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Nearly everyone inherits a vulnerability for addiction to mind-altering chemicals, according to new research. Between 60 million and 70 million Americans have tried an illegal drug some time in their lives, and 4.2 million have become addicted. Sixty-five million Americans drink alcohol, and slightly more than 8 million have become dependent. Thirteen percent of Americans 12 and older are heavy cigarette smokers, more than a pack a day, and 57% of them say they find it difficult to quit.

"It appears that the genetic vulnerability for substance and alcohol abuse is fairly general in our society," said Dr. David Goldman, chief of neurogenetics at the National Institute on Alcohol Abuse and Alcoholism. "It’s probably not wise for anyone to think that they’re not vulnerable to some substance of abuse," he said. "If you tested an entire population and identified everybody who’s vulnerable to drug abuse, you could probably put everybody’s name on the list."

But not all types of addiction are equal when it comes to the impact of genes. The study, which appears in the Archives of General Psychiatry, found that genes accounted for more than half of the risk of heroin addiction but only 26% of the addiction to psychedelics. The biggest factor influencing addiction to psychedelics is the nonfamily environment, including friends, schoolmates and co-workers, which accounts for 53%.

For marijuana addiction, the nonfamily environment also has the biggest influence, accounting for 38% and with genes at 33%. "Some of these addictions - for example, alcohol and opioid abuse - are more heritable than susceptibility to coronary artery disease or obesity," Goldman said. "While there’s a clear environmental component, it nevertheless works out that for somebody living in modern society, a prediction as to whether they would have a problem with alcohol or another substance would be substantially dependent on their genetic background," he added.

Although addiction-predisposing genes are not yet known, finding them has become a major goal of the National Institute on Drug Abuse. Early success by researchers in linking some genes to alcoholism has persuaded the drug abuse institute to do the same thing for the genes of addiction. "With the emergence of new genetic technologies and ways to actually look inside the human genome, we decided the time is right to mount a major initiative to try to understand the role of genetic influences on the vulnerability to addiction," said Dr. Alan Leshner, director of the drug-abuse institute. The ability to diagnose genes that make a person more susceptible to specific addictions could result in the development of medicines that block a drug’s action.

The study, headed by Harvard’s Dr. Ming Tsuang, also overturns the old belief that the use of less-addictive drugs such as marijuana sets people on the path to becoming hooked on cocaine or heroin, the so-called "marijuana gateway." Although some people abuse every drug they can, because these drugs affect a major chemical path in their reward system, others have genes that make them addicted to one type of drug that affects only a very specific part of the reward mechanism. "There are genetic effects that make some people predisposed to substance abuse," said Dr. Jack Goldberg of the University of Illinois at Chicago School of Public Health, who participated in the study. "It doesn’t mean that addiction is predetermined by genes. It just means that some of us are more susceptible than others to abusing drugs if we try them."