The increasing public perception that marijuana’s harmfulness is minimal is partially based on legalization efforts and its medicinal availability in some states, explained Dr. Kerry M. Green, community health specialist.
“But what do we really know the effects of marijuana? A lot of it is based on personal experience,” said Green, an assistant professor at the University of Maryland’s Department of Behavioral and Community Health and long-time researcher of substance use and prevention.
Results of a 36-year-long observational study of urban African-Americans that Green worked on suggest that perpetual adolescent pot smokers are at a higher risk than non-users or light-users for non-violent criminal involvement later in life.
In other words, while researchers did not connect smoking pot 20 or more times as a teen to violence or violent adult crimes, it was linked to drug-related crimes and property crimes. Green, who is concerned with health disparities and disadvantaged populations, is primarily interested in using this research as a grounds for prevention.
"If we could get those kids to not use marijuana heavily- to delay their use or eliminate their use- we should be able to lower their risk of getting arrested later for non-violent crimes," she said.
The study consisted of four periods in 1966, 1976, 1992, and 2002, in which data was collected from participants. Roughly 700 subjects completed all four stages. Only this information was used to calculate results.
First, teachers and mothers of African-American first graders in the Woodlawn neighborhood of Chicago were asked about the children’s personalities and behaviors. When the subjects were 16, they self-reported their drug and alcohol use patterns. Finally, they were asked about the adult crimes they had committed by ages 32 and 42, and criminal records were collected.
Researchers isolated the teenage pot use and adult crime variables using an advanced statistical method. In other words, they accounted for other observable factors, such as poverty and dropping out of high school, which also could have lead to criminal involvement.
When considering only the drug as the cause of the crimes, they found a statistically significant relationship between extensive teenage marijuana smoking and non-violent crimes committed in adulthood.
Although the common warning “correlation does not imply causation,” may come to mind for non-experimental data, this method actually does measure causation. It eliminated the most prominent factors that might have contributed to criminal involvement instead of marijuana use. These are called confounding variables.
“It’s a very advanced technique, and the whole reason we use it because we can’t infer causation from this observation data,” said Green. “Usually if we want to know if x causes y, we do an experiment, but with something like drug use, we can’t make kids do marijuana and see what happens.”
Residents of Woodlawn, a then-all-African American neighborhood on the south side of a segregated Chicago, were chosen as the population in 1966 because of the area’s notoriously high poverty and unemployment rates. Another factor was that African-Americans of various socio-economic statuses lived there, so there was opportunity for comparing such factors.
But to what extent can these findings be generalized? Green believes that it can be generalized to at least other urban African-American populations, but admits uncertainty.
“For marijuana, we don’t think there is any difference in drug using behaviors in different cities, although we would have to test it in other cities to confirm this,” she said.
And as with any scientific study, there are limitations to consider.
“Even though we have evidence consistent with a causal interpretation, it’s not a definitive study saying that marijuana causes crime,” said Green. “The evidence suggests that marijuana does not increase your risk of violent crime, nor does it cause violence. But it does seem to increase your risk of property and drug crimes.”
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