October 19, 2009
Humans Evolving Toward Earlier Childbirth?

Here's a study that finds that humans are still under selective pressure.

Durham, NC – Although advances in medical care have improved standards of living over time, humans aren't entirely sheltered from the forces of natural selection, a new study shows.

"There is this idea that because medicine has been so good at reducing mortality rates, that means that natural selection is no longer operating in humans," said Stephen Stearns of Yale University. A recent analysis by Stearns and colleagues turns this idea on its head. As part of a working group sponsored by the National Evolutionary Synthesis Center in Durham, NC, the team of researchers decided to find out if natural selection — a major driving force of evolution — is still at work in humans today. The result? Human evolution hasn't ground to a halt. In fact, we're likely to evolve at roughly the same rates as other living things, findings suggest.

Taking advantage of data collected as part of a 60-year study of more than 2000 North American women in the Framingham Heart Study, the researchers analyzed a handful of traits important to human health. By measuring the effects of these traits on the number of children the women had over their lifetime, the researchers were able to estimate the strength of selection and make short-term predictions about how each trait might evolve in the future. After adjusting for factors such as education and smoking, their models predict that the descendents of these women will be slightly shorter and heavier, will have lower blood pressure and cholesterol, will have their first child at a younger age, and will reach menopause later in life.

"The take-home message is that humans are currently evolving," said Stearns. "Natural selection is still operating."

Higher fertility is being selected for. But that's to be expected. Selective pressures work to boost reproductive fitness. In an environment where humans are not faced with starvation or much disease it makes sense that selective pressures will work toward reversing declining fertility.

Unless either robots take over or a world government controls human reproduction I expect long term human population growth to push humanity back up against Malthusian limits.

As I've previously argued:

In industrialized countries fertility rates have dropped. But I consider this a temporary phase. Selective pressures will increase the frequency of alleles that boost fertility. Much like Elvis, Darwin is everywhere. An Australian twins study (see here and here) found evidence that alleles that boost fertility are getting selected for in Western populations. Here are the reasons I expect to see rises in fertility in industrialized countries:

  • Selective pressures will increase the frequency of alleles that cause women to have more babies.
  • Continued advances in reproductive technology will allow infertile women to have babies and for older women to have babies.
  • The development of rejuvenation therapies will eventually allow women to remain young and fertile for centuries. The reproductive potential of each woman will skyrocket.
  • Some parents will genetically engineer their offspring to want to have kids and the parents will do that to ensure they'll get desired grandchildren.
  • Some religious cults will arise that are very pro-fertility.

Share |      Randall Parker, 2009 October 19 10:59 PM  Trends, Human Evolution

jp straley said at October 20, 2009 6:06 AM:

No reason why parents will not be able to have their own grandchildren (from F1 gametes). My book Smart Kat explores this.

anonyq said at October 20, 2009 10:45 AM:

The advantage with grandchildren is that you don't have to raise them but that your children will. I would argue that by definition that means you can't do it yourself

Mika said at October 20, 2009 12:37 PM:

One thing I don't understand is, if fertility-bossting alleles are indeed being selected for, won't their effect only be apparent in the future - decades, possibly a century or two away? In the meantime, the populations of most Western countries are decreasing rapidly.

As for pro-fertility cults-- American Christians already have one started, it's named "quiverfull". Other religions and sects simply icorporate high fertility into the general ideology, without making it into a central issue.

kurt9 said at October 20, 2009 1:53 PM:

So, Randall, you do not think we will become a partying-oriented society of childless post-mortals, heh? That's too bad, because that's what I and all of my transhumanist friends look forward to, especially the ones from down-under.

anonyq said at October 20, 2009 4:17 PM:

Age of first child is completely cultural decided so i don't see how any evolutionary pressure would work. And i see the same problem to a lesser extend with hight and weight. Hight is in aggregate an environmental and not genetic expression and poor people just get more kids

rob said at October 20, 2009 9:19 PM:

Great we've seen the future: a future of short squat women.

But it's pretty obvious that women are under selection for short. If they weren't the selection on men for height would have made us giants a century ago.

anonyq, of course it is not. Very stupid women have babies when they drop out of high school. Women who happen to hit puberty several years later than average are biologically incapable of having children very young.

You are exactly wrong about height in the US: almost all variation is genetic. Over the range of environments in which American children grow up, environment affects height hardly at all.

We do agree on the importance of stopping poor people from having children.

mabirch said at October 21, 2009 10:10 AM:

> We do agree on the importance of stopping poor people from having children.

Rob, by 'stopping' can I assume that you are in Randall's camp of having some sort of coercive government entity to do the stopping?

Also, by 'poor people' can I assume you mean poorer than you?

Randall Parker said at October 21, 2009 5:34 PM:


Can I assume you are in the camp that assumes a planet with 50 billion people on it would be a better planet than the one we live on today? Would 100 billion people be even better? 200 billion better still?

mabirch said at October 21, 2009 9:49 PM:

While sarcasm and mockery are usually effective tools for deflecting the questioner into a defensive mode,I don't think I'll go there right now.

