Bryan Caplan  

Who Wants More Kids?

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It may well be the biggest puzzle in evolutionary psychology: Why do humans have fewer kids when they get richer, when their extra resources allow them to support more? Robin blogs a new explanation from Ted Bergstrom:

Because of a genetic conflict of interest between mates, evolution could have shaped preferences so that "human females would fail to bear the optimal number of children in the absence of pressure of mates and kin." ... Thus men would desire more children and women fewer children than their own genetic interest dictates. Differences in birth rates across time and between cultures would occur as one side or the other gains increased leverage in this tug-of-war. In modern economics, women have increased influence in household decisions and, together with improved contraceptive technology, have gained greater control of their own fertility.
This doesn't sound right to me. Other men are usually surprised by my pro-natalist views. When couples I know disagree about family size, it's almost invariably the case that the woman wants more than the man.

In fact, I know a lot of guys whose first choice - at least before the endowment effect kicked in - was to have no children at all. One of the few women to express this preference became the subject of much controversy.

Is my sample biased? Remember, data is the plural of anecdote...

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COMMENTS (14 to date)
Bruce G Charlton writes:

I think the best answer to this involves re-framing the question.

Probably, mens and womens opinions on the number of kids they want is irrelevant (biologically speaking) because reproduction is a bye-product of sex, and the family size is determined mainly by female factors such as menarche, menopause, and breast feeding (which spaces-out the children) - plus infanticide/ abandonment as a last resort.

In other words, the reason for such diversity of opinion between sexes and cultures regarding the desirable family size is that such opinions are mostly culturally determined.

Here are some data:


Population and Development Review. March 1997, Vol. 23, No. 1. Sex and the Birth Rate: Human Biology, Demographic Change, and Access to Fertility-Regulation Methods

Malcolm Potts, Bixby Professor, School of Public Health, University of California, Berkeley.

Success, in evolutionary terms, means contributing more surviving offspring to the next generation than competing individuals of the same species in the same population. Human conception is a probabilistic event occurring against a background of frequent, usually infertile sex, which helps bond parents together. Humans have an innate drive for sex and for nurturing their children as they arrive, but they have no biological predisposition for a specific number of children. In preliterate societies, in the absence of artificial means of fertility regulation, pregnancies are spaced several years apart by unconscious physiological mechanisms based on breastfeeding. In preliterate and in preindustrial urban societies, socially successful individuals commonly had larger than average families. Once people have unconstrained access to a range of fertility-regulation options (including safe abortion), family size falls in all groups and in all societies. In such a context, social success tends to be associated with the accumulation of material wealth, rather than with having more children. The argument that development causes fertility decline is flawed because people cannot make choices about family size without realistic access to fertility-regulation technologies, and such access is historically recent and remains geographically limited. Where access to fertility regulation is constrained, the richer and more educated are usually better able than the less privileged to surmount the barriers between them and the needed technologies, hence the common inverse relationship between income and family size. Policies derived from this perspective are discussed. [23, no. 1 (Mar 97) 1-39]


Felix writes:

Maybe people have figured out that they can let other peoples' kids support them in their old age.

How many people do you know who profess Malthusianism to explain why they don't have kids?

Or, maybe it's just low self esteem - a failure of public schools?

mgroves writes:

Is this a correlation? If so, maybe they are richer *because* they have less kids.

Buzzcut writes:

I have three children. My wife says that she's done. I'd like to have another.

Karl Smith writes:

Barry has it right.

You have to remember to confuse proximate and ultimate mechanisms.

People do not behave as if they are maximizing their evolutionary success. They behave according to desire honed under evolutionary pressure.

There is also a larger point that many people miss. Both the industrial revolution and agriculture were radical disequilibrium events. A huge fraction

Cyrus writes:

If a forager does not work enough, then it will not extract sufficient resources from its environment to reproduce at replacement level.

But it is also possible for a forager to work too hard, if it converts surplus resources into greater reproductive success. It is possible for a forager to work hard enough that is degrades its environment to the point that its descendants will have to work just as hard to get even a subsistence return.

Karl Smith writes:

Sorry Bruce, not Barry. Was just reading The Big Picture

Floccina writes:

Responsibility and urbanization. IMHO most of the drop in birth rates in the modern world is due to urbanization not higher income. Higher income correlates with lower birthrates because in our ecologically minded society it is currently considered irresponsible to have more children BTW this is wrong IMO. More responsible people have higher incomes.

Floccina writes:

BTW an easy way to get your genes out:

Have just to children but get your children to have 5 or 6 each.

Robert Book writes:


You might be experiencing sampling bias. It's not that "most men" are surprised by your pro-natalist view -- it's that most men who express their views to you are surprised by them.

I'm not surprised by your pro-natalist views, and I agree with them -- which is why it never seemed important for me to mention my agreement to you (until now).

(Datum: I have 3.5 children.)

Matt writes:

We are seeing a rapid evolutionary phase triggered by a revolution in contraception, and tens of millions, if not hundreds of millions, of people are removing themselves from the gene pool.

Matthew c writes:

Remember, data is the plural of anecdote...

You're joking!?

Jason Malloy writes:

Why do humans have fewer kids when they get richer, when their extra resources allow them to support more?

'Humans' do not have fewer kids as they get richer - women have fewer kids as they get richer. Men have more kids as they get richer because they can support more, and there is a higher demand for their seed. This is the case in both contemporary and historical times. Women have fewer kids as they get richer because the time they would otherwise spend on birthin' and raising kids gets spent on, well, getting richer. And this new status for women is the almost entirely 20th century+ phenomenon that reversed the historical relationship between wealth and fertility. And increasing assortative mating by income will probably increasingly drive down the fertility of rich men and raise the fertility of poor men.

another bob writes:

I don't want more kids. I want more sex.

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