>> Irrelevant ad hominem; the report (https://www.federalregister.gov/documents/2016/08/12/2016-17960/denial-of-petition-to-initiate-proceedings-to-reschedule-marijuana) cites plenty of scientific evidence for that statement. <<
Nonsense. That report admits the existence of several studies showing that marijuana use was a strong indicator of the future use of other, harder addictive drugs... but dismisses them all in total on the argument that they only tested for whether people who used marijuana regularly were more likely to USE other drugs, not necessarily more likely to become dependent on them once they did use them. What it refutes is that something inherent about the nature of marijuana makes them more likely to become addicted to other drugs than other people who try those other drugs.
But that’s not the point! The point is that marijuana makes people more likely to try other drugs, giving them the opportunity to become addicted to them, which is clearly demonstrated by numerous studies! What the DEA refuted is not what people mean by “gateway drugs” Here are some definitions (emphasis and parenthetical statements are mine in all):
Wikipedia: Gateway drug effect ... is a comprehensive catchphrase for the often observed effect that the use of a psychoactive drug is coupled to an increased probability of the USE of further drugs.
Dictionary.com: any mood-altering drug, as a stimulant or tranquilizer, that does not cause physical dependence but may LEAD TO THE USE of addictive drugs
Collins: A gateway drug is a drug such as marijuana that is believed by some people to LEAD TO THE USE of more harmful drugs such as heroin or cocaine.
Center for Disease Control: a drug that is thought to LEAD TO THE USE of more dangerous drugs (such as cocaine or heroin).
National Institute on Drug Abuse: Some research suggests that marijuana use is LIKELY TO PRECEED USE of other licit and illicit substances46 and the development of addiction to other substances. For instance, a study using longitudinal data from the National Epidemiological Study of Alcohol Use and Related Disorders found that adults who reported marijuana use during the first wave of the survey were more likely than adults who did not use marijuana to develop an alcohol use disorder within 3 years; people who used marijuana and already had an alcohol use disorder at the outset were at greater risk of their alcohol use disorder worsening
National Institutes on Health: The gateway drug hypothesis refers to the pattern of substance use during adolescence whereby legal substances, such as nicotine and alcohol, PRECEDE THE PROGRESSIVE USE of illicit substances like cocaine and heroin.
No, more likely than non-marijuana users.
But that’s not the point! The point is that marijuana makes people more likely to try other drugs,
No causal link has been established - just a correlation.
giving them the opportunity to become addicted to them
Which is why we care about an alleged "gateway" - making the DEA's approach a reasonable one.
Research shows that the correlation between earlier marijuana use and later use of other drugs can be explained by a "common-factor" model, that is, a third factor that causes both results, such as individuals' opportunities and unique propensities to use drugs, or more broadly a social or psychological predisposition towards anti-social behaviour. (http://www.rand.org/pubs/research_briefs/RB6010/index1.html, http://rds.homeoffice.gov.uk/rds/pdfs2/hors253.pdf)