Biological factors are often the starting point for understanding criminal behavior; the idea is that some people are psychologically predisposed to committing crime. Even if that were true, it does not explain why the crime rate varies so markedly in different cities, counties and states. No one knows for sure what causes crime. However, researchers have identified a number of factors that influence crime rates and typically are present in jurisdictions where crime rates are high.
TL;DR (Too Long; Didn't Read)
Factors typically present in jurisdictions where crime rates are high include poverty levels, job availability, police policy and the average age of the population.
Poverty Level and Job Availability
Statistically, poverty goes hand-in-hand with crime. Where poverty is prevalent in a community, that community will experience higher levels of crime. Generally, it's not the poverty itself that leads to higher crime rates but the factors associated with poverty, such as chronic joblessness, less access to quality schools, employment, role models and the real or perceived lack of opportunity. Crime offers a way for less-advantaged people to access goods they may not otherwise afford. Often, the prize outweighs the risk of being caught, since impoverished people may believe they have less to lose than does a wealthy person.
Social Level of Morality
Different homes and communities have different degrees of morality. In some families and communities, deviant behavior is tolerated and encouraged. In others, even minor transgressions are reported and corrected. People's upbringing and social environment can shape their view of the world and directly affect their decisions in the future. For example, research shows that people who have been physically, sexually or emotionally abused as children are three times more likely than non-abused adults to commit acts of violence. In communities where crime is tolerated, a person may commit a crime simply to fit in with his peers.
A well-resourced police force coupled with tough sentences for perpetrators may help to reduce the crime rate. The U.S. crime rate was much higher in the 1960s and 1970s, before lawmakers responded by enacting tough-on-crime policies and building prisons. Mass incarceration removes people from the streets who would otherwise be committing crimes. Visible policing may also have a deterrent effect. When New York Mayor Rudy Giuliani implemented a policy of "broken-windows policing" – cracking down on minor crimes to make neighborhoods feel safer – crime declined at a precipitous rate. Some argue against credit given to Giuliani's policies, suggesting that a drop in unemployment rates may have been the reason for the crime reduction.
Age of the Population
There's a correlation between the crime rate and the age of the population. Specifically, most crimes are committed by people in their teens, 20s and 30s, especially in areas where the population is both young and transient. Violence in particular is a young man's crime; it has been said that the most effective law enforcement tool is a 30th birthday. Some commentators suggest that the country's aging population is the primary reason why the U.S. crime rate has fallen in recent years.