Monday, October 04, 2004

Does Capital Punishment Deter Homicide?

Ted Goertzel wants to believe that it doesn't. In "Capital Punishment and Homicide: Sociological Realities and Econometric Illusions," he asks and answers the question:
Does executing murderers cut the homicide rate or not? Comparative studies show there is no effect. Econometric models, in contrast, show a mixture of results. Why the difference? And which is the more reliable method?
His argument for comparative studies is simplistic, to say the least:
The first of the comparative studies of capital punishment was done by Thorsten Sellin in 1959. Sellin was a sociologist at the University of Pennsylvania and one of the pioneers of scientific criminology....

Sellin applied his combination of qualitative and quantitative methods in an exhaustive study of capital punishment in American states. He used every scrap of data that was available, together with his knowledge of the history, economy, and social structure of each state. He compared states to other states and examined changes in states over time. Every comparison he made led him to the “inevitable conclusion . . . that executions have no discernible effect on homicide rates”....

Sellin’s work has been replicated time and time again, as new data have become available, and all of the replications have confirmed his finding that capital punishment does not deter homicide (see Bailey and Peterson 1997, and Zimring and Hawkins 1986). These studies are an outstanding example of what statistician David Freedman (1991) calls “shoe leather” social research. The hard work is collecting the best available data, both quantitative and qualitative. Once the statistical data are collected, the analysis consists largely in displaying them in tables, graphs, and charts which are then interpreted in light of qualitative knowledge of the states in question. This research can be understood by people with only modest statistical background. This allows consumers of the research to make their own interpretations, drawing on their qualitative knowledge of the states in question.
This is laughable. You compile a bunch of data and display it in "tables, graphs, and charts which are then interpreted in light of qualitative knowledge of the states in question." In other words, you see what you want to see, and scores of researchers who want to believe that capital punishment doesn't deter homicide have simply squinted at their "tables, graphs, and charts" in just the right way, so that they could "prove" what they already believed. Qualitative, gut-feeling, ouija-board analysis -- in Goertzel's view -- is superior to rigorous statistical analysis, which he pooh-poohs. Worse than that, he seems to pooh-pooh it without understanding it:
Econometricians inhabit the mythical land of Ceteris Paribus, a place where everything is constant except the variables they choose to write about. Ceteris Paribus has much in common with the mythical world of Flatland in Edwin Abbot’s (1884) classic fairy tale. In Flatland everything moves along straight lines, flat plains, or rectangular boxes. In Flatland, statistical averages become mathematical laws. For example, it is true that, on the average, tall people weigh more than short people. But, in the real world, not every tall person weighs more than a shorter one. In Flatland knowing someone’s height would be enough to tell you their precise weight, because both vary only on a straight line. In Flatland, if you plotted height and weight on a graph with height on one axis and weight on the other, all the points would fall on a straight line.

Of course, econometricians know that they don’t live in Flatland. But the mathematics works much better when they pretend they do. So they adjust the data in one way or another to make it straighter (often by converting it to logarithms). Then they qualify their remarks, saying “capital punishment deters homicide, ceteris paribus.” But when the real-world data diverge greatly from the straight lines of Flatland, this can lead to bizarre results.
They don't adjust their data to make it "straighter," they introduce relevant controlling variables, which cannot done in any way other than through econometric (multiple regression) analysis. Try looking at "tables, graphs, and charts" of comparative homicide data while mentally accounting for such factors as income, age, race, gender, and population density, and see what you come up with. Nothing at all, unless you choose to ignore those and other relevant factors. Goertzel is either stupid or he simply chooses to misrepresent multiple regression analysis.

Goertzel does acknowledge and discuss some econometric analyses, for the purpose of contrasting their results:
...Mocan and Gittings...concluded that each execution decreases the number of homicides by five or six while Dezhbaksh, Rubin, and Shepherd...argued that each execution deters eighteen murders. Cloninger and Marchesini...published a study finding that the Texas moratorium from March 1996 to April 1997 increased homicide rates, even though no increase can be seen in the graph....The moratorium simply increased homicide in comparison to what their econometric model said it would have otherwise been....[Exactly. That's what econometric models do.]

Cloninger and Marchesini concede that “studies such as the present one that rely on inductive statistical analysis cannot prove a given hypothesis correct.” [That's simply a standard scientific disclaimer, which Goertzel wouldn't understand.] However, they argue that when a large number of such studies give the same result, this provides “robust evidence” which “causes any neutral observer pause."...[But Goertzel isn't a neutral observer, as you can tell.]

Econometricians often dismiss the kind of comparative research that Thorsten Sellin did as crude and unsophisticated when compared to their use of complex mathematical formulas. But mathematical complexity does not make for good social science. The goal of multiple regression is to convert messy sociological realities into math problems that can be resolved with the certainty of mathematical proof...[No, the goal is to take relevant factors into account.]
Enough of Goertzel. The econometric evidence is there, for those who are open to it: Capital punishment does deter homicide. See, for example, the careful analysis by Hashem Dezhbaksh, Paul Robin, and Joanna Shepherd, "Does capital punishment have a deterrent effect? New evidence from post-moratorium panel data," American Law and Economics Review 5(2): 344–376 (available in PDF format here). Dezhbaksh, Rubin, and Shepherd argue that each execution deters eighteen murders. That number may be high, but the analysis is rigorous and it accounts for relevant variables, such as income, age, race, gender, population density, and use of the death penalty where it is legal. It's hard to read that analysis and believe that capital punishment doesn't deter homicide -- unless you want to believe it. I certainly wouldn't take "Ouija Board" Goertzel's opinion over that of careful econometricians like Dezhbaksh, Rubin, and Shepherd.

Now, I must say that I don't care whether or not capital punishment deters homicide. Capital punishment is the capstone of a system of justice that used to work quite well in this country because it was certain and harsh. There must be a hierarchy of certain penalties for crime, and that hierarchy must culminate in the ultimate penalty if criminals and potential criminals are to believe that crime will be punished. When punishment is made less severe and less certain -- as it was for a long time after World War II -- crime flourishes and law-abiding citizens become less secure in their lives and property.

Related posts:
Libertarian Twaddle about the Death Penalty
Crime and Punishment
Abortion and Crime
Saving the Innocent?
Saving the Innocent?: Part II
More on Abortion and Crime
More Punishment Means Less Crime
More About Crime and Punishment
More Punishment Means Less Crime: A Footnote
Clear Thinking about the Death Penalty
Let the Punishment Fit the Crime
Another Argument for the Death Penalty
Less Punishment Means More Crime
Crime, Explained