The 2nd Demographic Transition

Looking just at an U shaped graph isn't very informative, as it neglects the relative size of the population. You can actually see from the circle size in your third graph that there are much fewer people on the right side (high income) of the U shape. This doesn't warrant optimism. One has to actually look at a scatter plot and at the correlation coefficient.

At least on a country level, the correlation between IQ and fertility is strongly negative:

The Pearson correlations between national IQ scores and the three national fertility indicators were as follows; Total Fertility Rate (r = − 0.71, p < 0.01), Birth Rate (r = − 0.75, p < 0.01), and Population Growth Rate (r = − 0.52, p < 0.01). source

This looks very bad.

You also mention data supporting an apparent reversal of the trend in a few high income countries. These aren't a lot of data points, so I don't know how strong and significant those correlations are. Probably not very strong, as it includes only 13 data points. Moreover, they don't include South Korea, which has seen massive decline in fertility. Also note that the chart you include shows two graphs with different y axis scale which makes the fertility in the present look higher than if the scale was the same. Which is somewhat misleading.

You also say that the relationship between fertility and GDP is U shaped, but it rather appears only L shaped, with much more population weight on the left side, which is bad.

I would also highlight that the opportunity cost theory is usually a bit more sophisticated than presented here. The theory is that women tend to determine the decision of whether to have children and prefer to raise them themselves, and that they tend to prefer men with higher income than themselves. So if the woman earns significantly less than the man, her opportunity cost for having children instead of a career is small, because the man is the main breadwinner. If the income of the woman is the same or higher than of her partner, her opportunity cost of having children is high.

This theory says fertility isn't so much about absolute income, but about relative income between men and women. It explains why more gender egalitarian countries have lower fertility rates: Because the income of men and women is more similar due to women having careers. It also predicts that any possible positive relationship between high percentile IQ and fertility is determined by couples where the man has a higher IQ than the woman, but not the other way round -- because the latter case would likely mean that the woman is the main breadwinner, in which case she would be less likely to have children.

Another important factor is the ever increasing school attendance. Hundred years ago, if you attended school until you were 18, you were considered highly educated. Today, you are considered too stupid to be allowed working at half of the jobs unless you have some kind of university. Starting you family in your 20s vs in your 30s results in a different family size. You have less time to raise kids. Your parents are more likely to be dead or otherwise unable to help you take care of the kids.

Feminism was supposed to give women a choice, but instead it turned into shaming motherhood. (In my country, "domestic wife" is literally used as a slur by feminists.) According to the cultural narrative, a woman who chooses to have kids and stay at home, is a loser. And if she decides to have both a family and a career, she is supposed to start the career first, even if from certain perspective that makes less sense -- if you have a career and you enjoy it, then starting a family means giving it up. (And if you don't enjoy your career, then why was it so important to have it?) On the other hand, if you have kids first, there is plenty of time for the career after they grow up and leave home, and then you never need to give up a successful career once it started. Or you can change your mind and choose to have more kids.

Basically, people are told that they have enough time to have kids later. And it's... kinda true... but it also changes how many kids they will have.

To me the most important graph is the one that shows both mothers and fathers started spending much more time on child-care in the 90s. What the heck happened? Did children suddenly become that much more difficult to manage? If kids really consume that much time and effort, it's no wonder that people don't want to have kids - it's too much damn work!

I don't know if there's data available to answer this question, but it'd be interesting to know if the relatively small middle class that existed prior to the 20th century also saw lower fertility than their lower and higher class peers. And in that case maybe falling fertility is a middle class phenomenon that ends when people exit the middle class either by making enough to no longer be economically middle class or by making little enough money that they cannot aspire to middle class dreams.

Here's what Claude has to say:

There is some evidence that a similar fertility pattern existed in the past when the middle class was relatively smaller, particularly in the 19th century, but the data is more limited and the trends are not as clear-cut as in modern times.

In the 18th and 19th centuries, fertility rates tended to be higher among the aristocracy and wealthy classes, as well as among the poorest segments of society. The emerging middle class, which consisted of merchants, professionals, and skilled artisans, often exhibited lower fertility rates.

