The United Nations (UN) publishes data on the total population and the urban population for 1950–2010.<sup>6</sup> It also groups the data into two broad aggregates: more developed regions (MDRs) and less developed regions (LDRs). For population, figure 1 shows the observed data points as dots and the fitted logistic curves as solid lines. The observation for a given year is the population in each region as a fraction of the terminal population for that region. The data do not pin down the terminal population with any precision. We assumed a terminal population of 1.35 billion for MDRs and 9 billion for LDRs, roughly in line with the medium fertility variant of the UN’s world population projections to 2100.

The goal here is not to test any hypotheses but merely to generate a natural set of summary statistics. Appearances notwithstanding, there is nothing especially impressive about the fit of these logistic curves. Many functional forms for distributions, including the standard normal, could yield an equally good visual fit. The advantage of the logistic is that its three key parameters—the terminal population, the initial rate of growth, and the lag between two curves—are easier to interpret.

The unconstrained growth rates for the two fitted curves in figure 1 are fairly close: 3.4 percent a year for the LDRs and 3.2 percent for the MDRs. This means that the 50-year lag between the two curves remains roughly constant over the last century and the coming one. The lag is consistent with other more granular data on health and demographics. For example, today’s life expectancy in low- and middle-income countries that correspond to the LDRs is about the same as that in 1960 in high-income countries that correspond to the MDRs.<sup>7</sup> Figure 2 shows the logistic fit for the urban share of the population in each region. In this case, we imposed a terminal urban share of 90 percent for both regions. This is on the high end of familiar estimates, but many projections of urban population look ahead only a few decades. Moreover, 90 percent is a value already seen in a few rich countries, notably Japan. Imposing a high terminal urban share lets us construct a useful upper benchmark for the scale of change that could be coming. It also lets us abstract away the concern that in many countries, actual governance might keep the potential urban share below the value justified by economic fundamentals. In this exercise, we are more interested in what the urban share could be than in what it is likely to be. At this aggregate level, the departures relative to more detailed countryby-country forecasting exercises are small. For 2050, the last year for which the UN provides projections, our simple procedure suggests a slightly higher share for the LDRs (65.3 percent) than does the UN projection (64.1 percent).

With our assumption about the terminal value, both logistic curves in figure 2 approach 0.9. Once again, the implied unconstrained growth rate, _g_ , of the urban share is similar in the two regions: 2.3 percent a year in the LDRs and 2.1 percent in the MDRs. The curve for the urban share in the LDRs lags farther behind the MDRs (80 years) than does the growth curve for total population (50 years).

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