Aren't cartoons supposed to be funny? In a New Yorker cartoon, a man and a woman are seated at a restaurant banquette and the man says, "OK, Cynthia. I'll tell you my hopes and dreams, my joys and my passions. But be forewarned—they all concern a particular sports team." What is funny about that?

It is not nice to joke about a neurological affliction. Fortunately, we can now comprehend the condition, thanks to a new book, "Your Brain on Cubs: Inside the Heads of Players and Fans," a collection of essays by doctors and others knowledgeable about neuroscience and brain disorders associated with rooting for a team that last won the World Series a century ago.

The sometimes terrible truth is that being a sports fan is a physical phenomenon as well as a psychological condition: It involves observable (with imaging technology) alterations of brain matter. Jordan Grafman, a senior investigator at the National Institute of Neurological Disorders and Stroke, was born and raised in Chicago, so he knows whereof he speaks when he speaks, politely, about the "paradox" of being a Cub fan even though baseball is supposed to provide relief from life's problems. Grafman has been to a pleasant purgatory, Wrigley Field, and returned with good news: Yes, rooting for the Cubs is a minority taste because it is an interminable tutorial in delayed gratification, but "there is some evidence that being in the majority (everyone loves a winner) reduces reflective thinking."

Rooting for a loser makes one thoughtful, or perhaps neurotic, which on Chicago's North Side may be a distinction without a difference. "The scientific literature," Grafman says, "suggests that fans of losing teams turn out to be better decision-makers and deal better with divergent thought, as opposed to the unreflective fans of winning teams."

Group memberships—in families, tribes, nations, religions—are so common and powerful as components of identities that they must be in some sense natural. That is, they are adaptive aspects of the evolution of humans as social creatures. But how does the group identity of Cub fans help them flourish? By giving them brain calisthenics. Grafman says that "given the complex situations and thinking that Cubs fans have had to engage in," their "frontal lobes are consistently activated" as they think about their affiliation.

Relative to the brains of other animals, human brains have disproportionately large prefrontal cortexes. Hence the human knack for planning, reasoning and experiencing subtle variations of feelings. Grafman says that when a fan's team wins, "the brain's reward system, including the ventral brain stem and basal ganglia," pumps dopamine into the brain, which gives—or perhaps is—the experience of intense pleasure.