When an act of seemingly inexplicable violence like the Arizona massacre occurs, everyone is desperate for explanations. Some look to political rhetoric, some look to mental illness, some blame the parents, others point to marijuana. But what’s really at fault?
The reality is clearly complex. As researchers searching for the causes of violence and the roots of psychosis know, Jared Loughner’s behavior cannot easily be explained by a gene or a drug or culture or parenting. While each of those factors can be shown to have measurable effects, their relative contributions are often small or moderate and can’t be easily teased apart into a recipe of rampage in any individual case. (More on Time.com: Neurosurgeon Nirit Weiss on Treating Gunshot Victims Like Rep. Gabrielle Giffords)
Take schizophrenia, which seems to be Loughner’s most likely diagnosis. If one identical twin has the disorder, about 40-50% of the time, the other twin will be affected as well— suggesting that about half the risk is due to genes. Specific genes associated with risk, however, only seem to account for a small proportion of the condition and although some environmental factors like childhood trauma and infections experienced in utero have been identified, these, too, don’t explain much.
Schizophrenia is associated with a genuine increased risk of violence: a recent systematic analysis of the data on the subject found that men with schizophrenia are 2 to 4 times more likely to be violent than men without that diagnosis, without taking alcohol or other drug abuse into account. Men with schizophrenia who also have drug problems—about half of those with the diagnosis by most estimates— by contrast are 12 times more likely to be violent.
As psychologist Vaughan Bell pointed out in Slate, however, that same meta-analysis found that people with substance misuse problems alone are more likely than people with schizophrenia to become violent. In the overall analysis, people with schizophrenia were 3.3 times more likely to be violent than those in the general population, but that figure was 5.5 for those with alcohol or other drug problems. Bell writes:
In other words, it’s likely that some of the people in your local bar are at greater risk of committing murder than your average person with mental illness.
So should we clear the mentally ill of being at risk for violence and put all our focus on alcoholics and addicts? Does this mean that the marijuana that Loughner apparently smoked is to blame for the shooting? Could it have even caused the schizophrenia itself? (More on Time.com: After Tucson: Why Are the Mentally Ill Still Bearing Arms?)
Here, too, it gets very complicated. Both substance misuse and psychosis are linked with violence and with each other and the causal pathways can go in both directions. For example, people may self-medicate psychosis with drugs and that makes it worse rather than better— and substance misuse can be a response to violence as well as a cause of it.
For marijuana, the bottom line is that if cannabis smoking can cause schizophrenia, it does so in only some of those who are already predisposed to the mental disorder. Otherwise, the massive increase in marijuana use seen in the West since the 1950′s would have been accompanied by a similar increase in the schizophrenia rate. That did not occur: schizophrenia rates have been stable, affecting roughly 1% of the population.
However, some people who are vulnerable to schizophrenia may become ill more quickly if they smoke pot: one study did find that about 36% of people with the mental illness were at risk for earlier onset disease by an average of three years if they used cannabis. (More on Time.com: See Images of the Mourners From the Tucson Shooting)
So, if Loughner was in that group, it is possible that his marijuana use affected the development of his psychosis. But is there really a link between marijuana and violence?
While alcohol and cocaine use are pharmacologically associated with increased risk of violence—due to their effects on the brain, these drugs themselves can increase the risk for violent behavior—marijuana tends to be, well, more mellowing. It can increase anxiety and paranoia, however, which can sometimes be linked to violence. And the illegality of marijuana is linked with drug-trade related violence.
Marijuana’s illegal status also prevents the government from regulating the levels of active ingredients in the drug. While marijuana’s best known component—THC—is linked with psychosis and paranoia at high doses, another compound, cannabidiol (CBD) actually seems to have antipsychotic properties. The levels in any particular joint, then, could affect risk— and the levels in what someone smokes repeatedly, particularly if they are at risk for schizophrenia, could make a difference. (More on Time.com: Phony Cancers and Self-Inflicted Acid Attacks: A National Outbreak of Munchausen’s?)
And just to make things even more complicated, there’s another factor that contributes not just to the risk for violence, but to the risk for both addiction and schizophrenia. That’s child abuse and childhood exposure to traumatic violence. Although we don’t know much about Loughner’s childhood, a neighbor told the New York Times that Loughner’s father is “very aggressive.”
Childhood exposure to violence may well be the single biggest risk factor for all of these negative outcomes. One study, for example, found that while early cannabis use and childhood trauma both significantly increased the risk that someone would develop psychotic symptoms, the two together made matters much worse than either alone. Other research has found that the risk of schizophrenia is doubled or more in those who have been victimized as children.
To top this all off, childhood experience takes place in a particular cultural context. Cross-cultural and historical research finds that the content of schizophrenic delusions is actually very much a product of its time and place. As anthropologist Gregg Downey noted on the Neuroanthropology blog, “delusional concepts may appear to be divorced from reality, but they are acutely sensitive to the zeitgeist, elaborating upon society’s own contemporary fears and anxieties.” (More on Time.com: ‘Caffeine Intoxication’ as Mental Disorder and Legal Defense)
That’s how paranoid delusions can come to center on, for example, the CIA at one time and religious figures in another. And how political rhetoric can play a role in these events.
Given all these complications, it’s highly unlikely that we’ll ever know the exact cause of Saturday’s awful events. But blaming pot, parents, politics or psychosis alone is insane.