Wired magazine printed an excellent overview of Alcoholics Anonymous last year. Here is what it had to say regarding the statistics of AA's effectiveness:
But how effective is AA? That seemingly simple question has proven maddeningly hard to answer. Ask an addiction researcher a straightforward question about AA’s success rate and you’ll invariably get a distressingly vague answer. Despite thousands of studies conducted over the decades, no one has yet satisfactorily explained why some succeed in AA while others don’t, or even what percentage of alcoholics who try the steps will eventually become sober as a result.
As a result of these complications [listed in the article but snipped], AA research tends to come to wildly divergent conclusions, often depending on an investigator’s biases. The group’s “cure rate” has been estimated at anywhere from 75 percent to 5 percent, extremes that seem far-fetched. Even the most widely cited (and carefully conducted) studies are often marred by obvious flaws. A 1999 meta-analysis of 21 existing studies, for example, concluded that AA members actually fared worse than drinkers who received no treatment at all. The authors acknowledged, however, that many of the subjects were coerced into attending AA by court order. Such forced attendees have little shot at benefiting from any sort of therapy—it’s widely agreed that a sincere desire to stop drinking is a mandatory prerequisite for getting sober.
Yet a growing body of evidence suggests that while AA is certainly no miracle cure, people who become deeply involved in the program usually do well over the long haul. In a 2006 study, for example, two Stanford psychiatrists chronicled the fates of 628 alcoholics they managed to track over a 16-year period. They concluded that subjects who attended AA meetings frequently were more likely to be sober than those who merely dabbled in the organization. The University of New Mexico’s Tonigan says the relationship between first-year attendance and long-term sobriety is small but valid: In the language of statistics, the correlation is around 0.3, which is right on the borderline between weak and modest (0 meaning no relationship, and 1.0 being a perfect one-to-one relationship).
“I’ve been involved in a couple of meta-analyses of AA, which collapse the findings across many studies,” Tonigan says. “They generally all come to the same conclusion, which is that AA is beneficial for many but not all individuals, and that the benefit is modest but significant … I think that is, scientifically speaking, a very valid statement.”
That statement is also supported by the results of a landmark study that examined how the steps perform when taught in clinical settings as opposed to church basements. Between 1989 and 1997, a multisite study called Project Match randomly assigned more than 1,700 alcoholics to one of three popular therapies used at professional treatment centers. The first was called 12-step facilitation, in which a licensed therapist guides patients through Bill Wilson’s method. The second was cognitive behavioral therapy, which trains alcoholics to identify the situations that spur them to drink, so they can avoid tempting circumstances. And the last was motivational enhancement therapy, a one-on-one interviewing process designed to sharpen a person’s reasons for getting sober.
Project Match ultimately concluded that all three of these therapies were more or less equally effective at reducing alcohol intake among subjects. But 12-step facilitation clearly beat the competition in two important respects: It was more effective for alcoholics without other psychiatric problems, and it did a better job of inspiring total abstinence as opposed to a mere reduction in drinking. The steps, in other words, actually worked slightly better than therapies of more recent vintage, which were devised by medical professionals rather than an alcoholic stockbroker.
AA is still far from ideal. The sad fact remains that the program’s failures vastly outnumber its success stories. According to Tonigan, upwards of 70 percent of people who pass through AA will never make it to their one-year anniversary, and relapse is common even among regular attendees.
[I retained the emphasis from the original but added links to the studies mentioned for convenience.]
The article has a lot of information on the history and details of the program, including some theories as to why it works for some people. It also addresses some of the issues raised in the question's comments, particularly regarding exactly how the spiritual component assists in treating addiction.
While I could spend years looking into the subject, I suspect that hundreds of studies later, I would end up drawing a similar conclusion to the article's author. Studies on AA are all over the map and it's impossible to draw a sound conclusion at this point in time. AA is not a panacea by any stretch of the imagination, but there is clearly some benefit to the program. More research is needed to pick out the most helpful segments of the program and combine them with other techniques, so many more people can receive successful treatment.