Adultery, an ancient crime that remains on many books

When David H. Petraeus resigned as director of the C.I.A. because of adultery he was widely understood to be acknowledging a misdeed, not a crime. Yet in his state of residence, Virginia, as in 22 others, adultery remains a criminal act, a vestige of the way American law has anchored legitimate sexual activity within marriage.

In most of those states, including New York, adultery is a misdemeanor. But in others — Idaho, Massachusetts, Michigan, Oklahoma and Wisconsin — it is a felony, though rarely prosecuted. In the armed forces, it can be punished severely although usually in combination with a greater wrongdoing.

This is yet another example of American exceptionalism: in nearly the entire rest of the industrialized world, adultery is not covered by the criminal code.

Like other state laws related to sex — sodomy, fornication, rape — adultery laws extend back to the Old Testament, onetime capital offenses stemming at least partly from a concern about male property. Peter Nicolas of the University of Washington Law School says the term stemmed from the notion of “adulterating” or polluting the bloodline of a family when a married woman had sex with someone other than her husband and ran the risk of having another man’s child....