Lempert on common sense and Lott

Richard Lempert comments on why he found Lott’s results implausible when they first came out:

To give another example, long before other research called their results into question, it was common sense that made me suspicious of John Lott and David Mustard’s claim in the Journal of Legal Studies that right to carry laws diminish violent crime. What made me skeptical was their finding that while right to carry laws diminish violent crimes like murder, rape, and aggravated assault they led to increases in non-violent property crimes. The authors had an explanation for this; namely, that the types of crimes are substitutes for one another, and as violent crimes are deterred for fear of meeting someone with a gun, crimes that involve non-confrontational thefts will be substituted for them. But a theory can be offered to fit any data, and when the theory is constructed post hoc rather than offered a priori, one must be especially cautious in accepting it. The theory Lott and Mustard offered was borrowed from economics where it often makes considerable sense of such behavior as purchasing decisions, career choices and the like, It may even sensibly explain choices criminals make between some crimes, for example the choice of whether to rob someone or burgle an apartment. But the idea that taking property by stealth might substitute for crimes like rape or murder ignores what we know about differences between these crimes, the motives for them and those who commit them. More bluntly, applying the substitution hypothesis to the crimes Lott and Mustard saw as substitutes defies common sense. Similarly some of the results in Lott’s later book, such as the suggestion in the data that reducing the number of black women over 40 would diminish certain crimes substantially are weird to the point of being incredible.

Of course Lott came up with a theory to explain this as well — those women are more likely to be crime victims. But his model found that an increase of 1 percentage point in the percentage of the population that was black, female, and 40-49 was associated with a 30% decrease in rape, and a 24% increase in homicide in the average county. Black females 40-49 are not 24 times as likely to get murdered as the average person. And the association with a 30% decrease in rape makes even less sense. Even if no women in this group were ever rape victims, this would only account for an association with a 1% decrease.

In other Lott news, he now a visiting professor at Binghamton.


  1. #1 ben
    August 10, 2006

    Speaking of the capital punishment part of the work, wasn’t there a new study that came out this year that showed a deterrent effect? Any comment on that?

  2. #2 Tim Lambert
    August 10, 2006

    I had a [post on the death penalty in March](http://scienceblogs.com/deltoid/2006/03/death_penalty_1.php). It’s possible that there is a deterrent effect, but if there is, the US isn’t executing enough people for it to be detectable.

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