LM Otero, Associated Press
A prison guard places an inmate in solitary confinement at the Beto Unit prison in Tennessee Colony, Texas, in this 2001 file photo. Crime rates have been decreasing over the past 25 years, but there is no consensus on why.

Extensive media coverage of the George Zimmerman trial, the rash of homicides on Chicago’s South side and the mass shooting at Sandy Hook Elementary school in Connecticut could leave the public with the perception that America is becoming an increasingly dangerous place.

Though this may seem like a reasonable conclusion, it's not how the facts shake out.

Crime rates have been going down for the last twenty years, according to figures released earlier this summer from the Bureau of Justice Statistics. Gun-related murders, for example, declined 39 percent between 1993 and 2011, while nonfatal gun crimes dropped 69 percent during the same period. Crimes committed by strangers have dropped 77 percent since 1993, while crimes committed by offenders known to the victim decreased 41 percent.

While the numbers make it clear that crime is going down, there is no consensus as to why. Any number of things could be going on, said Alfred Blumstein, professor of public policy at Carnegie Mellon University in Pittsburg. Some theories include higher rates of incarceration, reductions in exposure to lead, legalization of abortion and the stabilization of the crack cocaine market. Blumstein argues that these explanations apply only to reductions in crime between 1993 and 2000.

Higher rates of incarceration

The United States has one of the highest incarceration rates in the world. Nearly seven million adults, or 3 percent of the population, were under correctional supervision (jail, prison, probation or parole) in 2011, according to the Bureau of Justice Statistics.

Part of the reason America has such a high rate of people behind bars, experts say, are the nation's harsh sentencing policies. These include mandatory minimum penalties, three strikes rules and habitual offender rules, according to Nicole Porter, of the the Washington D.C.-based Sentencing Project, a nonprofit organization which advocates for reforms of sentencing policy.

While some, including experts like Porter, suggest that high incarceration rates represent both a miscarriage of justice and a significant financial burden on taxpayers, others argue these policies have made America a safer place. For example, a study from The Heritage Foundation, a conservative think tank based in Washington D.C, concluded that putting so many people behind bars has removed a criminal cohort from the streets and reduced crime.

Critics of this explanation argue that there isn’t a perfect relationship between crime and punishment. “Incarceration was growing in the late 1980’s which is when crime was also increasing,” Blumstein said.

America unleaded:

One of the newest explanations for the precipitous drop in crime over the last twenty years was outlined in a recent article for Mother Jones, a left-leaning national news magazine. In the piece the author Kevin Drum documents the well-established relationship between lead exposure and brain development, which is that lead exposure in small children is associated with many complications later in life, including lower IQ, hyperactivity, behavioral problems, and learning disabilities, and likelihood to take part in criminal activity.

Research done by Jessica Reyes, a professor of public health policy at Amherst College, shows a convincing relationship between lead exposure and crime. Reyes discovered demand for leaded gasoline declined unevenly in states around the country beginning in the 1970's, with some states banning leaded gasoline early while others continued selling the product well into the 1980’s. She found that in states where consumption of leaded gasoline declined slowly, crime declined slowly. Conversely, in states where it declined quickly, crime declined quickly.

Blumstein acknowledges the creativity of this theory, but said that just because lead emissions and crime rates have the same downward trend doesn’t mean that one caused the other; correlation is not causation. “It's hard to say whether it was lead exposure causing the higher rates of crime or if it was poverty or drugs,” he said.

Roe v. Wade

University of Chicago economics professor Steven Levitt and New York Times journalist Stephen Dubner popularized the idea that crime reductions in the 1990’s were a result of increased access to abortion after the 1973 Supreme Court decision in Roe v. Wade.

Their theory is based on two assumptions: first, legalized abortion leads to fewer "unwanted" children and second, unwanted babies are more likely to suffer abuse and neglect, increasing their risk for criminal behavior later in life. “The first assumption, that abortion reduces the number of unwanted children, is true virtually by definition. The second assumption, that unwanted children are at increased risk for criminal involvement, is supported by three decades of academic research. If one accepts these two assumptions, then a direct mechanism by which the legalization of abortion can reduce crime has been established,” wrote Levitt.

Using statistical data, Levitt and Dubner estimate that as much as one-half of the decline in crime in the 1990s is due to legalization of abortion.

Other researchers are skeptical of Levitt and Dubner’s conclusion. “It’s a very clever way of explaining the trend, but ultimately it doesn’t have a lot of explanatory power,” Blumstein said. He believes that if this explanation were true, the effects would show up in other places, for example decreased school drop out rates, teen pregnancy, and even academic performance. But there is absolutely no relationship, he said.

Another factor for which Levitt and Dubner didn’t account, according to Blumstein, is the fact that although abortion wasn’t legal prior to 1973, people still got them. “It could be the case that people who were going to get abortions would get them whether it was legal or not,” he said, a consideration which significantly diminishes the explanatory value of Levitt and Dubner's hypothesis

Crack goes flat

Blumstein’s preferred explanation for the 1993 to 2000 drop in crime is related to the decline of demand for crack cocaine. Between 1986 and 1993 demand for crack — a more potent mixture than regular cocaine — exploded, particularly among African-Americans. Crack was sold on the street so if drug deals went bad the violence would be visible, he said. As a consequence, law enforcement cracked down on this market, instituting harsh penalties for those found in possession of the substance. Five grams of crack would land a person in jail for five years. By contrast a person would have to be carrying 500 grams of regular cocaine to receive five years in jail. Harsh penalties may have deterred people from getting involved with the drug.

As the epidemic went on it became clear what crack did to users: short highs and crushing lows, hallucinations, irritability and paranoia. “Demand for the drug went down because potential users, the kids and grandkids of people on crack, saw what the drug did to people and stayed away,” Blumstein said.

The Obama effect

Criminologists expected the crime rate to go up significantly in 2009 because of the great recession, and as Blumstein points out, “we tend to expect crime to rise in a bad economy.” But that isn’t what happened. Crime actually went down nearly ten percent that year. By 2010 things had stabilized with crime falling again between one to two percentage points a year.

None of the explanations for the reductions between 1993 and 2000 make sense for 2009, according to Blumstein. “Those phenomena were all gradual so we’d expect to see gradual reductions like we did between 1993 and 2000,” he said. Crime just plummeted in 2009. Whatever caused it is likely a discrete event, not a policy change, he said.

Blumstein’s theory is that the election Barack Obama, the first black president, broadened young people’s vision of what is possible for their lives. This was especially true among young black men, whose crime rates during that year fell at almost twice the rate of their peers. Blumstein admits that more work needs to be done to evaluate this hypothesis. “It may just be a marginal effect and we need to do more research to see if [the Obama effect] shows up in other places,” he said, “But it is certainly plausible.”

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Several other prominent sociologists share with Blumstein the view that the 2009 dip in crime was a result of the election of President Obama. For example, Ohio State University’s Randolph Roth argues that Obama's election reengaged black Americans in the political process. “The inauguration of the first black president...re-legitimized the government in the eyes of many African-Americans." he wrote, "It is likely that their greater trust in the political process and their positive feelings about the new president led to lower rates of urban violence.”

Yale sociologist Elijah Anderson believes Obama's election had a more psychological impact on Black Americans. “Now we have a sense of future,” he said in a 2011 interview with Slate. “All of a sudden you have a stake. That stake is extremely important. If you have a stake, now there’s risk — you realize the consequences of compromising an unknowable future.”