THE DEVIL you know is better than the devil you don't, or so we're told. But how the devil do you find who the devil is? On this, the ancient epigrammatists are silent.

Enter Harvard psychologist Martha Stout, whose nerve-janglingly titled "The Sociopath Next Door" (Broadway Books, 2005) claims that "1 in 25 ordinary Americans secretly has no conscience and can do anything at all without feeling guilty." (No word on how many in 25 are shamelessly flaunting this fact.)

Anyway, Stout's book examines the ice-cold veins of several noted serial killers but also the subtler sociopaths, who terrorize spouses and family members in ways that, though insidious, won't ever land them on the front page. The author is even more illuminating on the subject of their neighbors. You know -- those folks who later say they never suspected a thing. In a brisk 218 pages, Stout argues persuasively that though evil has a human face, it's not just any old face, there being certain behavioral cues that the alert neighbor can learn to identify.

None of this would have anything to do with "Othello" at Shakespeare Theatre, were it not for Iago -- the conscienceless villain at its core -- and the actor playing Iago, Patrick Page, who brought Stout's book to director Michael Kahn's attention well before rehearsals began.

"I think Michael and I were having lunch at one point last year, and he said, 'Do you think Iago's a psychopath?' " Page recalls. "I said, 'I don't know, because I don't know what the word means. I've got to find out.' "

Thus began Page's year-long odyssey. "I've probably read 30 books on the subject," he says, discovering, among other things, that the issue of what to call these people, socio- or psychopaths, is a matter of some dispute. Jeffrey Dahmer, John Wayne Gacy and Ted Bundy are closely linked in the public imagination, but "there are very, very different patterns of behavior." Some psychopaths prey on children, others on the denizens of society's margins. Still others take their violence to the workplace.

That last would be Iago, whose evil without credible motive has been the subject of debate for 400 years. Yes, it stings to be passed over for a promotion, as Iago is in favor of Cassio (Gregory Wooddell). But most of us weather such storms without becoming homicidal. Shakespeare's great villain, instead, launches a revenge plot of cool brutality, one all the more terrifying for the near total lack of suspicion it arouses.

"It's astonishing to what degree he fits in as a psychopath," Page says, "and how Shakespeare seems to have known every single aspect of behavior that would manifest itself." Page notes the ghoulish yet persuasive charisma. ("The intense charm of people who have no conscience," Stout writes, "has been observed and commented on by countless victims.") Also the guilt-free stabbing of co-workers in the back -- in this case literally -- and telling "lethal premeditated lies to people who trust you" (Stout again). And on the evidence of the Bard's "drown cats and blind puppies" line, Iago's childhood history may even have included the torturing of animals, Page says, another of Stout's sociopathic alarm bells.

But locating Iago in the DSM-IV is far from the point, of course. What "Othello" affords is the rare chance to ponder such psychology from a safe distance and to meditate on the essential unknowability of other people.

"And that's scary," Page says. "The case I've been following with near-obsession, to my wife's great despair, is the BTK case in Wichita. What is most interesting to me is that Dennis Rader lived for 30 years . . . his first murder was 31 years ago, and he had in that entire time a stable marriage. We don't know how happy it was, but he lived with his wife and raised two children during that period of time. Apparently they had absolutely no clue, nor did anyone have a clue."

It's that strangers-among-us theme that sells books like Stout's and tickets to plays like "Othello." Though 96 out of 100 people are essentially harmless, we remain mesmerized by those other four who "live in a permanent and absolute moral nighttime," writes Stout -- undetected, unknowable and possibly next door.

"There have been nightmares," says Page of his somewhat macabre reading list of late. ("You can ask me about pretty much any serial killer, and I can give you every single detail that you will ever, ever find.") But the actor is cultivating a safe distance from Iago, too. The proper enjoyment of "Othello," it seems, for audiences and actors alike, depends on a fascination with both "doing anything you want as long as you could get away with it," and "letting go of it when the curtain goes down."

Unbeknownst to Othello (played by Avery Brooks, left), Iago (Patrick Page) is the face of evil in the drama "Othello."