“The Drunkard’s Progress,” a print by Kelloggs & Thayer of New York, c. 1845. Illustration: Library of Congress, LC-USZC4-10005

Debate over the causes of addiction has a long history. In the early 17th century, British clergy warned their congregations about the “disease” of alcoholism, which according to one cleric was “so epidemical” that “all the Physicians in England know not how to stop it.” The clergy called alcoholism a disease because they assumed their parishioners would not knowingly engage in self-destructive behavior. They had no medical evidence for their claim, and the public, as well as most of the medical world, took the alternate view that people got drunk because they liked to, not because they had to.

Starting in the late 18th century, though, a number of doctors in the United States and England began calling self-destructive drug and alcohol use a disease. In America, this movement was initiated by Benjamin Rush, a signer of the Declaration of Independence and professor of medicine at the University of Pennsylvania, who, in his 1785 “Inquiry into the Effects of Ardent Spirits on the Human Body and Mind” wrote that “drunkenness resembles certain hereditary, family, and contagious diseases.” More recently, Dr. W.D. Silkworth, a Manhattan physician and early supporter of the Alcoholics Anonymous program, proposed in his preface to the 1939 volume, Alcoholics Anonymous, the program’s basic text, that alcoholics have an “allergy.” The allergen is alcohol, and the allergic reaction is loss of control over drinking. One drink leads to another, which leads to another, just as ragweed pollen initiates a fit of sneezing.

Today, reams of clinical texts and articles in medical journals refer to addiction as a chronic, relapsing illness that should be classified with diseases such as Alzheimer’s and Type 2 diabetes. This perspective was captured in a 2003 comment delivered at the world’s foremost conference on addiction research, sponsored by the National Institute on Drug Abuse (NIDA, a division of the National Institutes of Health). Speaking from the floor, a much-published specialist in the field stated that addiction is a disease “because it has a genetic basis, and we do not choose our genes.”

The remark expresses a widely held idea: If an activity has a genetic basis, it is not voluntary. There is an intuitive appeal in this claim. But is it true? Aren’t there activities we engage in that are voluntary and have a genetic component? If so, far from being beyond control, addiction may be a voluntary act.

Research on the role of genetics in addiction has focused on alcoholics because it is easier to conduct multigenerational studies when the drug in question is legal. The fundamental finding has been that alcoholism runs in families, even when the family members do not live together. Dr. Robert Cloninger of Washington University Medical School in St. Louis led a project, reported in 1987, that illustrates this point. The research was carried out in Sweden, a country in which adoption is not uncommon and where the medical histories of biological and nonbiological parents are a matter of record.

Cloninger studied 1,700 men who had been given up for adoption at an average age of four months. He found that the rates of alcoholism among individuals whose biological fathers were “severe” alcoholics were nearly identical regardless of whether their adoptive father was an alcoholic or a teetotaler (18 versus 17 percent). In other words, if the biological father was a severe alcoholic, the drinking behavior of the adopting parent did not matter.

Genetic studies of illicit drug users yield similar findings. In a representative analysis from 2000, Dr. Kenneth Kendler and his colleagues at the Medical College of Virginia and Virginia Commonwealth University tabulated the correlations in illegal drug use and illegal drug addiction for fraternal and identical twins. Their hypothesis was that if a genetic basis for addiction existed, the actions of identical twins would be more alike than those of fraternal twins. The results for casual, nonaddictive drug use showed little difference between identical and fraternal twins. If one twin had experimented with an illicit drug, there was about a 75 percent chance the other had done so. But the numbers for drug addiction were markedly different. If one fraternal twin was dependent on drugs, there was only about a 25 percent chance that his twin was also dependent; whereas, if one identical twin was dependent, there was a 40 percent chance that his identical brother was also dependent.

Studies such as these leave little doubt that genes play an important role in the etiology of addiction. But they also suggest that genes may not play the leading role. It’s worth noting that the correlation for addiction among identical twins in the Virginia study was far from 100 percent, and that fewer than 20 percent of the biological sons of serious alcoholics in the Swedish survey became alcoholics, even if their adoptive fathers were alcoholics. The pathway from DNA to addiction appears to be indirect, with genes programming proteins that affect the probability of addiction rather than ensuring a particular outcome. If this is so, then genes do not preclude choice.

The late cartoonist Charles Addams (creator of the Addams Family) once published a cartoon of identical twins separated at birth who encounter each other in the waiting room of a patent attorney. Dressed alike (down to the pens in their pocket protectors), they hold on their laps identical gizmos. In recent years, researchers have focused on the notion that interests, occupations, and even personal idiosyncrasies run in families, and that genes play an important role. They’ve examined, for instance, the function of heredity in social attitudes, such as support for the death penalty and whether women should have nondomestic professions. One of the most interesting studies, carried out in 1990 by University of Minnesota professor Neils Waller and associates and reported in Psychological Science, evaluated the role of genes in the religious beliefs of twins.

