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Darwin Now pod 7: Evolution and the human family

Updated Tuesday, 17th November 2009

What can evolution tell us about falling birth rates, the menopause or the role of parents in childcare? Biological anthropologist Ruth Mace highlights the interplay between biology and culture in evolving family structures.

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Copyright British Council

Transcript

Music: Variations on a Theme by Giuliani

Rissa de la Paz:

Welcome to Darwin Now. In this podcast, we’ll be looking at how evolutionary biology can shed light on aspects of human behaviour. We’ve already seen how in the rest of the natural world, evolution is often the result of a trade-off between competing demands for major tasks such as growth, development and reproduction. Can something as seemingly complex as human life history or even the origins of family and social systems also be looked at in terms of evolutionary trade-offs? Ruth Mace, Professor of Evolutionary Anthropology at University College London, applies just such an approach in her study of diverse human populations. She explains what motivated her to shift her own research focus from animal to human subjects.

Ruth Mace:

I trained originally as a behavioural ecologist, which is someone who tries to understand how evolution has shaped behaviour to respond to different environments. And it always occurred to me that it could be a very useful framework for trying to understand human behaviour, which includes culture because so much of human behaviour is culturally learned, so it seemed to me to be a very natural step.

Rissa de la Paz:

What are the challenges of studying the evolution of human behaviour?

Ruth Mace:

Well obviously one of the difficulties about studying human behaviour compared to animal behaviour is that you can’t do experiments on humans, you can’t manipulate their reproductive success or things that might influence how they survive. But then there are also all sorts of advantages, not least that people can talk and people can keep written records about themselves, so you might be able to interview someone about their birth history and find out exactly how many children they’ve had over the last 25 years, how many of them died, and you can only get that kind of data from a red deer or primate if you watch them every year for 25 years.

You may also find the data just in records, so people keep legal records, they keep demographic records, they keep medical records, and all these sources are now being potentially exploited to find really quite useful information

Rissa de la Paz:

For Mace and her colleagues, key questions about human family organisation - when to begin reproducing as a woman, how to choose a mate or how much to invest in your children – can be studied fruitfully through the lens of evolutionary biology. And this can have broader cultural implications.

Ruth Mace:

Nearly all human behaviour is at least to some extent learnt from others so it’s cultural to some extent, and that’s led some people to think that natural selection and evolution are really not useful concepts. But first of all, an awful lot of what we do is clearly influenced by exactly what you would predict from natural selective theory. So for example, people devote most of their energy throughout their lives to raising a family, looking after their children, seeking mates, all these things are direct predictions from natural selection.

When cultural institutions try to enforce rules that don’t allow people to do those things people don’t accept those cultural institutions. And in fact really the cultural institutions are more likely to be designed to create some kind of social norm which helps us do those things in whatever environment we’re living in.

And the other thing to say is that cultural traits, even if not subject to natural selection, could be undergoing a process of cultural evolution in that they are transmitted from one individual to another, and cultural traits that have properties that make them very successful about being transmitted are going to spread. And in fact Richard Dawkins in the final chapter of The Selfish Gene talks about the spread of cultural traits and how that also has evolutionary properties.

Rissa de la Paz:

The parallels between biological evolution and cultural evolution also allow us to borrow tools from evolutionary biology to the study of cultural variation. One approach is the comparative method.

Ruth Mace:

We can look at different societies living in different environments and see if their behaviour varies in the way that we would predict given a different environment, and there’s a certain amount of evidence that cultures can be related through a branching tree-like pattern in a very similar way to the way species are related on trees of species, which means that some of the comparative methods that have been designed to test evolutionary hypotheses across species can also be used in anthropology to test evolutionary hypotheses about evolution across cultures, and some of these do seem to work quite well, in fact the basis of anthropology always has been some element of cross-cultural comparison.

Rissa de la Paz:

Another approach is to look at variation not across different populations but within a single population.

Ruth Mace:

The tools of behavioural ecology that are really looking at within a population – what causes variation in reproductive success – that’s been applied very usefully to humans. iSo basically who’s having more babies than somebody else, whose children are dying more or less than somebody else, and how does this correlate with various environmental features, and that will help us make models of what are the costs associated with living in a particular environment, what kind of behaviours lead to the highest reproductive success.

