For 20 years, crime in the US has been falling and new figures from the FBI show a sharp drop in the last two years, despite the recession. Why?

Through Democratic and Republican administrations and through booms and busts, crime has been falling since 1991.

Image caption There is little evidence the recession has influenced crime rates

Murder and robbery rates nearly halved from 1991-98, a phenomenon that has saved thousands of lives and spared many more potential victims of crime.

The pace of the reduction slowed in the late 90s but new FBI figures show the sharp drop in crime that began around 2008 continued last year, despite high unemployment.

No-one agrees on the reasons for this. Here are 10 possible theories.

1. The Obama effect could explain the increased pace of the reduction of the last few years, says one of the country's top criminologists, Alfred Blumstein. "The prior expectation was that the recession would have the opposite effect. The question then is what distinctive event occurred in '09?" The election of a black president could have inspired some young black men, who are disproportionately involved in arrests for robbery and homicide, says the professor. It's very speculative, he adds, and probably only one factor of many, as one of the cities with a huge drop in crime is Phoenix, in Arizona, which does not have a large black population. "In the field of criminology, you don't get consistent indicators as you would in physics. There are so many factors that could have contributed." A separate study on school test scores supports the view that some black teenagers were motivated to try harder by the new presidency.

2. The fall in violent crime that began in the early 90s can be partly explained by the fall in demand for crack, says Prof Blumstein, co-author of The Crime Drop in America. Word got round about the dangers of crack use and - aided by aggressive policing - the gun violence associated with its supply decreased. The converse had happened in 1985, when the incarceration of dealers led to a spiral of violence, as younger and more reckless suppliers took their place.

Broken windows theory In 1982, social scientists argued that repairing vandalism in urban areas prevented further crime

The New York Subway adopted this premise in the late 80s

And then Mayor Rudy Giuliani (above) used it as part of his "zero tolerance" policy

Supporters of the theory point to falling crime rates in cities like NY as evidence

But sceptics say crime fell in many cities over that period and it's hard to prove a link

3. Smarter policing helped the border city of Laredo in Texas to reduce car theft by 40% last year. Police spokesman Joe Baeza says they introduced a scheme whereby motorists could register their car number plates into a police database and this empowered patrol cars to stop these cars if they were spotted late at night, to verify the owners. Mr Baeza adds that they also targeted car theft networks, educated the community about prevention and promoted anti-theft devices.

4. Number crunching has also helped in Laredo, where overall crime fell 16% last year, says Mr Baeza. "CompStat is a crime mapping project that pinpoints crime peaks in different parts of the city. The police chief then sends a team of officers to reinforce hotspots for burglaries or thefts or robberies, and they hold steady the flow of criminality." The CompStat methods began in New York City and featured heavily in gritty television drama The Wire, set in Baltimore.

5. There is a controversial theory put forward by economist Steven Levitt that the increased availability of legal abortion after the Supreme Court ruling in 1973 on Roe v Wade meant that fewer children were born to young, poor, single mothers. This, says the theory, stopped unwanted babies in the 1970s and 80s from becoming adolescent criminals in the decades that followed. But some of his peers have questioned whether the evidence really supports the theory.

6. A sociologist at Tufts University, John Conklin, says a significant factor behind the fall in crime in the 1990s was the fact that more criminals were behind bars and therefore unable to offend. In his book Why Crime Rates Fell, he says sentencing was lenient in the 60s and 70s, when crime rose, and then more prisons were built and more offenders were imprisoned. But others question why crime has continued to fall recently when budget constraints have kept the prison population relatively flat.

What's happening in the UK? There are several sources of crime figures and they all point to a decline in violence

Hospital admissions for violent injury have been falling for 10 years

The British Crime Survey indicates a 7% drop in 2010 Violence falling for a decade

7. An economist at Amherst College in Massachusetts, building on earlier research, has linked the fall in violent crime to a decline in children's exposure to lead in petrol. Jessica Wolpaw Reyes says: "Even low to moderate levels of exposure can lead to behavioural problems, reduced IQ, hyperactivity and juvenile delinquency. You can link the decline in lead between 1975 and 1985 to a decline in violent crime 20 years later." About 90% of American children in the 1970s had blood levels that would today cause concern, she says. Her research also found a link at state level between the timing of laws banning lead and subsequent crime statistics.

8. The baby boomers grew up. With birth rates peaking between 1957 and 1961, the proportion of men in the US in their late teens and early 20s was highest in the late 70s and early 80s. As time went on, the proportion of people at "criminal age" decreased.

9. A study released last month suggested video games were keeping young people off the streets and therefore away from crime. Researchers in Texas working with the Centre for European Economic Research said this "incapacitation effect" more than offset any direct impact the content of the games may have had in encouraging violent behaviour.

10. Some people have suggested to Professor Blumstein there is another technological deterrent and that is the proliferation of camera phones, which makes some criminals think twice before risking possible incrimination on film. The impact of other kinds of cameras is unclear. In the UK, the influence of CCTV on crime is disputed.