On Nationwide Falling Crime Rates: A Surprise Hypothesis (Hint: Immigration)

by Dr. Reynaldo Guerra

First, let me say that this blog entry was motivated by an unfortunately too short conversation with Bill King and Greg Wythe on the set of Kim Davis’ Beyond the Headlines this past week.  This is my effort to keep the conversation going.

It has been in the news recently that despite widespread economic hardship, the nation’s crime rates have continued to fall….but nobody can figure out why.

Let’s frame the discussion.

It’s well known that the nation’s crime rate peaked around 1991 (Wiki “Crime in the United States” for a quick primer).  The crime rate has decreased steadily ever since. Now, what’s been drawing the notice of the press recently is that the pace of the decrease seems to be increasing; According to a recent FBI report violent crimes fell 0.7% in 2007, 1.9% in 2008, an impressive 5.3% in 2009, and 5.5% in 2010.

The nation is still in the throes of a dismal economy.  Conventional wisdom has held that our nation’s economy and crime rates are inversely proportional, i.e. if the economy goes down, crime rates go up, and vice versa.  At first glance, this line of reasoning seems rational.  If there are no jobs around, people may be more inclined to steal in the name of survival, or worse, be in a bad enough mood to commit a violent crime. Well, empirical data has disproven this line of reasoning over and over again throughout the years.  The very low crime rates during the Great Depression is a popular example.

So, why have crime rates fallen?  A couple of plausible, and very provocative, reasons have been formulated…and rejected.  The following outlines some of the rejected reasons and the rationale for their rejection.  I propose a hypothesis of my own toward the end of this article.  If you can’t contain your anticipation, feel free to skip to the A Surprise Hypothesis section of this article.

Law Enforcement Reform

As rising crime rates became a national epidemic in the early 90’s, reforms of local law enforcement strategies began to take place all around the country, beginning in New York City.  The Broken Windows Theory and various other reforms on local law enforcement strategies subsequently received deserved credit for the falling rates.  However, major crime rate drops that we are seeing today are occurring in cities that have not enacted major law enforcement reforms, suggesting other contributing factors.

Large Prison Populations

With an incarceration rate four times the world average, a record 2.3 million Americans were behind bars in 2009 (2009, US Bureau of Justice Statistics).  With respect to explaining falling crime rates, the idea behind incarcerating people is that most robberies and violent crimes are committed by a small number of criminals either repeating crimes or likely to repeat crimes; by incarcerating these types of people, crime rates should fall, or so the theory goes.  Again, it’s not that simple.  States with the largest rates of prison population growth in the 1990’s actually experienced lower crime rate drops than the rest of the country.

Now that we’ve ruled out the silly notion of law enforcement having anything to do with the recent crime-rate drops, some of the more viable explanations have proven to be the most scientifically investigated, and the most provocative.

Lead Poisoning

The government banned lead-based paint from housing in 1978 (Note: If your house was built pre-1978, you may want to look to the EPA for more information on the negative effects of lead, especially if you are considering remodeling: http://www.epa.gov/lead/).  Scientists have studied the psychological effects of lead on human behavior for some time now and the link between lead and aggressive/impulsive behavior is well established.  Two of the 20th century’s worst crime eras corresponded to peaks in children’s exposure to lead 20 years prior (first due to lead-based paint and then due to leaded gasoline).  The phasing out of lead therefore seemingly would justify a drastic reduction in crime.


Steven Levitt famously makes the case, in his best-selling book, Freakanomics, for the legalization of abortion in 1973.  Levitt argues that Roe v Wade reduced the number of unwanted babies born into troubled homes and, subsequently, abortion deserves the lion’s share of the credit for the drastic crime-rate reduction 20 years later in the early 90’s.

While compelling, the effects of these policy changes in abortion and lead are inherently transient and don’t explain the sustained reduction in crime that we’ve been experiencing the last couple of years.

A Surprise Hypothesis

It is here where I propose we examine the effects of immigration.

Several scientific studies have monitored and analyzed crime rates among immigrant and non-immigrant populations in the United States.  The statistics have overwhelmingly shown, both recently and for the last 100 years, that crime rates among immigrants are drastically lower than non-immigrants.

Crunching some Bureau of Justice 2008 incarceration numbers, the rate of incarceration for non-native born persons 18-39 years of age (the typical crime-committing age range) is somewhere around 0.5%.  In contrast, the rate of incarceration of native-born persons in this age range is an order of magnitude higher at approximately 5%.

The undocumented immigrant population in the United States is currently held to be somewhere around 12 million, double the undocumented population in 1994.  Since 1994, again, the same time period in which the undocumented immigrant population has doubled, the violent crime rate in the US has declined 40%, with the whole country scratching their heads trying to figure out why.  This decline occurred during a time period when the purported effects of Roe v Wade and changing lead policies should have been ebbing.

It turns out that cities with higher immigrant populations are also some of the safest cities in the country.  The most notable of these cities is El Paso, TX.  Despite a poverty rate approaching 30% (twice the national average) and it’s close proximity to the city of Juarez (with more than 4,000 murders since 2008), El Paso regularly ranks as the safest city in the country (Congressional Quarterly, 2011).  El Paso tallied 5 murders in 2010.  Cities of similar size tallied up to two orders of magnitude higher: Milwaukee (72 total murders in 2009), Memphis (132 total murders in 2009).  New York, the largest city in the country and a historically large immigrant city, was the 3rd safest city in the country in 2010, followed by San Jose and San Diego.

The low crime rate data is jaw-dropping and all the more revealing considering that immigrants are much more likely to settle in poorer, disorganized communities, with a historical propensity for crime…and their presence drastically reduces the crime rate in these communities.  This seems reasonable.  Immigrants leave their home country and family in search of jobs and a better livelihood for themselves and for their families back home.  They have much to lose by committing a crime.

With recent immigration numbers dropping, I wouldn’t be surprised to see crime rates shoot back up in the next couple of years.

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