The logic for this designation, as explained by former director Alan I. Leshner, is that "addiction is tied to changes in brain structure and function." True enough, repeated use of drugs such as heroin, cocaine, and alcohol alter the neural circuits that mediate the experience of pleasure as well as motivation, memory, inhibition, and planning -- modifications that we can often see on brain scans.

The critical question, though, is whether this neural disruption proves that the addict's behavior is involuntary and that he is incapable of self-control. It does not.

Take the case of actor Robert Downey, Jr., whose name was once synonymous with celebrity addiction. He said, "It's like I have a loaded gun in my mouth and my finger's on the trigger, and I like the taste of gunmetal." Downey went though episodes of rehabilitation and then relapse, but ultimately decided, while in the throes of "brain disease," to change his life.

The neurocentric model leaves the addicted person (Downey, in this case) in the shadows. Yet to treat addicts and guide policy, it is important to understand how addicts think. It is the minds of addicts that contain the stories of how addiction happens, why they continue to use, and, if they decide to stop, how they manage. The answers can't be divined from an examination of his brain, no matter how sophisticated the probe.

It is only natural that advances in knowledge about the brain make us think more mechanistically about ourselves. But in one venue, in particular - the courtroom - this bias can be a prescription for confusion. The brain-based defense ("Look at this fMRI scan, your Honor. My client's brain made him do it.") is now commonplace in capital defenses. The problem with these claims is that, with rare exception, neuroscientists cannot yet translate aberrant brain functions into the legal requirements for criminal responsibility -- intent, rational capacity and self-control.

What we know about many criminals is that they did not control themselves. That is very different from being unable to do so. To date, brain science cannot allow us to distinguish between these alternatives. What's more, even abnormal-looking brains, have owners who are otherwise quite normal.

Looking to the future, some neuroscientists envision a dramatic transformation of criminal law. David Eagleman of the Baylor College of Medicine's Initiative on Neuroscience and Law, hopes that "we may someday find that many types of bad behavior have a basic biological explanation [and] eventually think about bad decision making in the same way we think about any physical process, such as diabetes or lung disease."

But is this the correct conclusion to draw from neuroscience? If every troublesome behavior is eventually traced to correlates of brain activity that we can detect and visualize, will we be able to excuse it on a don't-blame-me-blame my-brain theory? Will no one ever be judged responsible?