The twins studied by Waller were all separated in their first year and grew up in different families. Questionnaires gathered their views about the nature of God, the literal accuracy of biblical stories, and the role of prayer in their lives. Fraternal twins agreed at about a chance level while identical twins agreed with significant—though not total—consistency. These results do not stand alone. There are many studies on the heritability of attitudes and beliefs, and they typically show that beliefs reflect genetic as well as societal influences. The same is true for other interests. Stories of precociously talented musicians, for example, suggest that genetic endowment affects musical ability. Yet a career in music also depends on environmental influences and many choices—the musical program at one’s school, for example, and the teachers one selects—plus some luck. In short, genes play a role in what ultimately amounts to voluntary behavior.

The picture is similar with drug addiction. Large-scale population studies, intensive neighborhood studies, and one-on-one interviews all reveal that familiar motivations—a wish for more respect from family members, worry about finances, the desire to be free of the hassles that accompany illegal activity—eventually persuade most drug addicts to quit. Epidemiological studies show that most drug addicts quit by about age 30, without the assistance of clinical intervention. By contrast, personal, financial, and legal concerns have little or no direct influence on the symptoms of schizophrenia, Alzheimer’s disease, and other true brain diseases. Worries about job security do not halt a schizophrenic’s hallucinations, but the record shows that a threat to job security often bring addicts’ drug use to a stop.

Not all addicts quit, to be sure. Life’s complications lead to different choices by different people. However, individual differences notwithstanding, the major epidemiological surveys show that addiction has the highest recovery rate of any psychiatric disorder.

A more recent argument in favor of the addiction-as-disease notion springs from technological advances in studying the structure and function of the brain. It is now possible with magnetic resonance imaging to observe the brain as it experiences emotions, say, or commits a fact to memory, or makes a decision to take a drug. The drug-related studies have shed new light on how drugs work and in recent years have become the primary pillar supporting claims that addiction is a disease. Alan Leshner, director of NIDA from 1994 until 2001, put the case most succinctly in a 1997 paper in Science: “That addiction is tied to changes in brain structure and function is what makes it, fundamentally, a brain disease.” Glen Hanson, Leshner’s successor at NIDA, makes the same point in slightly different words: “Addiction comes about as a result of the long-lasting effects of drugs on brain function. Therefore we say it is a brain disease.”

Hundreds of experimental reports published during the last decade or so validate the idea that drugs alter the brain. This makes sense. Drugs change behavior, mood, and thought; the brain is the organ controlling these actions; hence drugs change the brain. Indeed, given our biological nature, all factors influencing behavior do so by altering the brain. The research supporting this logic helps explain why addicts can have such high recovery rates.

A study on brain plasticity and obsessive-compulsive disorder (OCD) is revealing. Individuals with OCD are plagued by disturbing thoughts. They find temporary relief by engaging in ritualized behaviors that address their obsessions. For example, the idea that one’s hands are swarming with infectious bacteria can be relieved by washing. But then the thought returns, and with it the motivation to wash one’s hands. Therapists have developed behavioral and cognitive techniques that help OCD patients ignore their disturbing thoughts. Follow-up studies show that these techniques are effective. This implies that the techniques must have changed the brain. A 1998 study by Jeffrey Schwartz, a psychiatrist at UCLA’s Semel Institute who specializes in OCD treatment, confirmed this inference.

Schwartz and his colleagues trained OCD patients to disregard their obsessive thoughts. This led to a marked reduction in compulsive rituals and also in the intrusive thoughts. The researchers measured neural activity in areas of the brain associated with OCD symptoms. For those patients who learned to ignore their obsessions, activity levels in the critical brain areas were similar to those of OCD-free control subjects. (Drugs that help ameliorate compulsive behavior produced similar changes in brain activity.) Summarizing these findings, Schwartz writes: “Change your behavior, change your brain.” This comment captures the dynamic quality of brain and behavior interactions, but it leaves out the clinicians and their contribution. They gave their patients encouragement and techniques for replacing compulsions with less disruptive activities. A more accurate summary of the OCD treatment results is: “Change the conditions, change your behavior, change your brain.”

If OCD patients can learn to dismiss their disturbing thoughts, then it seems reasonable to sugest that similar processes are taking place when motivated addicts learn to ignore drug cravings. Ex-addicts often report that they discovered techniques for not giving in to their cravings, such as going to the gym or reminding themselves that they quit their addiction for a good reason. Just as genetic influences do not imply a disease state, drug-induced brain change is not a sufficient sign of disease. Drugs alter the brain, but the evidence shows that they do not do so in ways that prevent addicts from quitting.

Gene M. Heyman is an adjunct associate professor of psychology at Boston College. He also has appointments at McLean Hospital and Harvard Medical School. His essay is drawn and adapted from Addiction: A Disorder of Choice (copyright © 2009 by the President and Fellows of Harvard College), by arrangement with Harvard University Press.