the rewarding neurochemical dopamine actually changes depending on whether or not that vole has a partner. This means they're far more motivated to form a bond when they're single than when they're already paired up. Of course, we can't say for certain that the same happens in humans.

But perhaps this neurochemical shift is part of the reason most humans choose monogamy, alongside avoiding jealousy and the emotional effort of managing multiple relationships. That said, for some people, those challenges are outweighed by the benefits, like greater emotional and economic support and stronger social connections.

What's special about being human is that our behaviour can be so flexible. Here's anthropologist Katie Starkweather again. Humans have evolved to be flexible, and that includes the way that we... have relationships and the way that we get married. You know, we live in every type of environment on the planet, and that's because of the flexibility and behavior that we have. That's all for this edition of CrowdScience from the BBC World Service.

The question was from me, Alina, in the UK, and the show was presented by Caroline Steele and produced by Priya Sippy, Ben Motsley, and Imon Moyn. Thanks for listening. Bye. Hello, it's Ray Winstone. I'm here to tell you about my podcast on BBC Radio 4, history's toughest heroes. I've got stories about the pioneers, the rebels, the outcasts who define tough.

And that was the first time that anybody ever ran a car up that fast with no tires on. It almost feels like your eyeballs are going to come out of your head. Tough enough for you? Subscribe to history's toughest heroes wherever. you get your podcast.