Monday, January 31, 2011

The curious case of Dr. Glaeser

Edward L. Glaeser is smart guy. Despite being in his mid-forties, Glaeser has already made a number of important contributions to economics. He has been a fixture in the Boston Globe and has recently joined the lie-up of the New York Times' Economix blog (slogan: "explaining the science of everyday life."). All of which makes this recent column on population shifts all the more inexplicable.

The article is classic "ice cream causes murder" analysis. In order to get the desired conclusion, you have to ignore numerous confounding factors and discard a number of alternative hypotheses that better explain the data and even then you have to be selective with your metrics and graphs (or at least the labeling) to maintain even the appearance of credibility.

Joseph has already discussed this at some length but just to review, percent change can be tricky to work with, particularly when dealing with state populations which can vary by well over an order of magnitude. Add to this the matter of population density -- certainly a concern when talking about home prices but notably absent from Prof. Glaeser's post.

Let's take Glaeser's comparison of California and Texas. If we look at percent population change as he does, Texas does much better but Texas has more land and fewer people. If we look at absolute change, the results are much closer and if we take density into account with something like change by area, California may actually come out ahead. (Glaeser's use of California is also interesting in other unintended ways, but more on that later.)

If you look at this table from Wikipedia, the role of population density becomes harder to ignore and Glaeser's case becomes harder to buy. Consider these two paragraphs:

"More generally, population isn’t moving to high-income areas. The four fastest-growth states were Nevada, Arizona, Utah and Idaho (in order of growth), all of which have earnings below the national average.

"Our richest states, Connecticut, New Jersey and Massachusetts, grew by 4.9, 4.5 and 3.1 percent, respectively, far below the national average. People are not following the money."

Could available land be playing a role here? Probably. Nevada, Utah and Idaho are among the ten least densely populated states in the union. Arizona is is in the next ten. By comparison, Connecticut, New Jersey and Massachusetts make up three of four most densely populated.

Glaeser's response to this point (in a related post) doesn't really help his case:

"Why is housing supply so generous in Georgia and Texas? It isn’t land. Harris County, Tex., which surrounds Houston, has a higher population density than Westchester County, N.Y."

To people with even a passing familiarity with these two areas, the fact that Harris County is more dense than Westchester County is hardly surprising. What is surprising is that someone would say that Harris 'surrounds Houston.' For most purposes, Harris IS Greater Houston. The city does include two other counties but Harris contains all but a fraction of the metropolitan area's population.

In other words, the county that includes Houston is more dense than a suburb of New York.

But perhaps the most damning rebuttal comes from Glaeser's prime example:

"My interpretation of Red State growth is that Republican states have grown more quickly because building is easier in those states, primarily because of housing regulations. Republican states are less prone to restrict construction than places like California and Massachusetts, and as a result, high-quality housing is much cheaper.

"There is a strange irony in this: more conservative places do a much better job in providing affordable housing for ordinary Americans than progressive states that are believed to care about affordable housing.

"Progressive states, of course, have other objectives beyond affordable housing, and some involve blocking building. California environmentalists have been fighting construction for more than 40 years, and regulations in Massachusetts are barely less intrusive."

According to Glaeser's hypothesis, California should be scraping the bottom, particularly given that, as mentioned before, the metric being used tends to understate the growth of highly populous states. And keep in mind that he previously used Oklahoma as one of his examples of Red State growth. Oklahoma's growth rate was 8.7%. California's was 10%. That puts California slightly above the national average. (You'll notice that, unlike the other states discussed, California's actual growth rate does not appear in the article, nor is it labeled in any of the graphs.) California also beat a number of McCain states in the region of Oklahoma and, being a good ol' Arkansas boy myself, I can tell you that if a lack of building codes and environmental regulations were driving immigration, the whole area would be packed.

[To get a good picture of what's going on here, take a few minutes to look at this helpful interactive map. Pay close attention to the red states in the center of the country like Missouri.]

Population density alone does a good job explaining population shift patterns (considerably better than Glaeser's hypothesis). Add in the growth of the Hispanic population (something any competent demographic analysis should include) and it does an excellent job. And when you lay on top of that the expected impact of Katrina (how much of Texas' growth was diverted from the anemic Louisiana?) and the Nevada real estate bubble, the map we see looks almost exactly like the map we would expect.

If you don't want to use population density and the growth of the Hispanic population, how about the graying of America? Traditional retirement migration patterns look a lot like what we're seeing here and I'll bet we can come up with a few more explanations that outperform Glaeser's hypothesis.

George Polya once observed* that, when given a theory, scientists and mathematicians tended to look for cases that contradicted the theory while almost everyone else looked for cases that confirmed it (others had made similar points before he did, but I'm a Polya fan). Glaeser's approach here falls into the 'everybody else' category. He went looking for confirmation and he found it, but his theory maintains the appearance of validity only as long no one looks for contradictory evidence.

Perhaps Economix needs a new slogan.

*Quoting from memory so I may have to revise this later.

1 comment:

  1. This case is an excellent illustration of when different explanations would result in different policy responses. If growth in these states is a result of the regulatory environment than that would suggest relaxing regulations would promote growth. If, on the other hand, growth in the sunbelt is due to the "graying of America" or "patterns of hispanic immigration" then this suggests other policy responses to promote growth.

    The details matter in this case.