When Robert R. Provine  tried applying his training in neuroscience to laughter 20 years ago, he naïvely began by dragging people into his laboratory at the University of Maryland, Baltimore County, to watch episodes of "Saturday Night Live" and a George Carlin routine. They didn't laugh much. It was what a stand-up comic would call a bad room.
So he went out into natural habitats — city sidewalks, suburban malls — and carefully observed thousands of "laugh episodes." He found that 80 percent to 90 percent of them came after straight lines like "I know" or "I'll see you guys later." The witticisms that induced laughter rarely rose above the level of "You smell like you had a good workout.""Most prelaugh dialogue," Professor Provine concluded in "Laughter," his 2000 book, "is like that of an interminable television situation comedy scripted by an extremely ungifted writer."He found that most speakers, particularly women, did more laughing than their listeners, using the laughs as punctuation for their sentences. It's a largely involuntary process. People can consciously suppress laughs, but few can make themselves laugh convincingly.
"Laughter is an honest social signal because it's hard to fake," Professor Provine says. "We're dealing with something powerful, ancient and crude. It's a kind of behavioral fossil showing the roots that all human beings, maybe all mammals, have in common."