Rissa de la Paz:

Certainly as far as reproduction is concerned, human females pose something of a puzzle for evolutionary biologists.

Ruth Mace:

We have a slightly different life history so we have a very long childhood, then we have a period of reproduction that seems to be quite hectic in that females have very short inter-birth intervals, they have babies every three years or so, which for a primate or an ape of our size is quite unusual. I mean maybe an orang utan of a similar size might not have a birth about every six to eight years.

And then females, but not males, have menopause at the age of about late 40s/early 50s when reproduction ceases altogether, but there’s still at least 20 years normally of post-reproductive life.

Rissa de la Paz:

What aspects of being a human has led to this unusual life history, which is so different from that of our nearest common ancestors?

Ruth Mace:

One hypothesis that’s been put forward by a number of people is this idea that humans are actually cooperative breeders to some extent, so the reason you can reproduce at such a high rate is because you’re being supported, either by a male partner, so in primates males don’t generally provision at all whereas in humans males bring back food, they’re subsidising their spouses’ reproduction. But there’s also been a belief that you’re supported by wider kin, especially grandmothers, and if grandmothers are important it might be that menopause is actually an adaptation to grandmothering in that after a certain age it’s better to stop investing in your own immediate reproductive success and help your daughters or even your sons reproduce.

And it’s very noticeable that as you go through childhood as a woman, as you hit puberty your mother is more or less hitting menopause, as you hit menopause your mother is more or less dying, so it’s almost as if there’s this three stages of life which are all part of a big cooperative enterprise to maximise the inclusive fitness, in other words get the genes out there that can do that.

Rissa de la Paz:

In collaboration with the Medical Research Council, Mace and her colleagues have been collecting data on reproductive patterns in rural Gambia. Their results, confirmed by several studies, show that other women do help with the rearing of children. In particular, having maternal grandmothers available significantly improves the survival chances of children. Mothers may also rely on older children, especially daughters, to help care for younger siblings. So it looks as though female relatives who aren’t themselves reproducing – whether because they haven’t yet reached puberty or because they’ve already hit the menopause – can help mothers in child-rearing. The tendency for such investment from relatives on the mother’s side is precisely what evolutionary theory would predict.

Ruth Mace:

If you have children by more than one male in your lifetime, either because you change mates during your lifetime or because there’s paternity uncertainty, so maybe there’s some extra pair mating outside of marriage, then on average a mother’s going to be more closely related to her offspring than the putative father, and therefore her relatives, on average, are going to be more closely related to female relatives’ offspring than they are to male relative offspring because they just don’t always know whether they really are exactly their offspring.

So, for example, estimates of maybe 10% of children are not fathered by the person that they might have thought they’d been fathered by, that can generate a 10% bias, and that seems to be what we see. So we don’t know the mechanism for the bias, it’s really just that evolutionary theory predicted that such a bias would exist and we see it in the data.

Rissa de la Paz:

The role of different relatives in child survival has been studied in populations spanning Europe, North America, Africa, Asia and the Caribbean. It turns out that the contribution of certain relatives may vary with the ecological context. For instance, the father’s contribution to child survival appears to be more important in hunter-gatherer societies than in agricultural communities.

Ruth Mace:

From what we know of hunter-gatherer society infanticide was a common risk, and not having a father alive could be putting children at great risk of infanticide, because children are very costly. We think that raising children in agricultural communities is probably less costly than raising them in hunter-gatherer communities. Hunter-gatherer children are very dependent on input from their parents right up to their teenage years, and if you don’t have two parents willing to feed you, then the community might not want you around. Whereas in agricultural communities they can often help with the farming enterprise from quite a young age and so they can start paying back their costs a little bit earlier, which also probably explains why farming communities have larger families than hunter-gatherers because the costs of children are actually possibly a little bit less.

Rissa de la Paz:

A range of population studies has shown that we humans have evolved flexible family systems that vary with ecological conditions. This seems to make adaptive sense, because by spreading the responsibility of childcare over a number of individuals, a mother minimises the risk of losing childcare support should any one relative die before she herself has finished reproducing. But our own patterns of reproduction are changing and new questions have begun to emerge.

