A theory of menopause

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<p style="padding: 0px 0px 0.5em; margin: 0px;"><em>Romesh Vaitilingam interviews Aloysius Siow for Vox</em></p>
<p style="padding: 0px 0px 0.5em; margin: 0px;"><em>October 2010</em></p>
<p style="padding: 0px 0px 0.5em; margin: 0px;"><em>Transcription of an VoxEU audio interview [http://www.voxeu.org/index.php?q=node/5968]</em></p>
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<p><strong>Romesh Vaitilingam:</strong> Welcome to Vox Talk, a series of audio interviews with leading economists from around the world. My name is Romesh Vaitilingam, and today's interview is with Professor Aloysius Siow from the University of Toronto. Aloysius and I met at a conference on sex selection at the Center for Market and Public Organization at the University of Bristol. During his visit he also presented a paper called &lsquo;A Theory of Menopause.&rsquo; I began by asking him why menopause was something that needed explaining.</p>
<p><strong>Aloysius Siow:</strong> What's kind of surprising is that menopause is not common among mammals, and certainly not present in any other primate species other than humans. So one thing, it's rare. And from an evolutionary theory perspective, it is also anomalous. Post menopausal woman still consume resources which could be otherwise given to her offspring. So from evolutionary theory you have an organism that is consuming resources but not adding to future fertility and depriving resources from her offspring which could increase future fertility. So that's a problem from a theory point of view.</p>
<p><strong>Romesh:</strong> And it&rsquo;s this issue around resources really which is why and how an economists comes to be thinking about these things. That's kind of linked with the evolutionary biological questions.</p>
<p><strong>Aloysius:</strong> Right. So let me just say that there is an existing theory that evolutionary biologists have thought about, and they call it the grandmother hypothesis. Their hypothesis basically says these post reproductive grandmothers essentially help their offspring to survive and maybe to have more kids and also to look after the grandkids. So while they are not producing kids directly themselves, they further the survival of their offspring and grand-offspring. That theory, I think is interesting. The evidence on it is mixed. Part of the reason why it is difficult to get conclusive evidence is (1) even though post reproductive grandmothers have more grandkids part of the reason could be that women who live long enough to be post reproductive have long-life genes which also says that their grandchildren are going to be healthier. So the fact that they have more grandchildren is not necessarily what they do directly, but the fact that they may have healthy genes.</p>
<p>Another issue that comes up is the question of reverse causality, whether a post reproductive grandmother who is consuming resources but not producing future fertility justifies consuming those resources by helping out their offspring and their grand-offspring. So then again you have a correlation but the causality is reversed.</p>
<p><strong>Romesh:</strong> So you feel that the grandmother theory is unsatisfactory which is why you started thinking about an alternative theory, and you started off by thinking about in terms of looking at chimps which are our nearest primate relatives.</p>
<p><strong>Aloysius:</strong> Yes. Biologists have studied chimps, and suddenly most of the evidence points to these chimps not going through any period of menopause at all. It turns out they are our nearest relatives also among the primates. The fertility of chimps is kind of interesting. They live in troops of mixed gender of about 15 to 50 chimps in a troop. When a female chimp is about eight to ten years old, she becomes sexually active. Menarche actually begins at age ten which is when she begins to menstruate. The first birth occurs somewhere between age 12 to 20.</p>
<p>It turns out that these female chimps are quite promiscuous, and they would essentially mate with every male chimp in the troop each time they ovulate. As a consequence, males have no idea of who the father of an infant that is born. Because they don't know who the father of the infant actually is, male chimps don't contribute much to survival of the offspring, nor do they contribute much resources to the females because they are not sure actually who produced the offspring.</p>
<p>So female chimps essentially bear a burden both of child bearing, child birth, and also investing in their own children. So in general you would think that if the female chimps are aware of this whole cost of child bearing they wouldn't want to have as many kids as nature wants them to have. The male chimps of course are happy to have more kids because they don't bear any of the cost of having those kids.</p>
<p>But nature solves that problem by of course making chimps have a desire for sex when the female ovulates. So chimps have sex. Of course as a consequence of having sex they have kids. So to put it another way, two chimps are not consciously able to control how many offspring they'll have.</p>
