A psychologist friend once described a series of questions one of her professors had posed to her abnormal-psychology class. "A woman has one cat," she began. "Is that unusual?" Of course not, was the overwhelming consensus. "OK, what about two cats?" she continued. "Would that raise flags about her mental state?" Again the consensus was no. "Three cats?" she asked. A few students expressed a little discomfort with the three-cat threshold. "How about four? Or six? Or 10?" she continued.

For some, three cats suggested that the woman might have attachment issues. Could the 10-cat owner be a hoarder?

The cat escalation concluded with this query: "How many cats indicate crazy?"

Just what kind of crazy 10 cats illustrate might be answered in Sharon Begley's compelling new book, "Can't Just Stop: An ­Investigation of Compulsions."

This ­investigation by the Stat writer and former science columnist for Newsweek and the Wall Street Journal explores the meaning and the neuroscience of some of the hardy perennials of compulsion — obsessive-compulsive disorder, for instance, and hoarding — but also those that might be less debilitating.

When might the selfless act of donating a kidney be considered a compulsion to do good? It would when the donor feels "irresistibly, often inexplicably driven to engage in" this level of selflessness, especially given that this act is, perforce, possible only once.

Is labeling so much of what we do as "compulsions" just another way to pathologize the full spectrum of human behavior?

The answer is clearly no. But one of the strengths of this book is Begley's rigorous clarity about her subject matter. She draws a bright line between our collective and nearly universal smartphone-scanning, computer-dependent actions and those that are evidence of a deeper dysfunction.

"We describe as 'compulsive' someone who reads, tweets, steals, cleans, watches birds, lies, blogs, shops, checks Facebook, posts to Instagram, eats, or Snapchats not only frequently but with the urgency of someone not fully in control of his behavior."

Donating a kidney is an extraordinarily selfless gesture, and yet, Begley notes, for some the compulsion to do this is not simple generosity but the need to make bearable a nearly unbearable mortal ache that is created by anxiety — the unifying emotion shared by all compulsions and one that seems to define the 21st century.

It is hardly surprising that during a time when anxiety is far more common than depression, wild and varied compulsions should define and upend so many lives.

Hoarding, gaming and more

In her vast reporting, Begley introduces us to a compulsive shopper, Sophie, whose attempt to purge the mountains of stuff in her house was cut short by a call from her abusive father — after which she bought eight vacuum cleaners.

We meet a compulsive gamer, Ryan Van Cleave, who changed his name to that of a character in "World of Warcraft," a video game he played up to 80 hours a week, more than 12 hours a day, at the obvious expense of his family and professional life. Only a failed suicide attempt loosened the grip of his gaming compulsion.

And there's Tom Somyak, whose worry about his door being unlocked shaped his waking hours and was only a precursor to his fanatical fear of being unsafe after his son was born. Somyak — like the handwashers, the sidewalk-crack-avoiders, the number-obsessed — suffered from obsessive-compulsive disorder (OCD).

I am sure no one who reads this book will ever again blithely refer to themselves as having "a little OCD," so harrowing are Begley's descriptions of those who "know their thoughts are mad, yet awareness of the madness brings no power over it."

She deftly draws a distinction between OCD and obsessive-compulsive personality disorder (OCPD), whose numbers include the organizers, the meticulous performers, "whose drive for competence has hypertrophied into perfectionism," eventually making them unbearable but utterly self-righteous colleagues, partners and friends.

Interestingly, she explains that for those with OCD, some external force seems superimposed on them, forcing their compulsive actions, but those actions are at odds with their true identity. In contrast, the compulsions of those with OCPD are in weird harmony with their own temperament.

These may be the disorders of our anxious age, but we hardly created them. Begley takes us on a tour of compulsions through history and the fascinating transition as they moved from being seen as a religious phenomenon to being regarded as a medical illness. The grand sweep illustrates the real takeaway from this fine work: While some of us may have 10 cats, in the end, "there is no bright line between mental illness and mental normality."

Marianne Szegedy-Maszak is a senior editor in the Washington bureau of Mother Jones and the author of "I Kiss Your Hands Many Times: Hearts, Souls, and Wars in Hungary."