Ruth Mace:

Another puzzle that’s arisen during the course of the last century is that we’re actually stopping reproducing generally quite a long time before we hit menopause, and also even starting reproducing quite late. So family size has gone down to really quite small numbers, at least in the western world, and in fact that’s a trend that’s now beginning to happen all over the world, so it’s a global trend. And speaking as an evolutionary anthropologist, that is a bit of a puzzle because if we’re all about maximising fitness, maximising inclusive fitness or reproductive success then why would we have such small families.

Rissa de la Paz:

It’s interesting to note that this decline in fertility is happening fastest in urban areas. That’s not surprising when you consider that a shortage of affordable housing, the transitory nature of many communities and a lack of family support make the rearing of children a costly business.

Ruth Mace:

As a behavioural ecologist, I think the most obvious explanation would be that children are just becoming much more costly because children now need to be well educated, they need to have housing which has got very expensive, we’re not necessarily living with our families so we don’t have the grand-paternal/maternal support that we used to have. Older children are at school so they can’t help us raise younger children, so we’re doing it more on our own. And the level of investment now required through education and being competitive in the job market is so high that individuals are going for quality over quantity.

Rissa de la Paz:

So for parents there seems to be trade-off: do you spread your resources more thinly over a larger number of children, or do you have fewer children and invest more in each one? One way to explore this problem is to study the effects of family structure on parental care and see what impact this has on a variety of outcomes for children. Mace and her colleagues looked at data collected from over thirteen thousand UK children and their families. They looked at how much time both the mother and her partner spent in certain key care activities during the first 10 years of a child’s life. How did family size affect the degree of parental care and what impact did this have on children?

Ruth Mace:

We did recently do an analysis of UK children to look at whether having lots of siblings did have any measurable effect on various child outcomes, and this was looking at a cohort of children who’ve only just reached their teenage years, so we don’t know the long term effects, but one of the things we were able to do with this particular cohort study was we were able to look across a range of measures, so we could look at physical height, we could look at how you did in school tests, we could look at how much time your parents spent with you, and that kind of data is actually rather rare.

We found lots of effects of having siblings, including the more siblings you had the slightly shorter you were, and the big one was the more siblings you had the less time your parents could spend with you, so there’s definitely still competition for parental resources, even in the western world where we’re all relatively wealthy and no-one’s dying of starvation or anything like that. But resources such as time spent with children, money, and something even that’s relating to physical height is still limited by having a large family.

Rissa de la Paz:

One might imagine that families on a higher income would have more children since they have more resources to invest. But the picture isn’t that simple.

Ruth Mace:

There is a bit of a puzzle that when we do studies on, for want of a better word, traditional societies, we normally find there’s some kind of relationship between resources and reproductive success. So wealthier households will have more children or more children will survive, or wealthier men might have more wives and therefore have more children. And that’s what you would expect from a kind of behavioural ecology perspective. But when you look in most western countries, or countries that have switched over to having small family sizes, you very often find that the relationship between wealth and reproductive success or family size has disappeared, or it may even be negative.

That’s something of a puzzle, although one thing we did find in the study of UK children is that even though wealthy families weren’t necessarily having any more children than poor families, they were investing more in each child. So it seems to be the level of investment that’s going up rather than the quantity of children that’s going up.

Rissa de la Paz:

In fact, Mace’s own research has shown that children of such families benefited from several advantages working in synergy.

Ruth Mace:

One of the things that came out of this study was certain children seemed to have all the luck. In other words, if you had parents who were wealthy they were more likely to be well educated, they were more likely to spend more time with you, even on measures that don’t cost any money like reading your child a bedtime story. If you had a mother who was a high investor you were more likely to have a father who was a high investor. So the overall effect could be that a lot of these children from the kind of higher socioeconomic status and higher educated parents were getting a lot more investment on a range of measures.

