Legal minds like numbers. They prefer the ?bright line? to the blurry curve. This causes problems in cases where things are hard to define precisely.

Like ?mental retardation.? Last year the U.S. Supreme Court ruled that states could not execute people who were mentally retarded. The Supremes left it up to states to define mental retardation for themselves, but noted that many mental health experts consider anyone with an IQ of less than 75 to be retarded. Never mind that the IQ test is notoriously subjective in itself, and the same person can achieve wildly different scores on multiple tests.

The state of North Carolina decided to use the number 70 as its magic divider between the killable and the retards. If your IQ is 69, you get to live. If it?s 71, you don?t. The state also chooses to look at factors like ability to function in society. But as some recent questionable cases prove, it?s hard to agree on who is fair game for the state?s axe. Take Travis Walters, who scored as low as 64 in one IQ test and qualified for Social Security payments from the federal government on the grounds that he was mentally disabled. But Walters also held down a job at one point and had a girlfriend. Apparently the ability to date proves that you?re sharp mentally.

The country prosecutor argues that 70 is too high a cutoff number, and Walters did score a 72 on one of his IQ tests. But nobody?s questioning the wisdom of using arbitrary numbers to judge ripeness for slaughter in the first place.