<p><strong>Romesh:</strong> So the key element to you theory is then taking this idea of the chimps and then you add a certain kind of intelligence to that which would make them more like us. And then thinking about the effect that that intelligence will have on their approach to thinking about having offspring.</p>
<p><strong>Aloysius:</strong> That's exactly right. So, I was beginning to think about what happens if you give this chimp a very specific intelligence, in particular, an understanding of where children come from. That if they did have sex that sex in a couple of months will turn into a kid. And then they would have to bear the consequence of both childbirth and child rearing. The second part of the intelligence which of course is also that even if they realized the process of reproduction, that they care about the consequence of having sex now and the cost the mother will have to pay later.</p>
<p>If the chimp had that sort of intelligence, the argument is that these mothers or these female chimps would choose for one, first, to delay child bearing. And second, to have a total smaller number of children. Now, from the evolutionary perspective, that is inefficient because the mother doesn't take into account that the chimp bears also half the genes of the father.</p>
<p>So, from nature's point of view, the child is a public good, half the genes belong to the mother, half the genes belong to the father but the mother is not going to take that into account in making her child bearing decision. So, she's going to choose to have less kids than will be optimal from an evolutionary perspective.</p>
<p>So, to get things back to maximizing fitness which is what evolutionary theory predicts, nature then reacts to the fact that these chimps became intelligent by developing menopause. And what does menopause do? What menopause does is, of course, is basically retards the ability these chimps then to delay childbirth and child rearing because if they wait too long, they won't have any kids at all.</p>
<p><strong>Romesh:</strong> So, it's kind of a signal to young humans or young chimps that require the intelligence, it&rsquo;s a signal that you have a biological clock.</p>
<p><strong>Aloysius:</strong> That's exactly right so to the extend then we have young women who complain about biological clock and what it means, the whole point here is that nature imposes the clock basically to get young women to get going on building a family. Now fair or not, we still see that it works in the right direction.</p>
<p><strong>Romesh:</strong> Is there any way we can test this theory, Aloysius? Is there any evidence we can go back and look at from primitive societies for example?</p>
<p><strong>Aloysius:</strong> This is, like I say, the theory is certainly speculative and it's kind of interesting but it also says that looking at hunter gatherers today is not the right way to test this theory because the theory basically says, look this problem is going to arise from evolution prospective as soon as chimps or other primates get a level of intelligence. And certainly hunter gatherers already know, understand the reproductive system and the consequences of reproduction. And in fact, hunter gatherers try very much to control the reproductive process. They practiced contraception by both rhythm methods and other methods which it doesn't work very well. They also practice homemade abortions which is also dangerous and don't necessarily work very well either. But what that does tell you is that hunter gatherers clearly want to control their reproduction.</p>
<p>So certainly testing the models using hunter gatherers is not the right way to go. Now, the theory does have a prediction though. Now whether we can test it in our lifetime or whether you want to see the validation of tests in your lifetime is not necessarily something you want to experience.</p>
<p>Recall that I said that the whole point of having sexual desire is that these chimps don't understand that you have to sex to have babies so nature directly makes chimps value sex. So, when a chimp ovulates, when the female ovulates, the desire for sex increase.</p>
<p>And in fact, males also know that the female chimp's ovulating. So they have sex and then the baby comes naturally. Since humans know where babies come from, having the desire for sex is redundant and especially when they have contraceptives, then having just desire for sex is not enough to have babies.</p>
<p>They have to want their babies. So, therefore having sex by itself doesn't serve any purpose in terms of maximizing fitness. So one of the predictions of this model essentially will be human sexual desire essentially becomes obsolete from a fitness maximization point of view.</p>
<p>Now, whether we will ever be able to live long enough to see that, I don't know.</p>
<p><strong>Romesh:</strong> Aloysius Siow. Thank you very much.</p>
<p><strong>Aloysius:</strong> Thank you.</p>