More on fertility and the HDI

James Holland Jones, a biodemographer at Stanford has posted a critique of Edward's critique of the recent paper by Kohler, Myrskylä and Billari et al. in Nature arguing that development beyond a certain level reverses the fertility decline usually associated with development. Jones' response is a good and fair one and well worth reading, because it gives a good overview of some of the issues involved. However, much of the response amounts to "you work with the data you have, not the data you'd like to have". This is true, but it troubles me that Myrskylä et al. are making a fairly strong claim based on fairly weak data. I would like to add some of my own critique of the Nature paper, which I think complements the points Edward raised.

First, maybe I should say something about why Edward and I are devoting so much attention to this article. If the claims are correct, it really would have very important consequences. Almost everybody would agree that excessive aging is an important impediment to economic growth, both because of the sheer fiscal cost of paying for a large population of retirees and because of structural issues associated with aging, like increasing export dependency, increased savings, reduced consumption and lackluster growth. If the article is correct, this would imply a couple of things: first, that to a certain extent (but not completely) this is a problem that resolves, or at last alleviates itself somewhat as long as one can attain a certain level of development. Second, it suggests a different kind of low-fertility trap where countries who pass a certain tipping point in development can raise their birth rates whereas countries who fail to get to that point may enter a vicious economic circle.

This may in fact be the case, but I dont think Myrskylä et al. make a convincing case for it. Let's have a look at their graph:




Now what's important here, as Jones, Hugh and I all agree on is the black line, which shows adjusted total fertility rate that is not distorted by birth postponement. Looking at, it does indeed seem like there's a positive correlation between adjusted total fertility rate and the Human Development Index. However, in this case correlation does not mean causation. If you look at the countries with an HDI above 0.9, you can distinguish a few important groups. The Nordic countries ( 5 countries, average aTFR in 2005: 1.99 range: 1.92-2.2), English-speaking countries (4 countries, average aTFR: 2.01, range:1.91-2.2 Canada and New Zealand are not included in the data set, so UK, US, AUS and Ireland). These are two cultural groups that both have a great deal of gender equality, the Nordic countries also have a similar social model. There's a bit more variation in the Anglophone group in terms of economic model, but also many similarities. Together, these two groups dominate the upper range of the HDI and they all also have high fertility. Since Canada is not in the data set, they have the top five spots in terms of the HDI.

On the other hand, you have groups countries with more conservative family models: German-speaking countries (3 countries, average atfr 1.58, range 1.47-1.72) and Mediterranean countries (Spain, Portugal, Greece and Italy :average aTFR 1.51 range 1.37-1.65). Mostly, these countries rank slightly lower on the HDI but even when these countries rank high like Switzerland and Spain, they maintain low fertility. Ditto for Japan which has a very HDI, but an extremely conservative family structure.

My point is that cross-country comparison between high-HDI countries are distorted by groups of countries that are culturally similar with similar fertility regimes. The Anglosphere and the Nordic countries have had high and stable fertility -and moreover they had the same level of fertility they do now even at lower levels of development. The presence of these two groups largely explains the correlation between adjusted TFR and HDI. The more conservative countries have had fertility rates that are low and stable. It's therefore difficult to see the impact of HDI on fertility that Myrskylä et al. are claiming

Myrskylä et al. do try to do a longitudinal survey, where they try do look at developments within a country over time and they get similar results. However, the manner in which they conducted the study is, I think, deeply flawed. Since they didn't have a time series of aTFR that went far back enough in time for a sufficient number of countries, they put together a combined series, using TFR when they didn't have adjusted TFR and then controlling for the change in level in the year that they switch between the series. This really seems problematic since TFR and aTFR don't really measure the same thing. Edward pointed this out by looking at the Cohort Fertility rates, but even looking at just the ajusted fertility rate series, it's hard to discern any trend at all. They fact that they got a statistically significant result must be a statistical artifact of the method they used and the limited amount of data available. Just have a look at the graphs for adjusted TFR for the 25 countries in their time series (this is just aTFR and not the combined series they use):











I find it hard to find any support for their argument here. Estonia has experienced an upswing but it happened before the level they claim would induce a fertility increase. The UK series is too short to really tell, but you might be able to make a claim for the US. Either way, the data does not really support their conclusion. I take Jones's point that we can't expect all the hard data we want, but in the absence of it, it would perhaps be better if we all refrained from strong claims without much empirical evidence.