Data in Everyday Life – The Curious Case of Correlation, Causation, and Crime: Part 2

Last month’s blog focused on the messiness of determining causation and correlation when studying data trends throughout history. This month, we’ll be looking at the 90s Crime Drop as an example of this messiness and discussing some of the theories. I’ve chosen three, but there are many. Vox has summarized 16 of the theories and why they do or don’t work.

One thing to consider when looking at all the theories is that the key demographic is people who would have been born in the 1970s as “peak criminality” occurs in the early 20s.

Theory 1: The Lead Reduction Theory

In 2013, Kevin Drum from Mother Jones provided a detailed summary of the Lead Reduction Theory. Lead exposure in childhood is linked to lower IQ, hyperactivity, learning disabilities, and behavioural problems. The cause of lead exposure came from leaded gasoline and in 1994, Rick Nevin proposed that the large increase of leaded gasoline use in the post World War II era correlated with the spike of violent crime in the 1960s and 1970s. In 2000, Nevin followed up on this theory but proposed that the drop of lead resulted in the drop in violent crime. In 2007, Jessica Wolpaw Reyes provided further evidence by looking at the differences in crime rates and lead reduction by the state level. States where leaded gasoline use declined quickly had quicker drops in violent crime. States where leaded gasoline use declined slowly had slower drops in violent crime. Nevin also started looking at international data found similar trends in Australia, Canada, the UK, New Zealand, Finland, France, Italy, and West Germany. In 2013, Mielke and Zahran looking at US cities, and even the neighbourhoods of one city and found correlations.

Wow… there’s a lot of evidence here isn’t there? Is this the answer?


This is a strong correlation and the reduction of lead exposure probably did contribute to the crime drop, but not to the extent that the researchers suggest. In particular, in the 1990s, people who were exposed to lead started committing fewer crimes. As well, crime continued to drop well into the 2000s when more of the population had experienced less lead exposure, which weakens the theory.

Theory 2: The Abortion Theory

In 2004, Steven D. Levitt (author of Freakonomics) proposed that access to legal abortions in the 1970s in the US meant that less unwanted children were born and less children were exposed to neglect, abuse, or poor environments; therefore, less children would be exposed to environments and conditions that correlate with criminality.

A plain level summary of the Abortion Theory is available from Allison Tarmann of Population Today and from Levitt’s follow up dialogues. Levitt raises four key pieces of evidence that support the theory. First, in the US crime rates started to drop 18 years after Roe vs. Wade legalized abortion in the US, which is when the first round of children born under legalized abortion would reach peak criminality. Levitt follows up by looking at the data from states that legalized abortion before Roe vs. Wade saw a drop in violent crime earlier than the rest of the United States. Third, states with higher legal abortion rates saw a quicker decline in violent crime.

Interesting. Does this work?


Levitt only explored the drop in violent crime in the US, when we know that violent crime dropped in several countries, including countries where abortion access was still tightly controlled or the laws were very unclear. For example, in Canada, until 1988 only limited abortion was permitted, meaning it could only happen if a committee of doctors decided that the pregnancy was too dangerous for the mother, but crime rates still dropped. The theory also makes some big assumptions about abortions. First, it doesn’t consider abortions that happened prior to 1973. Second, it assumes that unwanted children are more likely to commit crime. Third, it doesn’t take into account that violent crime rates dropped among multiple age ranges in the 1990s, not just 18 to 24.

Theory 3: The Technology is Keeping Us Inside Theory

As outlined in Maclean’s, new technologies including televisions, gaming consoles, cell phones, and the internet mean more people are staying inside for entertainment purposes; therefore, creating less potential victims and offenders on the street. The theory works in two ways. First, people who could potentially be victims of crime are less likely to be victims because they are staying inside for entertainment. Second, people who might have engaged in criminal behaviour are less motivated to do it because there are less people to exploit and the criminals themselves are being entertained by the technology. This theory also proposes a modern example in our post-smartphone world, people commit fewer crimes because everyone has a camera and it’s easier to get caught. The Vox article also highlights how video games, especially violent video games, reduce violent behaviour because games provide opportunities to act on violent impulses without actually committing violence.

Does this theory work?


Like the abortion theory there’s several big assumptions to this theory. First, it implies that violent crime is the result of people looking for entertainment, which might account for some violent crime at “peak criminality,” but definitely not enough to account for the drop in violent crime in the 1990s. Second, there’s another theory about technology and violent crime. In 2019, Edlund and Machado proposed that technology, especially cell phones, reduced violent crime by making drug dealing easier thus reducing “turf wars” from gangs fighting over physical locations to deal drugs. Third, there’s just not enough data on this theoy.

So there we have it. Three theories, each suggesting interesting correlations, but none are strong enough to explain causation. All three have different types of data analysis to provide support to their theory and all three have data countering the theory. The truth is all three proposed theories probably played a role in the violent crime drop of the 1990s, but there are more theories that have been proposed and some that haven’t been discovered yet.