More on Fertility Rates

Back in March, I mentioned that postponement of childbirth could create an illusion of declining fertility and provided a crude proof-of-concept example. However, it was never really clear to me what the magnitude of this effect would be or whether it could plausibly account for a significant portion of the recent decline in European fertility rates. I finally got around to creating a somewhat more realistic model this weekend.

I found that the effect could be fairly significant. When mean maternal age is increasing at a rate of one month per year, total fertility rate is understated by 8%. At two months per year, TFR is understated by 14%. At three and four months per year, it's understated by 20% and 25%, respectively. At six months per year, it's understated by a full third. Increases in mean maternal age at first birth (presumably a reasonable proxy for median maternal age at all births) has been roughly one to two months per year in much of Western Europe, so this could presumably account for perhaps a 0.1-0.3 point reduction in TFR---probably not enough to bring them up to replacement levels, but maybe enough to convince some people that the sky isn't falling quite so rapidly as they thought.

Methodological details below the fold.

Within each birth-year cohort, I generated birth rates using a normal distribution. The standard deviation was the same for all cohorts (I used 4, but the actual value doesn't matter), but I increased the mean by a fixed amount for each cohort to simulate postponement of childbirth. For example, for women born in 1920, the births were normally distributed with a standard deviation of 4 around a mean maternal age of 24. If I were increasing the mean maternal age by two months per cohort, then the 1921 cohort would have births distributed with SD=4 around age 24.17, and the 1922 cohort would have births distributed with SD=4 around age 24.33. Excel or CSV file available upon request.

I don't know whether maternal age is actually normally distributed for births within a given birth-year cohort, and even if it is I suspect that standard deviation may be increasing. I don't believe that either of these issues poses a significant problem for the validity of this model, but as always I welcome arguments to the contrary. This model also does not address the possibility that one commenter brought up in response to the original post: Women who think they are postponing childbirth may for one reason or another never actually have children, or at least not as many as they otherwise would have.

Share this

What's barbaric about the

What's barbaric about the lotion thing?

Long term predictions about

Long term predictions about human behavior are among the most difficult to create. Added to the possibility that some women might voluntarily not reproduce after initially planning just to postpone pregnancy, are the many women who find that their fertility has declined or even disappeared.

There have always been women who don't reproduce. The anguish some feel about educated women not adding to the gene pool is not new. Educated women, other than the anomaly known as the Baby Boom years, have always produced fewer children than the less-educated. In the old days, these women were known as Bluestockings.

The bigger issue is that, in general, women are not living up to their normal role as dispensers and protectors of the culture. They have farmed out their children to others for a good portion of the day, and are not working to shield their young from coarse influences. As a teacher, I am seeing children who have no clue why barbaric behavior might be unacceptable (i.e., putting one's feet on a table, slathering on lotion over most of one's body, talking about one's sex life in vivid detail in mixed company).

Moisturization is of the

Moisturization is of the debbil.

I followed your argument,

I followed your argument, but a postponement of births could not explain a phenomenon that has been evident for generations. I mean, Germany┬┤s low fertility is now 50 years old, I presume, and its beginning was announced already in the twenties. In Spain low fertility continues now for 25 years, and no increase is in sight. Moreover, its effect is cumulative, causing an accelerating collapse of numbers.

If women keep postboning

If women keep postboning parenthood until they reach they age where they biologically can't reproduce, the decline then becomes quite permanent.

And indeed this has already happened, so many women in their late thirties and even forties desparately trying to have children.