Bryan Caplan  

Why Doesn't Parenting Affect Fertility?

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Twin and kinship studies find that parenting has little influence on fertility.*  While there is some family resemblance - big and small families run in families - heredity accounts for all or almost all of it. 

This is a little counterintuitive even to me, because parents vary widely in their helpfulness.  Some are willing to provide decades of free childcare to their grandchildren; others won't lift a finger.  It's one thing to say that in the long-run, nurture effects fade out.  It's another to say that directly changing the cost of kids doesn't make a difference.

Here's my favorite way to resolve the paradox: Grandparents' assistance and interference are highly correlated.  Their assistance increases their kids' fertility by lowering the cost.  Unfortunately, their interference has exactly the opposite effect: Each kid becomes another flashpoint for unwanted advice and family strive.

As long as the assistance and interference are highly correlated and have roughly equal and opposite effects, twin and adoption studies will fail to detect a nurture effect on fertility.  There just won't be enough quietly helpful (or loudly lazy) grandparents in the data.  This is very hard to test, but it makes sense to me.  What do you think?

* Check out the following titles on google scholar: "Is Fertility Behavior in Our Genes?," "Behavior Genetic Modeling of Human Fertility," "Genetic and Environmental Influences on Fertility Expectations and Outcomes Using NLSY Kinship Data," and "Natural Selection and Quantitative Genetics of Life-History Traits in Western Women."

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COMMENTS (14 to date)
Prakhar Goel writes:

Here's another possibility: People don't perform a cost-benefit analysis prior to having babies. Rather they rely on beliefs, instincts, and generally (using Pareto's terminology) non-logical actions.

It destroys your pet theory but has the benefit of being a much closer fit to the facts available.

RL writes:

Parenting has little influence on fertility? That's ridiculous. My parents didn't have any kids, and neither did I...

jbs writes:

As an economist, it's a shame you fall ito the same narrow minded trap as the social scientists (a generous term for their soft credentials as scientists) you cite, trying to distill similarities in family size (family size being a more appropriate term than can be plenty fertile and not have kids) across generations of genetically related individuals to a universal equation - you imply that helpful, yet non-invasive grandparents would result in more children for the subsequent generation.

The decision to have children is multi-factorial, and often not an well rationed decision at all, as Goel notes above. And even with significant grandparent interference free help, even someone from a large family may not decide to have more of their own children, but rather, give more to a few number of children.

Then you run into the real biological (real science) issues surrounding fertility (realy fertility, not family size). You may have the most helpful parents in the world, but if your reproductive system doesn't work, you can't have more kids.

Just because your grandmother bugs the heck out of your mom porportional to the help she provides doesn't explain the broader phenomenon.

Fenn writes:

I'm gonna assume you did not post this to buttress Kling's post about the lengths people go to to not change their minds.

Sounds very unlikely. Tangible support > nagging, or so it seems to me.

Look forward to the book.

agnostic writes:

Having a grandmother around *does* boost her daughter's fertility, likely via her grandmothering efforts. Google "grandmother hypothesis."

There's actually pretty good data on this from -- where else? -- Scandinavia. (Such good record-keepers.)

The idea is that this fitness boost that the grandmother gives her daughter is why women go through menopause. It's not random wear & tear; it's an intricately orchestrated cascade that makes her infertile decades before dying. Couldn't have persisted unless it helped her fitness elsewhere, e.g. through helping her daughter thrive.

In modern Western samples, this grandmothering won't vary wildly -- most women live long lives and can provide help, so this source of environmental variance is small. Hence the variance in fertility due to genetic differences goes up.

Why is that genetic-related variance there in fertility? Well, why is it there in the fertility differences between mice and penguins? The environments they're adapted to are different. Even within an environment, there isn't necessarily one best strategy.

There could be one niche that higher-fertility women exploit and another that lower-fertility women exploit. Not necessarily geographical; could be different social niches.

SWH writes:

In the majority of cases, people do not "decide" to have children.....evolution has designed us to preempt contemplation in that matter; and it usually works. Children "happen". The few parents who have children as a consequence of contemplation are too few to have significance in any economic discussion....or most any other.

tom writes:

"Each kid becomes another flashpoint for unwanted advice and family strive."

I think strive is even better than strife here.

