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.
Is it still true that “more gender egalitarian countries have lower fertility rates” if you only include industrialized / wealthy countries? Thinking off the top of my head, some of the least gender egalitarian OECD countries, South Korea and Japan, also have some of the lowest fertility rates.
That chart doesn’t prove much because there were a ton of confounding variables. The late 1940s were an extremely strange time for Japan due to multiple years of near famine, American occupation, new constitution, total reorganization of its industrial economy, etc., as well as the beginning of 4 decades of extreme economic growth, which is associated with lower fertility. Wikipedia even says that the “Western practice of ‘dating’ spread” implying it was not a thing before. All this had to have moved fertility in one direction or the other.
The entirety of Japanese society, government, politics, economy, etc, changed dramatically beginning in the late 1940’s, in a rather sudden way largely due to the influence of a foreign power. Thus I think that the fact that formal legal equality for women in Japan began in the late 40’s, and Japan’s fertility rate decline also began in the late 40’s, provides extremely little information on the relationship between gender equality and fertility in developed countries.
Edit: Thomas Kwa already said basically the same thing, I just didn’t see his comment when I wrote mine.
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:
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.
Is it still true that “more gender egalitarian countries have lower fertility rates” if you only include industrialized / wealthy countries? Thinking off the top of my head, some of the least gender egalitarian OECD countries, South Korea and Japan, also have some of the lowest fertility rates.
Unfortunately I don’t have such data. But regarding Japan, I found this chart:
That chart doesn’t prove much because there were a ton of confounding variables. The late 1940s were an extremely strange time for Japan due to multiple years of near famine, American occupation, new constitution, total reorganization of its industrial economy, etc., as well as the beginning of 4 decades of extreme economic growth, which is associated with lower fertility. Wikipedia even says that the “Western practice of ‘dating’ spread” implying it was not a thing before. All this had to have moved fertility in one direction or the other.
Of course this isn’t a proof, but it’s the best explanation for the drop I heard so far.
To the person who disagree voted: I assume you know a better or equally good explanation for the drop?
The entirety of Japanese society, government, politics, economy, etc, changed dramatically beginning in the late 1940’s, in a rather sudden way largely due to the influence of a foreign power. Thus I think that the fact that formal legal equality for women in Japan began in the late 40’s, and Japan’s fertility rate decline also began in the late 40’s, provides extremely little information on the relationship between gender equality and fertility in developed countries. Edit: Thomas Kwa already said basically the same thing, I just didn’t see his comment when I wrote mine.