In a new working paper on Quality Adjusted Population Density (QAPD), PSTC economist David Weil, the James and Merryl Tisch Professor of Economics, introduced a new measurement of land area and population density that offers novel findings on income per capita and population growth around the world. 

Along with PSTC faculty affiliate J. Vernon Henderson of the London School of Economics, and Adam Storeygard, a former NICHD-funded PSTC trainee and associate professor of economics at Tufts University, Weil developed QAPD, a measurement of population density that takes geographic, climactic, and ecological characteristics of land into account. 

By creating a statistical model that takes these variables – such as ruggedness, temperature, precipitation, suitability for agriculture and distance to the coast – into consideration, Weil and his colleagues recalculated each country’s area based on how advantageous its land is for living and producing output relative to other countries. As shown in the image, some countries “shrunk,” meaning they have a smaller portion of desirable land than average, while countries that “grew” have more than average. Using this quality adjusted area along with national population data, the researchers calculated each country’s QAPD. 

Weil noted that when thinking about population density, we often adjust for such characteristics naturally. For instance, he explained, the United States and Canada have roughly the same area, but the U.S. has a much greater population. Yet we don’t generally think of the U.S. as significantly more “crowded,” because we know that much of Canada, especially in the North, occupies less habitable terrain and climate. Thus, the more populated parts of Canada feel roughly similar in density to those in the United States.

“We do those adjustments in our head all the time, so this paper is an attempt to do that more formally,” he explained. “To let the data tell us which characteristics of land are important and to do that in a quantitative way. The tool we deliver is this idea of quality adjusted area.” 

While the changes in area after adjusting for land quality did not particularly surprise Weil, he found surprising correlations between QAPD and income per capita. While there exists no correlation between the standard measure of population density and income per capita, Weil and his colleagues found a negative correlation between QAPD and income per capita. Additionally, countries with higher QAPD and lower income per capita tend to have experienced more population growth over the last 200 years than countries on the other end of the spectrum. Essentially, countries that are more populated relative to the quality of their land, which also tend to be poorer, underwent demographic transition and experienced population growth later than those with lower QAPD and higher incomes. 

Weil hopes that these findings are only the beginning of QAPD’s contributions to population studies. “If someone was doing a measurement and their ruler was bent and I came and straightened it out… that would be our contribution,” Weil said. “The fact that we can construct this quality adjusted area and people can use that for whatever they want is, I think, a contribution of a tool to the field.”