Most likely the baby boom that started after World War II was the main reason why crime increased in the U.S. in the middle of the 20th century. More than anything else, the percentage of the population that is in their late teens and twenties appears to be most significant factor in the crime rate. This is because most street crimes are committed by this age group.
In the 1990s, mayors around the U.S. bragged about cutting down crime rates. Some of the crime rate reduction may have had to do with the better economy of that time and better policing policies, but really the baby bust that was coming of age in the 1990s is the most significant factor in the decline in the crime rate. There was a smaller pool of criminals out there commiting crimes. Some even point to the Rowe V. Wade court decision in the early 1970s that reduced the number of unwanted babies that grew up to be criminals in the 1990s, but I that is too controversial to discuss in detail.