India, Oct. 10 -- We laugh at the same jokes (usually for the same reasons), blush at innuendo, bow to dictators (at least temporarily), all because of a sense of implicit understanding: ie, common knowledge. We maintain social contracts such as friendships and kinship, and group together as mobs, based on common knowledge too.

The idea of common knowledge is key to understanding human social constructs, says Steven Pinker, 71, a Canadian psychologist and psycholinguist at Harvard University.

"As a cognitive scientist, I have spent my life thinking about how people think," Pinker adds. "So the ultimate subject of my fascination would have to be how people think about what other people think, and how they think about what other people th...