Most people (and particularly my mother) think that economics is only about money. That we economists are basically counting money and telling people where to invest. This post will be the first evidence that there is no boundary to economics. Today I will go as far as to fertility. Yes, we economists have a lot to say about fertility.
Fertility has interested economists for a long time. At least since Malthus' theory on population growth in 1798 economists have been interested in fertility. Malthus basically thought that as people became richer they would have more kids, which would mean less resources for everyone (he expected technological growth to be quite small), leading distress to knock on everyone's doors (though louder on the poorer doors). Fortunately, this is also an example of economists failure at predicting the future.
On top of human capacity to increase resources, which Malthus undervalued, the other assumption that does not hold in Malthus idea is that the higher your income, the more kids you have. But we will get to that later. Let's see the broad picture first. How has fertility evolved in the US in the last 200 years? Here is a plot of number of kids for (married) women born in different years (cohorts in the figure):
The dark blue line shows that the overall average of children born has decreased from a high 5.5. to an average of just below 2. More impressive is the fact that initial differences between groups (black vs. white, urban vs. rural) have narrowed substantially. Differences that used to be as high as 1.5 kids are now smaller than one-fifth of a child. Notice that this data is for married women, so hypothesis that are based on the reduction of marriage as a cause for the reduction in the number of kids are in trouble. It is also interesting to note that most of this compression is coming from the reduction of women having lots of children.
The number of women having either none or one kid has been quite stable around 10-20%, but the number of women having 4 or more children has diminished from almost 70% to below 10%. What seems to be behind this? There are many theories out there (see the paper cited below if interested), but I will stick here to the one of the most popular among economists: money, money, money...Here is a plot of number of kids for different levels of income for all the different cohorts.
The shocking thing about this picture is that all the women born from 1828 to 1958 seem to be gravitating around a constant relation between (real) income and number of kids had. In other words, in either century, women with incomes of around X apples (real income) would have in average a very similar number of kids. More impressively, there seems to be no difference in this relationship for either urban or rural areas. (Caveats aside) This suggests that the main difference between the average number of kids women have in the 1800s versus the 1900s (or in rural compared to urban areas) is mainly income. People are richer today and, for some reason, richer people tend to have less kids. The question is then why?
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