A crime is any behavior that is punishable by imprisonment or fine (or both). In the United States, an act is considered criminal when Congress or a state or local legislative body has defined it as such. But why are some acts defined as crimes while others aren't? While whole books have been written on this subject, here a few straightforward reasons why crimes are crimes:

• Many acts that we consider crimes today were considered crimes under English law when the United States became a country. In large part the U.S. adopted English law as its own.

• Many crimes have their origin in moral precepts that originally were enforced by churches and were eventually taken over by the secular state.

• Acts carried out with an antisocial or "evil" intent are usually considered worthy of punishment.

• Acts that may have been acceptable at one time (such as physical punishment of a child, drinking while driving or sexual harassment) are redefined as crimes when society convinces lawmakers to criminalize them.

Ultimately, what is and is not a crime is, to an extent, arbitrary and a reflection of who has the power to decide. But with some notable exceptions -- for example, drug laws -- most common crimes have been considered crimes for centuries and most people agree that they should be.

On the other hand, in recent years, the U.S. Supreme Court has struck down certain federal crimes, finding that Congress had no authority under the Constitution to create them. Examples of federal crimes that have been disallowed are statutes forbidding the sale of firearms within a certain distance of schools and allowing rape prosecutions in federal court. Under these decisions, the question of which crimes may be created by Congress, and which crimes must be left to the states, remains an open one.