Rissa de la Paz:

If parental investment is a key influence in a child’s future success, and if the ability to invest effectively in children can be passed on from parents to their offspring, could it be that these factors are driving the fertility rate ever downwards? After all, in biological evolution, there are examples where intense competition, for mates, for instance, can lead to evolution of elaborate or extreme behaviours. It’s a process called runaway selection.

Ruth Mace:

If parental investment is helping your children compete in the modern world it may be that our levels of parental investment are just going up and up and up. We’re just trying to be better than everybody else, trying to give your child an edge over all other children. Biologists talk about runaway sexual selection, by which they mean the trait concerned gets evolved to be more and more elaborate, and you almost wonder whether parental investment might not be undergoing some kind of runaway parental investment; as those around you invest more and more, then in order for your child to be competitive you yourself are investing more and more and more, and that helps you find mates who are higher investors and the whole process seems to go on and on and on. So rather than having more and more children, we’re having a small number of children but investing more and more in them as time goes on.

Rissa de la Paz:

Some people might argue that we don’t need an evolutionary explanation for the decline in birth rates: they’re simply a reflection of women making the choice to limit families through artificial contraception. Mace argues that factors other than mere technological convenience have been at work.

Ruth Mace:

In historical times people were trying to limit fertility long before there were any very effective means of modern contraception. So I think you can’t explain entirely in terms of the technological change. I actually believe that it was the demand for small families that led to the demand for contraception to be invented rather than contraception coming first on the scene and people thinking oh, that’s a good idea.

Ruth Mace:

It’s difficult to say in every case that there’s any one variable that predicts the onset of fertility decline, but very broadly it was associated with a kind of post-industrialisation, so people were moving into cities, infant mortality was going down and therefore population density was going up. And the more professional classes started the decline in fertility before the working classes where the sort of perceived cost of what you need to invest in your children was more in some of those societies, so in family-run businesses and things like that people didn’t want to have such large families. So I think tying it back to the idea of perceived cost of children, relative competition between children and parental investment to me still seems to be the underlying causation for why the demand for contraception was there.

Rissa de la Paz:

These studies illustrate the dynamic interplay between social and biological factors in any study of human behaviour. And there are more tantalising areas still to be explored.

Ruth Mace:

There’s been a lot of interest recently in trying to think about whether cultural evolution can cause different kinds of behaviours to emerge than is purely genetic evolution. So for example, humans are very good at coordinating behaviour amongst large numbers of unrelated individuals, whereas in animal species we normally only see coordinated behaviour across related individuals.

Some people have argued that maybe cultural traits have slightly different properties from genetic traits. So for example, you can change cultural traits during your lifetime, if someone coerces you or encourages you to change your behaviour you can. So cultural groups might be able to instil certain rules which they enforce through punishment that could make the group do well at the expense of other groups, and because we have the cognitive abilities to keep track of who’s doing what, we can force people to change their behaviour, if it’s cultural behaviour. Then certain kinds of behaviour might be commonly observed in humans that are not commonly observed in other species, and there’s been a lot of modelling work trying to show how these kind of high level cooperation amongst unrelated individuals could evolve in humans through cultural evolution.

Rissa de la Paz:

Might an evolutionary perspective even shed light on the development of more complex behaviours such as altruism, morality or even religious belief?

Ruth Mace:

If we have emotions that are designed to be pro-social to the wider group or to be very sensitive to punishment, and keeping local norms because the consequences of deviating from local norms are serious, then those kind of emotions could explain, for example, whyall humans are very susceptible to religious belief. So some of these models might explain some of the wider issues about morality, religion, these kind of things that all human cultures seem to place very high emphasis on.

So those are all areas I think where there’s a lot of interest and they may prove more difficult to study than some of the reproductive questions where can get data on fitness, but a lot of people are thinking about those kind of questions now.

Rissa de la Paz:

So whether it’s cooperative breeding and parental investment on the one hand or morality and altruism on the other, human behaviour will continue to pose fresh challenges for evolutionary anthropologists.

Music: Variations on a Theme by Giuliani

Rissa de la Paz:

The insights they yield will shed light on what makes us distinct from our nearest primate relatives and what it means to be human.

This podcast was produced as a collaboration between the British Council and the Open University.

 

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