[Ken H]

The following is directly from the JAMA article itself.

Conclusions  Associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs. JAMA Abstract of this article

Is the headline of this article--

Researcher's Study of Twins Bolsters 'Gateway Theory'

justified by the conclusions in the JAMA article?

[aruanan]

The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.

So much for the idea of causality. This reduces marijuana to a pleasant attendent or reinforcing circumstance.

As I said earlier, the "twin study" as presented in the article did not do what twin studies are used to demonstrate, an inherent predisposition toward disease or behavior. This one didn't claim an association between twins and their drug-using behavior. That is, if one did or did not use drugs, the other was more likely than not to follow suit; thus, the lack of such association suggests that one is not genetically predisposed to recreational drug use.

The claim that use of marijuana at a young age tended to predict later experimentation with other drugs did not, though, demonstrate that marijuana possessed any sort of inherent perniciousness that would cause a user to progress to harder, more dangerous drugs. The researchers, as they admit above in the excerpt from the abstract, failed to control for social effects, which can have a greater influence on human behavior than either genetics or pharmacology. This is a major failing of the study.

For all we hear about drugs causing people to become brain-fried and given over to pro-drug enthusiasm, the sheer level of lack of analysis and thought in this thread among the anti-anti-war on drugs crowd is telling. It demonstrates that such a condition is a trait shared at least to the same degree by all those who are going to maintain their chosen position no matter what.

They're in good company, though. This mindset is common both in religion as well as science. It appears to be, unfortunately, a default condition of the human mind either to persist doggedly in a belief demonstrated to be wrong or to embrace precipitously whatever appears to support whatever position one happens to want to maintain.

[shrinkermd]

From the AMA abstract, the following probably explains why they reached their conclusion.

Results Individuals who used cannabis by age 17 years had odds of other drug use, alcohol dependence, and drug abuse/dependence that were 2.1 to 5.2 times higher than those of their co-twin, who did not use cannabis before age 17 years. Controlling for known risk factors (early-onset alcohol or tobacco use, parental conflict/separation, childhood sexual abuse, conduct disorder, major depression, and social anxiety) had only negligible effects on these results. These associations did not differ significantly between monozygotic and dizygotic twins.

Conclusions Associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.

I fail to see the conflict.