Academic Legal Writing - Eugene Volokh [93]
Relying on such assumptions doesn't make your argument fallacious. You might have evidence that shows your assumptions are plausible; and in any event, we often have to make decisions based on hypotheses that are less than mathematically proven.
But you should make your assumptions explicit, and defend them explicitly, so the reader is persuaded that they're justified. You should certainly never hide them by misstatement—or let such common misstatements in others' articles dupe you.
To see these principles more clearly, try the following exercise: Assume that a study showed that 15% of New York drivers aged 16 to 25 drive drunk at least once a month. The Minnesota legislature is considering new penalties for drunk driving by 16–to–18–year-olds, and a commentator who supports the law writes “Drunk driving has reached epidemic proportion among teenagers, with 15% of driving-age teenagers driving drunk at least once a month.” What errors or unstated assumptions can you find in this statement? See p. 365 for the answers.
I. Make Sure Your Comparisons Make Sense
People often use comparisons to draw inferences about causation (“women earn 72.3 percent of what men earn in median annual earnings,” so we should infer that this roughly measures the amount of sex discrimination by employers) or about costs and benefits (“a gun in the home is 43 times more likely to kill its owner or a friend than an intruder,” so we should infer that the costs of gun ownership for the owners exceed its benefits). Such comparisons are often valid steps in your argument, and can be quite effective rhetorically.
But many comparisons that sound good at first collapse on closer examination, either because they don't consider alternative explanations for the disparities, or because they don't sensibly quantify the costs and the benefits.
1. Consider alternative explanations for disparities
Consider one example, a concurrence in the Supreme Court case Ring v. Arizona, which argues that juries, not judges, should decide whether to impose the death penalty (an eminently plausible position, which ultimately got seven votes). The opinion summarizes arguments against the death penalty, and the second-to-last paragraph reads:
Many communities may have accepted some or all of these claims [that reflect badly on the death penalty], for they do not impose capital sentences. See A Broken System, App. B, Table 11A (more than two-thirds of American counties have never imposed the death penalty since [the death penalty was reaffirmed to be constitutional in 1976] (2,064 out of 3,066), and only 3% of the Nation's counties account for 50% of the Nation's death sentences (92 out of 3,066)). Leaving questions of arbitrariness aside, this diversity argues strongly for procedures that will help assure that, in a particular case, the community indeed believes application of the death penalty is appropriate, not “cruel,” “unusual,” or otherwise unwarranted.
At the heart of the argument is a comparison—“only 3% of the Nation's counties account for 50% of the Nation's death sentences”—from which the opinion infers that there is a diversity of views about the death penalty.
But one can't simply infer from (1) some counties' imposing more death penalties than other counties to (2) the conclusion that those counties' citizens have different views on the death penalty, because there's an obvious alternative explanation: (3) the counties that impose few death penalties might simply have few homicides.
To begin with, many counties have much smaller populations than others; there would be few homicides in those counties for that reason alone. Beyond that, many places have lower per-capita homicide rates than others, another reason that those places might rarely impose the death penalty.
In fact, in the 3% of American counties that impose the 50% of the death sentences, there were a total of 142,228 homicides in 1973–1995 (according to the table that the opinion itself cites), and according to the Uniform Crime Reports, there were