at "coffee shops", which the Dutch have allowed to operate for decades, and possession of less than 5 grams (0.18 oz). British Crime Survey statistics indicated that the proportion of 16- to 24-year-olds using cannabis decreased from 28% a decade ago to 21%, with its declining popularity accelerating after the decision to downgrade the drug to class C was announced in January 2004. The BCS figures, published in October 2007, showed that the proportion of frequent users in the 16–24 age group (i.e. who were using cannabis more than once a month), fell from 12% to 8% in the past four years. American teenagers are drinking and smoking less and doing fewer drugs than their predecessors in more than 40 years of tracking. Use of marijuana is down among 8th- and 10th-graders, though it is flat among high school seniors, according to the annual Monitoring the Future survey of American teens. A 2008 scholarly study found that intensified enforcement of marijuana laws does not achieve the stated goals of marijuana prohibition and that "decriminalizing marijuana possession or deprioritizing marijuana law enforcement does not appear to increase marijuana use". Gateway drug theory Arguments that cannabis is a gateway drug The US Drug Enforcement Agency's "2008 Marijuana Sourcebook" argues that recent research supports the gateway hypothesis that certain drugs (such as cannabis) act as gateways to use of 'harder' drugs such as heroin, either because of social contact or because of an increasing search for a better high. Proponents cite studies such as that of 311 same sex twins, where only one twin smoked cannabis before age 17, and where such early cannabis smokers were five times more likely than their twin to move on to harder drugs. Arguments that cannabis is not a gateway drug In the American Journal of Public Health, Andrew Golub and Bruce Johnson of the National Development and Research Institute in New York wrote that young people who smoked marijuana in the generations before and after the baby boomers did not appear to be likely to move on to harder drugs. Researchers from the independent Rand Drug Policy Research Center in Santa Monica, California, looking at data from the National Household Survey on Drug Abuse between 1982 and 1994, concluded that teenagers who took hard drugs did so whether they had first tried cannabis or not. A twin study (of 510 same sex twin pairs) which adjusted