In a 2005 study in The Journal of the American Medical Association, Dr. Shear presented evidence that the treatment was twice as effective as the traditional interpersonal therapy used to treat depression or bereavement, and that it worked faster. The study supported earlier suggestions that complicated grief might actually be different not only from normal grief but also from other disorders like post-traumatic stress and major depression.

Then, in 2008, NeuroImage published a study of the brain activity of people with complicated grief. Using functional magnetic resonance imaging, Mary-Frances O’Connor, an assistant professor of psychiatry at the University of California, Los Angeles, showed that when patients with complicated grief looked at pictures of their loved ones, the nucleus accumbens  the part of the brain associated with rewards or longing  lighted up. It showed significantly less activity in people who experienced more normal patterns of grieving.

“It’s as if the brain were saying, ‘Yes I’m anticipating seeing this person’ and yet ‘I am not getting to see this person,’ ” Dr. O’Connor said. “The mismatch is very painful.”

The nucleus accumbens is associated with other kinds of longing  for alcohol and drugs  and is more dense in the neurotransmitter dopamine than in serotonin. That raises two interesting questions: Could memories of a loved one have addictive qualities in some people? And might there be a more effective treatment for this kind of suffering than the usual antidepressants, whose target is serotonin?

Experts who question whether complicated grief is a distinct disorder argue that more research is needed. “You can safely say that complicated grief is a disorder, a collection of symptoms that causes distress, which is the beginning of the definition of a disease,” said Dr. Paula J. Clayton, medical director of the American Foundation for Suicide Prevention. “However, other validators are needed: family history and studies that follow the course of a disorder. For example, once it’s cured, does it go away or show up years later as something else, like depression?”

George A. Bonanno, a professor of clinical psychology at Columbia known for his work on resilience (the reaction of the 85 percent of the population that does adapt to loss), was skeptical at first. But, Dr. Bonanno said, “I ran those tests and, lo and behold, extra grief symptoms were very important in predicting what was going on with these people, over and above depression and P.T.S.D.”

Regardless of how complicated grief is classified, the discussion highlights a larger issue: the need for a more nuanced look at bereavement. The DSM-IV devotes only one paragraph to the topic.