There have been many theories as to why people become criminals. Criminologists—persons who study crime—believe that there is not any single cause, but many. Among these are poverty; poor and overcrowded housing; families broken by separation and divorce; parents who neglect their children; mental illness or mental deficiency; alcoholism or drug addiction; and racial or religious prejudice. Juvenile deliquency is often the first step toward a life of crime.

Crime has often been considered, especially in ages of deep religious feeling, to be due to an evil element in human nature. Modern Criminologists acknowledge the existence of moral weakness, but do not consider it to be a basic cause of crime. They do feel, however, that a morally weak person may be easily tempted by circumstances to commit a crime.

An Italian criminologist, Cesare Lombroso (1836–1909), asserted that there is a criminal physical type whose traits are passed on by inheritance. In Criminal Man (1876), he listed these traits as including a heavy jaw, sloping forehead, scanty beard, and, very often, lefthandedness. Modern Criminologists and biologists reject these theories. Another pioneer Italian criminologist, Enrico Ferri (1856–1929), asserted that crime is due to many causes. He stressed the effect of environment (a person's surroundings). Most later Criminologists tend to agree with this. The extent to which biological or environmental factors are responsible remains debatable.