The Roman philosopher Seneca may have put it best 2,000 years ago: "To be everywhere is to be nowhere." Today, the Internet grants us easy access to unprecedented amounts of information. But a growing body of scientific evidence suggests that the Net, with its constant distractions and interruptions, is also turning us into scattered and superficial thinkers.

The picture emerging from the research is deeply troubling, at least to anyone who values the depth, rather than just the velocity, of human thought. People who read...