I wouldn’t date the start of the first 20th century crime boom at 1890. The 1890s were a low-crime period in the big cities of the North. In the South, however, black violent crime rose and rural whites panicked, leading to the lynching and convict lease policies of that era. Northern cities started to suffer more violent crime in the first decade of the 20th century, partly because of the southern Italian migration to the U.S. The typical Italian immigrant crimes were murder, assault and threats of same by the so-called Black Hand, a proto-Mafia which mainly terrified the immigrants themselves. Then, following World War I, a Mexican migration to the U.S. added to the crime totals, as did a major spike in black migration out of the South. The war sparked a black movement to big cities for economic betterment, but, unfortunately, also brought with it high crime rates within the black community. In addition, Prohibition, which began in 1920, produced violence among the alcohol distribution gangs competing for turf (though this violence did not target ordinary citizens).
Violent crime peaked in the early 1930s, with a wave of bank robberies by “Pretty Boy” Floyd, “Baby Face” Nelson, John Dillinger, and Bonnie and Clyde Barrow. This was accompanied by the sensational kidnap-murder of the Lindbergh baby in 1932 and a spate of copycat kidnappings. J. Edgar Hoover made his name by directing the Federal Bureau of Investigation to hunt down and capture or kill these “Public Enemies,” as he labeled them, and by 1934 the FBI or local agents had successfully done so with each of them.
Crime rates started to decline in the mid-1930s, at the same time that the New Deal went into effect. This may seem like cause-and-effect: unemployment and poverty were reduced, so violent crime diminished. But this is not necessarily correct. First, Prohibition ended in 1933, and that helped reduce murder rates. Second, the spate of bank robberies and kidnappings declined, partly because law enforcement apprehended high-profile perpetrators. Third, migration by blacks and Mexicans and immigration by Italians declined dramatically when jobs became unavailable due to the Depression. Finally, there was a severe downturn in the economy in 1937 and 1938, yet violent crime continued to fall. The American public was terribly damaged by the Great Depression—68 percent of Americans were below the poverty line in 1939—but this produced no increase in violent crime.
During World War II, crime continued to drop, partly because the war removed hundreds of thousands of young men from the streets to the barracks. When the war ended there was a brief spike in violent crime, but the downturn continued after the war and well into the postwar boom of the 1950s. No one is sure why crime remained low in the 1950s, but several factors helped. Crime rates for African Americans, though higher than average, were historically low for that community. Drug and alcohol use also were down. The Depression had produced a birth dearth, so the young male population was reduced. And the supercharged economy created a massive and growing middle class in a short period of time; and middle-class people seldom commit crimes of violence. All in all, the 1950s was a golden age of low crime.