"Science":

Regular cannabis users are at greater risk of developing mental illness later in life, according to research.

One study found that the risk was seven times higher for heavy users, said Professor Robin Murray of the Institute of Psychiatry in London.

Speaking at the Royal College of Psychiatrists' annual conference in Edinburgh, he said: "In the last 18 months a number of studies have confirmed that cannabis consumption acts to increase later risk of schizophrenia. This research must not be ignored."

For his study, Professor Murray reviewed research in Sweden, Holland and New Zealand.

A recent Dutch study of 4,000 people in the general population showed that those taking large amounts of cannabis were almost seven times more likely to have psychotic symptoms three years later.

Another study, in 1987, of 50,000 Swedish Army conscripts, found that those who admitted at age 18 to having taken cannabis on more than 50 occasions, were six times more likely to develop schizophrenia in the following 15 years.

Professor Murray said these findings had been largely ignored.

The faultiness of the analysis is extremely basic, something that almost everyone would get in the most basic science or psychology class. A co-relation does not mean causation. The classic example from my own high school classes was noting the co-relation between hair length and English scores on the SAT. Does growing your hair long increase your verbal testing scores? No, the results might be more likely to do with gender differences. Girls score a little higher than boys on that side. Girls tend to have longer hair.

In this study, the obvious conclusion would be to suspect that people who were predisposed toward mental problems are more likely to smoke dope than people with nice, calm balanced mental connections. I bet you'd find a similar connection between mental illness and all types of drug use. It would be probably a better inference that people who are prone to serious mental problems are much more likely than the population as a whole to seek out mood altering drugs.

A trained scientist using these surveys to reach the conclusion that "cannabis consumption acts to increase later risk of schizophrenia" indicates bad faith. I would think that even a journalist would pick up something so basic.

Now, it could be that smoking dope increases your vulnerability to mental illness. It could be that it has no causal connection at all. It could be that smoking the chronic acts as a kind of self-medication minimizing the effects of pre-existing conditions. You just can't really tell from these kinds of studies.