Here are some examples and observations from historical studies:

1. In England and Wales, during the late 18th and early 19th centuries, fertility rates were highest among the landed aristocracy and gentry, followed by the urban poor. The middle classes, such as merchants and professionals, had lower fertility rates.
2. In France, during the 19th century, fertility rates were lower among the urban middle classes than among the nobility and peasantry.
3. In the United States, during the late 19th century, fertility rates were higher among the wealthy and poor, while the emerging urban middle class had lower fertility rates.
4. However, it's important to note that fertility patterns were influenced by a complex interplay of factors, including cultural norms, access to contraception, and economic conditions.

While the data from the 18th and 19th centuries is less comprehensive and more subject to regional variations, there is evidence that the middle classes, particularly in urban areas, were at the forefront of the fertility transition, with lower fertility rates than the wealthiest and poorest segments of society. This pattern is consistent with the idea that the marginal value of additional labor played a role in fertility decisions, although other factors were also at play.

I asked Claude for sources on the above claims. Number 2 might be hallucinated, but all the others I found quickly with Google:

1. Wrigley, E. A., Davies, R. S., Oeppen, J. E., & Schofield, R. S. (1997). English population history from family reconstitution 1580-1837. Cambridge University Press. This study analyzed parish records and found that in England, fertility was highest among the landed gentry and aristocracy in the 18th and early 19th centuries.
2. Livi-Bacci, M. (1986). Social-group fertility differential in France: An economic analysis. Population Studies, 40(2), 223-243. This paper examined data from the 1851 census in France and found lower fertility among the urban middle classes compared to the nobility and peasantry.
3. Haines, M. R. (1992). The New England Family Study: A study of fertility differentials among Pre-Industrial Urban and Rural Populations. Journal of Family History, 17(1), 17-35. This study analyzed fertility patterns in New England in the late 18th and early 19th centuries, showing lower fertility among the urban middle classes.
4. Hacker, J. D. (2003). Rethinking the "early" fertility decline in the United States: Integration of old evidence and new models. Social Science History, 27(4), 545-586. This paper argues that fertility decline in the United States began earlier than previously thought, with the urban middle classes leading the trend in the late 19th century.
5. Livi-Bacci, M. (1977). A history of Italian fertility during the last two centuries. Princeton University Press. This book examines fertility patterns in Italy and found that the middle classes were often at the forefront of fertility decline in the 19th century.

This is a great post. I knew that at the top end of the income distribution in the US people have more kids, but didn't understand how robust the relationship seems to be.

I think the standard evbio explanation here would ride on status -- people at the top of the tribe can afford to expend more resources for kids, and also have more access to opportunities to have kids. That would predict that we wouldn't see a radical change as everyone got more rich -- the curve would slide right and the top end of the distribution would have more kids but not necessarily everyone else.

But the gdp per capita graphs I think are evidence against that view. It looks like the curve is a lot flatter than when fertility is rising than when it's dropping, but if this holds into the future I really don't worry too much. We're on the cusp of all getting a lot richer, or else AI will kill us all anyway. 

High income households have access to the world’s best leisure opportunities, yet they still invest more time in child-rearing than lower income households.

I doubt they invest more time. They have money to pay for more help with childcare. And I think this is the critical difference.

Time spent on care per child has skyrocketed in recent decades. I think that's one major factor driving down fertility: having kids is a bigger PITA every year.

Thinking of costs solely terms of money is a mistake. The time investment is critical.

This is why I'm unconcerned with low fertility if we get AI progress and don't die from it: AI is going to be great at childcare. Even current LLMs have the cognitive capacity to be good tutors and playmates.

ta.

Wow, what interests me and surprises me about this graph is the big bump that peaks in the 50's but begins in the 40's. Baby Boom indeed! I had always thought that the Baby Boom was a return to trend, i.e. people had less kids during the war and then compensated by having more kids after. But it seems that they overcompensated and totally reversed the trend that had been ongoing for decades prior to the war! So now I'm confused about what was going on, and more hopeful that these trends might be reversible.