In the United States, for example, violent crime rates have fallen by over 50% in many major U.S. cities since these rates peaked in the early 1990s; in New York City, these rates had dropped by 75% from the early 1990s to 2010. In the United States, a second decline in the crime rate was also observed, with homicide rates declining first from 1994 to 2002, and then again from 2007 to 2011. The crime rate in Los Angeles decreased from 1993 onward, including e.g. a decrease in the crime rate of 10% during the first six months of 1998. 
On average, international crime declines from 1995 to 2004 were as follows: 77.1 percent in theft from cars, 60.3 percent in theft from person, 26.0 percent in burglary, 20.6 percent in assault and 16.8 percent in car theft. The crime drop since the early 1990s has occurred in many countries, including the United States, the United Kingdom, and New Zealand. However, no overall crime decline occurred in Western Europe during this period.
Many hypotheses have been proposed as to why crime has fallen, especially in the United States. Blumstein & Wallman (2006) conclude that a complex interaction between "prisons, drugs, guns, policing, economics," and "demography, including abortion" is the best explanation for the crime drop in the United States.
The lead-crime hypothesis proposed a link between elevated blood lead levels in children and later increases in crime. Children exposed to forms of lead at young ages are hypothesized to be more likely to develop learning disabilities, attention deficit hyperactivity disorder, and problems with impulse control. These problems are suggested to lead to the commission of more crimes as these children reach adulthood, especially violent crimes. 
Drug use and demand
Alfred Blumstein argues that part of the drop in the United States' violent crime rate is due to declining demand for crack cocaine. A 2014 report by the Home Office stated that changes in demand for illegal drugs (specifically, heroin) were a major contributor to the crime drop in the United Kingdom.
The mainstream view among criminologists is that unemployment and poverty are strongly related to crime, because a decrease in opportunities for legal employment, in theory, should increase the frequency of illegal employment. Multiple studies of the United States, for example, have found that the improvement of the American economy coincided with a drop in crime throughout the 1990s. A 2015 Brennan Center for Justice report, however, estimated that no more than 5 percent of the 1990s crime drop in the United States was attributable to changes in unemployment. The view that higher unemployment rates cause higher crime rates has also been challenged by the fact that the United States crime rate reached a 40-year low in 2010, despite America's lagging economy.
Studies of the United States have shown that increases in the concentration of immigrants are associated with decreases in violent crime rates, especially homicide and robbery. This relationship suggests that increasing immigration to the United States may be responsible for part of the recent drop in violent crime rates in the United States.
A 2015 Brennan Center for Justice report found that increased incarceration was responsible for about 5% of the crime drop in the United States during the 1990s, and for essentially none of the crime drop there since 2000. Commentators and academics who question the role of incarceration in the crime drop have noted that Canada's crime rates followed similar trends as those in the United States during the 1990s; in contrast, Canada's incarceration rate did not change significantly during this time, while that of the United States increased significantly. In 2009, Steven Messner and Richard Rosenfeld found that incarceration was negatively related to burglary rates "...only after unusual policy interventions, such as Italy's 2006 clemency measure that dramatically reduced the size of its prison population."
Some have proposed that changes in policing practices (e.g. the adoption of broken windows policing) were responsible for the crime drop in the United States, especially in New York City. However, Canada did not change its policing practices significantly prior to their crime drop, which casts doubt on the extent to which policing was responsible for this phenomenon. Some of the most popular claims about policing reducing violent crime are not supported by the evidence.
Levitt (2004) estimates that increases in the number of police accounted for between 5 and 6% of the crime drop in the United States during the 1990s. A 2007 study found that misdemeanor arrests were negatively associated with changes in total homicide rates in New York City.
A 2014 article in Crime and Justice reported that the "security hypothesis" was the best explanation for the drop out of the 17 hypotheses tested. This hypothesis proposes that improved and more widespread security devices, like electronic immobilizers and central locking, were responsible for a large part of the crime drop by preventing numerous crimes. Consistent with this hypothesis, attempted crime has also been declining, suggesting that would-be criminals are becoming discouraged by improved security.
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