In the United States, laws punishing adultery vary from state to state. Although adultery is rarely prosecuted, laws against it remain on the statute books in 23 states. Penalties vary from life (yes, life in prison!) in Michigan to a $10 fine in Maryland to a Class 1 felony in Wisconsin. In 2007, Michigan's second-highest court ruled that anyone involved in an extramarital fling can be prosecuted for first-degree criminal sexual conduct, a felony punishable by up to life in prison. The ruling proved particularly embarrassing for Attorney General Mike Cox, who subsequently confessed to “an adulterous relationship.”

Most societies prohibit adultery--sex between a married person and someone other than his or her spouse--at least, formally. A handful--the most researched is a tribe of Alaskan Inuit--have condoned affairs. Tolerance of adultery varies. In France, Prime Minister Franois Mitterrand's mistress stood next to his wife at his funeral. In some Muslim societies, adulterers are still stoned to death. The United States falls somewhere between these extremes.

Alfred Kinsey's famous 1948 survey of American sexual behavior found that seven out of 10 men had cheated on their wives. More recent research shows that the prevalence of adultery is not as high. A survey taken two years ago by the University of Chicago's National Opinion Research Center shows that slightly more than 11 percent of women and 21 percent of men admitted to having an adulterous affair during their lives.

All but seven states punish both people involved. Colorado, Georgia, Nebraska, North Dakota, and Utah only punish the married person. In the District of Columbia and in Michigan, when a married man sleeps with an unmarried woman, only the man is guilty, but when a married woman sleeps with an unmarried man, they're both guilty. Most laws make no exceptions for couples that are separated or in the process of obtaining a divorce.

In the Catholic Church, adultery is grounds for annulment, on the theory that if a person knew that his or her spouse had a predilection toward infidelity, the marriage would not have occurred. Until recently the church was stingy with annulments, but last year it granted more than 60,000. 9 out of 10 of those were in the United States.

The Old Dominion state brought a former town attorney before the bar of justice in 2004 on a charge of adultery. John R. Bushey Jr. was finally brought to justice in a small courthouse in Luray, Va. For 32 years, Bushey, 66, served as the attorney for the small Shenandoah Valley town of 4,500 people, and he had been married for about 18 years to Cindy Bushey, the town's clerk. However, he had an affair with Nellie Mae Hensley, 53, and after the affair ended, Hensley proved the adage "scratch a lover, find a foe," and she went to the police. While Hensley was divorced, Bushey was married and therefore subject to a criminal adultery charge, a misdemeanor. Bushey accepted 20 hours of community service as punishment.

Most societies condemn adultery, and there are three theories to explain why: One, evolutionary. Men must determine which children they sire, something only strict monogamy can ensure. Two, economic. Prohibiting adultery preserves monogamous relationships and thence families, whose labor was needed for agriculture. Three, religious. The Jews enshrined the adultery prohibition in the Ten Commandments--of which it is the seventh--to make their group more cohesive and to distinguish themselves from surrounding polygamous tribes.

Judges focus more on the economic impact of adultery. In most states, marital misconduct or fault, such as adultery, is not considered. For example, committing adultery typically will not have a bearing on whether a parent can get custody of his or her child because it takes two people to do it. No one ever committed adultery alone.

Judges now pay more attention to the economic impact of marital misconduct, such as the dissipation of assets or hiding assets. Judges increasingly disregard the marital misconduct by either spouse, focusing instead on a dispassionate analysis of what each party needs and what each party can afford. Even in those states where adultery and other marital misconduct can have an impact on the financial settlement, the impact is often less than people think. Rock solid evidence of an affair would shift the financial settlement by 10-15%. Rarely more.

A couple trying to repair a marriage after an episode of adultery should be tested for sexually transmitted diseases - STDs. STDs can be transmitted even with condom use. Adultery exposes both people to life-threatening illnesses such as AIDS and Hepatitis C. No matter what assurances a spouse offers, testing is necessary.