Bigamy is the act or condition of a person marrying another person while still being lawfully married to a second person. Bigamy is listed (and sometimes prosecuted) as a crime in most western countries. For example, in the United Kingdom, by law, a married person is not allowed to marry again as long as their first marriage continues.

Often the term bigamy is used where two or more spouses are unaware of each other, in contrast to polygamy where normally all spouses know about one another.

In the United States, the Model Penal Code (section 230.1) defines bigamy as a misdemeanor and polygamy as a felony. Having more than one spouse at the same time gets classified as polygamy, and bumped to a felony, if it is done "in purported exercise of a plural marriage..." According to Joel Feinberg in Moral Limits of the Criminal Law: "Righteously, flaunting one's illicit relationships, according to the Code, is apparently a morally aggravating circumstance, more punishable than its clandestine and deceptive counterpart."

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