WASHINGTON (AP) — People born after the 1960s are less likely than baby boomers to go from using marijuana to heroin and other hard drugs, according to a private study that challenges the so-called gateway theory of drug abuse.

The White House's drug policy office expressed doubts about the study and suggested it could undermine attempts to prevent drug use among young people.

The study published in February's edition of the American Journal of Public Health concludes that a rise in marijuana use among young people during the 1990s is unlikely to result in an epidemic of hard drug use in the near future.

"The drug subculture among inner city youth today encourages marijuana use but discourages use of hard drugs," Andrew Golub, the study's main author, said Wednesday. "Many of these kids witnessed the devastating effects of crack and heroin on their own families and neighborhoods."

The gateway theory doesn't contend there is a direct connection between different degrees of drug abuse, but says that those who use tobacco and alcohol are statistically more likely to go on to use marijuana and in turn are more likely to use cocaine, crack or heroin.

Golub said his research shows the theory "is not relevant to the kids who came before the baby boomers and those born during the 1960s, and it is increasingly less relevant to those who came after."

Bob Weiner, spokesman for the White House Office of National Drug Control Policy, said he hadn't read the study but expressed doubts about Golub's conclusions, citing recent research that found young people who regularly use marijuana are 80 times more likely to use cocaine.

"The parents of the children who have gone onto cocaine would have more common sense than his findings seem to come out with," Weiner said.

For the new study, researchers analyzed data about adolescent drug use reported by more than 100,000 people who participated in the government's annual National Household Survey on Drug Abuse between 1979 and 1997.

Golub said that before 1944, alcohol and tobacco use was common among youths, but progressing to other drugs was virtually unheard of. That trend peaked for those born in the 1960s, when the likelihood of progressing to marijuana use by age 17 reached as high as 47% and moving to harder drugs reached 20%. For people born at the end of 1970s, the risk of marijuana use declined to 36% and the rate of progression to harder drugs fell to 6%.

Susan Foster, a director with the National Center on Addiction and Substance Abuse at Columbia University, said the research was important and the results may reflect the overall national decline in drug use, although they may not yet account for the increase of the early 1990s.

"I would certainly caution against drawing conclusions that would lead us to abandon any efforts to stem alcohol, tobacco or illicit drug use," she said. "We do know that if you can keep youth from using before they're 21, it vastly reduces the risk that they're going to run into problems later."