I really would like for you to answer yes or no as to whether you personally are in favor of a coercive government entity that will enforce population controls. You quote experts who either explicitly or implicitly support the concept and you certainly don't resort to your witheringly sarcastic mode with other discussion participants who support that position. So what, exactly, is your position on that matter?

Bob Badour said at October 22, 2009 6:54 AM:


I do not perceive any sarcasm in what Randall wrote. His questions are very fair questions to ask.

One does not have to favour coercive governments sterilizing folks to recognize that, absent any other population-controlling incident, many people will eventually favour it or something very like it. The other alternatives like famines, deadly viral pandemics and wars are no more desirable.

rob said at October 22, 2009 9:43 AM:


Given the real-world constraints that some populations face, namely increased population results in reduced real income per person. I favor involuntary reductions in birth rates over involuntary increases in death rates. You may favor more Ethiopans born so they can starve to death later, but do not pretend it's obviously more ethical.

mabirch said at October 22, 2009 10:30 AM:

Thank you for making plain your position. You believe that taking active, coercive measures toward people who are alive now is the reasonable course of action to avoid what you believe to be an inevitable and disastrous future, and because it is future, speculative scenario.

Bob Badour,
It may be that Randall meant no sarcasm in his questions, therefore I will attempt to answer them as though they were fair and honest attempts to clarify my position as I have asked him to clarify his. The supportable size of the population on the planet, under present circumstances, technologies, social systems, etc. has some sort of upper limit. What that limit is can be debated to varying degrees of detail. Past doomsayers have posited upper limits which have now, not without difficulties, been exceeded.
However, given the otherwise optimistic outlook toward technological progress present on this forum and the amount of speculation surrounding life extension, AI, and other game changing break throughs, why is there the assumption that other lines of progress might not allow such a population to exist comfortably and productively? Why is there, at least as far as I can interpret the conversations that have taken place, the a priori assumption that there is NO way for that to occur and that coercive population control is the only reasonable alternative to famine, plague, and war? So, to be specific, I think people are great. The numbers that Randall has, no doubt without a trace of sarcasm, put forward are extreme scenarios that I hesitate to take seriously. But, since we are engaging in unchecked speculation, why not? Why couldn't 200 billion people with access to as yet unexplored technologies comfortably exist in such a way as to not destroy the earth's ecosystem?
Fear of the future is never an adequate excuse for the implementation of a present tyranny.

Still waiting for the clarification of your position.

rob said at October 22, 2009 11:48 AM:


Africans really have starved as populations increased. It is not speculative, Ethiopan famines really did happen. Zimbabweaens starved to death. Maybe this does not bother you, and that's fine. As to how much can be invested in each child, we know from constrained budgeting that more children means fewer resources per child. Ain't rocket science.

Now one could argue that while Africans are amazingly poor by Western standards, it is the way they want to live. That arguement is rarely made, because so many people think African poverty is a problem the West can and should fix.

anonyq said at October 22, 2009 2:37 PM:


individual peoples height is genetically decided but for groups of people it is the different environment they grow up in and not their genetic make-up.

The number of women that get children when they are 14 is statistically insignificant, number of women that get children when they are 17 isn't but at that age the women that haven't hit puberty probably will never and biologically 17 isn't very young, is is more the biological age to get a first born.

In the 20 century the only reason why people died of hunger was political, not the environment. Even in areas as badly overpopulated as the Netherlands it was war and not bad weather that lead to people dying of hunger.

ps. Are you really claiming that Africans want to be poor?

Randall Parker said at October 22, 2009 7:24 PM:


No sarcasm intended. I'm testing your moral posture. I hear lots of people claim they are taking principled stances on questions involving government power and forms of coercion are acceptable and they are very absolutest about it. I get it a lot from libertarians (e.g. frequent commenter averros). I test them to find out whether their absolute position is really absolute or is it situational.

For example, if a libertarian is opposed to the military draft as a form of coercion I ask them if the military draft was morally unacceptable in 1942. Or were government taxes to fund a military machine to fight the Nazis morally unacceptable?

I want to know what values underlie the moral position.

Starvation: There are more people hungry today than were alive in 1804.

200 billion people living compatibly with ecosystems: This is extremely unlikely. Why? It would go against human nature. People who are more affluent use more land. If we had enough money to produce enough wealth to support 200 billion some of them would have money to build mansions and huge manicured lawns. We'd have enough tech to support dense human populations in all sorts of places where dense human populations weren't previously possible.

Look around today. Are we leaving enough space for large numbers of bison or wolves? They exist in some parks and not much elsewhere. In a place like India with literally 10 times the population density of the United States the wild animals are even harder hit.