Dr. T writes:

There is much misuse of terminology. Fertility, for individuals, is the biological ability to reproduce. It does not mean having lots of children. There is no such thing as "Fertility Behavior" except for behaviors that result in damage to the reproductive system (such as acquiring a sexually transmitted disease that scars the uterine tubes). Perhaps the authors really mean sexual behavior but hate the "s" word.

When papers on fertility, reproduction, pregnancy rates, and birth rates cannot correctly use common terminology, I doubt the authors' abilities to correctly devise methodologies. I strongly doubt the existence of hereditary factors that alter birth rates via non-medical causes. A genetic factor that makes us want two children but abhor three? A genetic factor that makes us want as many children as we can produce? Are the authors serious? Without a plausible mechanism, the entire idea seems ludicrous.

Note for "fertility behavior" researchers: Please find a plausible mechanism (endocrine, neurochemical, brain area concerned solely with reproductive behavior, etc.) before generating wild hypotheses.

-- A skeptical pathologist

Doc Merlin writes:


In the US at-least, only half of children are unplanned, the other half are planned. They find no correlation between family size of parents and family size of children... its probably because of the change due to easy availability of birth control and abortion. There could be an effect if you look a few generations from now since both have already become commonplace.

The Cupboard Is Bare writes:

In terms of hereditary factors, I would say, "never say never." For example, a friend of mine appears to have an inherited trait that affects the women on her mother's side. They have the same complication that occurs either during or after delivery. They start to bleed out. Fifty years ago, this was a serious problem and on several occasions, proved to be fatal. Eventually, medicine improved sufficiently so that the women simply had close calls and heavy bleeding for their remaining cycles. Now there are specialists that would handle high-risk pregnancies such as these from the get go.

The result of all this risk is that most of the women decided to have only one or two children.

So, I'm not saying that the size of all families is regulated by hereditary factors, only that it does on occasion play a role.

Troy Camplin writes:

We also know that regardless of previous family dynamics, a more educated, wealthier person will have far fewer children than the average. So there are other environmental factors out there than just nature and nurture.

My father was raised in a large family (9 brothers and sisters), but he decided that he would rather raise a few children with middle class money than several with working poor money (he is a coal miner). Thus, there was my brother and me. My brother has one child, and one on the way. I just had my second 7 weeks ago today. He is working on his MFA, and I have a Ph.D. Both our wives have M.A.'s. Now, I don't know about my brother, but as I was aware of the research showing people with high IQs having few children and lower IQs having more children, I had decided that I wanted to have at least 2 (perhaps more) children with someone smart to try to help in the other direction (call me arrogant if you will, but we do need more smart people in the world, and that is how I can contribute). I have her, we have two children, and she wants a third. Actually, if we could become wealthy enough, we'd probably have even more (not going to happen on an adjunct professor's and Kindergarten teacher's salaries, though).

Jessi Stewart writes:

In Child and Adolescent Development classes, at least on Western Carolina's campus, it's being taught that grandparents are playing a larger role in the development of their children's children. Because i think that grandparent's prerogatives are different than those of today's society and their children's, they are influencing their grandchild's fertility rates including the age of childbirth.

Also because parents usually grew up with more siblings, they want to have smaller families, and grandparents had these large families, I feel that this affects the rate of birth and family size of the grandchild.

The economic status has changed from the time of our grandparents, but there are also many other new advances that make having children much more expense than it used to be. Not to mention that the purpose of having lots of children 60 years ago was so they could help on the farms and with every day chores. Now with child labor laws and the lack of agriculture in the United States we don't see as many large families here. But we have to take into account the foreign countries and how even as close as Mexico there are still families of 8 and 9 and grandparents are raising those children as well.

Jeff Erickson writes:

I went through a similar thought process as we started a family. There is some evidence that after a country passes a certain average wealth threshold people start having MORE children. The argument in brief is that they can now afford support (e.g., nannies, private preschool) to make the prospect of more children more appealing. The Economist had an article on the topic at:

Brandalyn writes:

Parenting has a big affect on fertility in my opinion. Of course if the grandparents are going to help out it makes the decision a little easier and it makes things flow more smoothly.

The two biggest things that people make fertility decisions about is parenting and wealth. I think besides parenting, money is one of the biggest factors because of how much it cost to have babies. Although some people rely on the gov't to pay for things I think that most average people would base that decision on whether they could afford it or not.

In my opinion, these are the two most important factors of choosing fertility.

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