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Danielle Kuczynski contributed to this post

A recent Economist on-line poll asked: Does the world have too many people? There was a predictable response: from their Blackberries or the comfort of their swivel chairs, 80% of Economist voters clicked on yes. You can be sure the respondents are neither the folks having lots of children, nor the ones suffering from the environmental impact of lots of people on the Earth. Yet the commentary accompanying the poll was intriguing, because the same people who agree that the world is over populated don’t agree about what that means, nor what to do about it.

Their disagreement may arise because important basic information about changes in fertility are poorly understood, even among the generally well-informed readers of the Economist. In my mind, the important questions are:

  • What are people’s fertility aspirations—that is, how many kids do people want?
  • Why are people in different countries seemingly so different in this regard?

Contrary to the views of many economists, the answers to these questions suggest that reducing population growth rates involves much more than simply waiting for rising incomes to bring down fertility rates.

Diverging Fertility Rates in the Developing World

A quick look at how fertility changed over 35 years in developing countries reveals some surprises. In 1970, more than 120 low-income developing countries had total fertility rates (TFRs) that exceeded 4.5 children per woman. More than 80 of these countries had fertility rates that exceeded 6 children per woman, and four exceeded 8 children per woman (World Development Indicators, 2009).

Today, no country in the world has a TFR higher than 8, and only 12 have a TFR greater than six children per woman. Conversely, 131 countries have fertility rates lower than 3.5, more than double the number of countries in 1970. Yet many of these countries continue to be very poor.

The chart below shows this remarkable transformation for 100 developing countries for which we have good data. The vertical axis is total fertility, the horizontal axis is per capita income. All 100 countries began with high fertility and low income. Click on the arrow below the graph. The countries divide neatly into two groups: the green, with big reductions in fertility (but remaining poor), and blue, which show little progress on both measures.

There is strong regional flavor to the changes over time. By 2005, all but two of the remaining high-fertility countries are in Sub-Saharan Africa (those two are Timor-Leste and Yemen). The global percentage decline in fertility for this period was 86%. In East Asia fertility fell by 198%, in South Asia it fell by 92% and in Latin America it fell by 117%. Sub-Saharan Africa managed only a 29% fertility decline.

What makes Africa different? Nobody knows for sure. Possible answers include fertility desires, unmet need for contraception, economic structure, and persistent high poverty. Moreover, Africa is not one uniform place with one set of macro or population policies. Differences abound, and they matter for fertility. For example, South Africa and Swaziland have 70-80 percent modern contraceptive prevalence, whereas Niger and Burundi have remained near 10 percent. But the dots do tell a story, and the end is yet to be written.

Uptick in Fertility in High-Income Countries

If the story in the developing world is more diverse than usually recognized, the story in the high-income countries has taken a surprising twist.

It’s a shock to learn from an Economist story about a recent article in Nature (subscription required for Nature article) that in some rich countries a 40-year old trend of declining fertility is reversing.

The graph below presents the same relationship we showed above between fertility and wealth (this time the Human Development Index rather than per capita income,) for countries that were rich in 1975. The blue dots, indicating each country for 1975, show a strong downward slope: richer countries had lower fertility. But the red dots, indicating each country for 2005, show that once countries score 0.9 or higher on the HDI, fertility rises.

Cross-sectional relationship between TFR and HDI, 1975-2005.

From Nature, Myrskyla et al. 2009From Nature, Myrskyla et al. 2009

What’s up with that? Could it be pro-natalist policies in countries worried about declining population? Maybe people recognize the improbability of social insurance to provide for their extended old-age? Regression to the mean of replacement level fertility? A shortage of family planning in the rich world? (just kidding about that one) Is it a natural biological phenomenon?

The Nature authors speculate there may be a biological response to environments that guarantee stability and comfort: people are so comfortable they want to share it with (more) children.

Just as in a real game of connect-the-dots, a picture will emerge as we try to tie together all of these points. Like the responses to the Economist poll, there may be one answer, and many different ways to get to it.