I see two ways for wildlife to survive:

1) Governments use far more power (coercion) to keep humans out of large areas and from killing much in other large areas. Force them into much denser living arrangements, force them to use less resources. This is what a lot of environmentalists on the Left favor. A lot of Objectivists, libertarians, and others on the Right detest the Leftist environmentalists who support these policies.

2) Governments use far more power (again coercion) to restrict reproduction.

Left to their own devices without some form of powerful coercion exercised by the state humans will wipe out tens or hundreds of thousands of species. So there's a third option that some people favor though most deny it:

3) Let other species get wiped out.

I even have a few commenters showing up who bluntly state they favor the 3rd option. I'm thinking about it because I figure large numbers of humans will oppose the first two options. I'm picturing that world of the third option and wondering if I can find a way to live a satisfying life in it. It seems like the most likely outcome to me.

One can't preserve ecosystems without coercion by humans against humans. We are smart omnivores with lots of technology and desire to kill, eat, consume.

I think the debate should be over which form of coercion to embrace. I write posts like this one because I want the people who morally posture against such coercion to see they've got a choice:

A) Coercion of humans.

B) Destruction of ecosystems and extinction of large numbers of species.

To get a sense of what we've already changed read Alan Weisman: The World Without Us. That's nothing compared to the changes humans are making and will make in the future.

Randall Parker said at October 22, 2009 9:25 PM:


Regards starving Ethiopians: Far more will starve than even existing in 1975.

According to the CIA World Factbook, Ethiopia’s population is over 85 million, up from 32 million in 1975, despite the famous killer famine of the 1980s. Its current total fertility rate is 6.12 babies per woman per lifetime. The annual population growth rate is 3.2%. At that rate of growth, Ethiopia’s population will be over 140 million in just 16 years.

They are making over $2 per day. Good times. (sarcasm alert: "Good times")

rob said at October 23, 2009 9:26 AM:

Anonyq, you did not understand the study: even if you were right, and all variation between the average height of population were environmental (Italians live in a worse environment than the Masai. Really?) the nurses study that Randall linked to was not between population. It was within one population, a population in which height has a 90% heritability.

As to whether Africans want to be poor: you think they do. Are Africans just as smart as whites? Just as hard-working? Just as capable of thinking ahead, building complex systems? Since you know they are, any difference in outcome results from difference in preferences. The reason Somalis don't live like the Dutch is simple: they don't want to. I really don't see any other possibilities.

People often claim that starvation only happens from politics. It happens from economics: if Ethipians were as productive as Germans, they would export very good cars to buy food. They don't do that.

Randall, I didn't know the rate was that high. Totally amazing.

not anon or anonymous said at October 23, 2009 2:13 PM:

While individual Africans may not "want" to be poor, successful wealth creation requires a favorable institutional framework. And African governments seem to be pointedly unwilling to adopt pro-growth institutions; they care more about favoring the military leaders and kleptocrats who are currently in power.

nmg said at October 23, 2009 2:23 PM:

Two things to keep in mind. Population growth rates are low or even negative in industrialized nations and the world produces a surplus of food. Calls for totalitarianism as a way to address hunger in the third world are premature.

mabirch said at October 23, 2009 9:38 PM:

Thank you for making your position more clear. I appreciate your taking the time to do so.

Government's nature is that it exerts force, coercion. The question then becomes, what actions or situations will it encourage and what actions will it discourage, by force, by preferential treatment, etc. That question directly leads to an exploration of the basis of governmental action or even existence. The founding fathers of the American experiment in government acknowledged that human nature is such that people, or groups of people will seek their own advantage at the expense of others if allowed to do so. Thus the separation of powers and barriers to action that are built into the Federal structure. Even with that, they acknowledged that such a separation would do no good if there were not a fairly widely held basis for self restraint and altruism on the part of the general populace.

I think that you have hit upon the truth when you say that "It would go against human nature". As you are testing my moral posture so I am testing yours. The problem I have with giving government no coercive authority is that there are many examples of societies and situations that descend into horrible violence and oppression absent a coercive check on individual behavior. The problem I have with giving the government too much coercive authority is that "human nature" is present in those humans into whose hand the authority is placed. Self seeking tyranny is, historically at least, the result.

So, given that "human nature" is, at least from historical example, often selfish, short sighted, and self defeating what method do you propose to change it so that real issues can be addressed without the solution being worse than the problem it was intended to solve? For without solving that problem you, I think, are left without the hope that a solution can be implemented.

Dave Gore said at June 10, 2010 10:13 AM:

In the short term, overpopulation is a problem in some parts of the world, and underpopulation is a problem in others. So it confuses the issue to talk about the world as though it were unified. What is likely to happen is that places like Africa will continue to experience disease and famine, which will limit population growth there. Places like Europe and Japan will experience zero population growth.

In the long term, differential fertility within each country will push up growth rates in some developed countries, especially in those which are multicultural (Monocultural countries such as Japan will be able to limit procreation by social pressure). Countries such as the US will either put legal limits on child bearing or face a declining standard of living.

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