Possible Explanation for the Decline in Murders

Since reaching a peak in 1980, the murder rate in the United States has declined by about half. In some areas, such as New York City, it has decreased even more. Why did it happen? Here are five very plausible explanations.

1. Ban of Leaded Gas

Leaded gas was introduced in the 1920s, curtailed in the 1970s, and finally banned in 1996. We know that lead exposure causes lowered IQs. We also known that people with lower IQs are more likely to commit violent crimes.

Interestingly, U.S. leaded gas production per capita has almost exactly mirrored violent crime levels:

2. Aging of the Population

Since 1980, the median age of the United States has increased from 30 to 38 years old. But most people who commit murder are quite young, with the average age being around 23. If we had the same age distribution today that we did in 1980, we would expect the murder rate to be about 16% higher.

3. Mass Incarceration

Due in part to tougher sentencing, the number of Americans in prison has risen risen significantly since 1980. This matters because a large percentage of murders are committed by career criminals. For example, in the city of Milwaukee, more than half of murder suspects had at least six prior arrests. It is likely that a small number of people are responsible for a large number of crimes. Keeping these people off the streets and in prison is likely to have reduced the murder rate.

4. Abortion

In 1973, the Supreme Court decided in Roe v. Wade that the states could not pass laws restricting abortion during the first trimester of pregnancy. The result was a huge increase in the number of abortions performed in the United States.

In 1970, there were 52 abortions per 1000 lives births in the United States. By 1984, this number had increased by a factor of seven. From 1978-1997, over 1 million fetuses were aborted each year.

However, these were not evenly distributed. As famously argued in the book Freakonomics, people who chose to have abortions were more likely to be in demographics (such as single-parent households) that produced higher levels in crime.

In fact, states where the abortion rate rose more rapidly saw higher reductions in their murder rate. According to a paper by the Freakonomics authors, perhaps 50% of the decline in murder rate can be explained by the increase in abortions.

5. Advances in Emergency Medicine

According to one study, murder rates in the United States were about the same in 1931 as in 1997. But the rate of aggravated assault increased by 750% during the same period. It's possible that advances in medicine, and the implementation of a nationwide 911 system, have turned murders into attempted murders as more shooting and stabbing victims are saved in the emergency room.

However, we are no longer seeing improvements in this regard. In the last two decades, the rate of aggravated assault has fallen faster than the murder rate.