This question is recurrently discussed in the popular press and by online bloggers. What I've discovered is that depending upon one's personal motivations for supporting or outlawing marijuana use, the available data allows one to make reasonable-sounding arguments on either side of the issue. The actual answer is far more nuanced.
Medical pharmacology textbooks from the 1970s actually stated that people who had a history of repeated frustrations, and deprivations, who were sexually maladjusted, especially homosexuals, or those who seek escape and sometimes possess major personality defects and are often psychopathic, are the kinds of people who smoke marijuana. Honestly, this is what physicians were taught forty years ago. In 1971, one publication reported that "moderate to heavy use of marijuana in adolescents and young people without predisposition to psychotic illness may lead to ego decomposition, ranging from mild ego disturbance to psychosis." However, this study defined a heavy user as someone who smoked up to 200 marijuana cigarettes per day! If you do the math, that's one joint every five waking minutes.
In a somewhat later publication, the marijuana psychosis theory was tested again by administering questionnaires to people who were being admitted to mental institutions in the state of Missouri. The researchers identified 38 individuals who had used marijuana prior to having shown psychiatric symptoms. A preliminary analysis of the results confirmed their hypothesis that marijuana smoking lead to psychosis. However, these researchers decided to dig deeper into their data and determine what else their patients were doing prior to being admitted into a psychiatric hospital. Indeed, ten other events occurred more often than marijuana use, including masturbation, driving a car, drinking beer, dancing, smoking tobacco products and kissing! The most dangerous behavior that correlated best with being hospitalized for psychosis in the state of Missouri in the 1970s was watching late night television! Finally, clear evidence that watching Jonny Carson was the equivalent of a gateway drug.
What do similar investigations tell us about this issue today? Does smoking marijuana make it more likely that you will develop schizophrenia? Forty years of research has led to the following answer: yes, and no. Overall, today's researchers are being more careful and are trying to bring less bias to their investigations. One recent study asked whether marijuana use was associated with an earlier age of onset for the first episode of schizophrenia. The researchers concluded that there was no significant relationship between the onset of illness and marijuana use that could not be accounted for by other demographic and clinical variables. Meaning, once again it's important to take notice of all of the other variables that contribute to developing psychosis. For example, it is now well understood that schizophrenia is an inherited disorder that is usually diagnosed during the late teens to early adulthood, exactly the time that most people tend to be using marijuana.
Recent studies have also demonstrated that the endogenous marijuana receptors in the brain of someone with schizophrenia respond differently than those in the brain of someone without a predisposition to this disorder. It may be that someone who is vulnerable to developing psychosis is simply more likely to indulge more often in marijuana smoking, experience more difficulty in controlling the urge to smoke and be more likely to ultimately develop a true psychosis. It now appears as though stimulating endogenous marijuana receptors may be able to unmask underlying symptoms related to schizophrenia as well as other mental disorders. For example, a recent report from Denmark stated that smoking marijuana reduces the age of onset of the initial symptoms of bipolar disorder. Similar to schizophrenia, bipolar disorder is also a highly inheritable genetic disease.
Given recent evidence, marijuana smokers face a challenging dilemma: do they feel genetically lucky? There are no accurate genetic tests currently available to inform someone whether they are at risk of developing psychosis or bipolar disorder. Therefore, the answer to my question posed above is still "yes, and no." It may simply depend upon the genetic cards you were dealt by your parents.
See also: Marijuana and Coffee are good for the brain. http://www.youtube.com/watch?v=2uVXs6CY2ps