CAMBRIDGE, Mass. — People spend 46.9 percent of their waking hours thinking about something other than what they’re doing, and this mind-wandering typically makes them unhappy. So says a study that used an iPhone web app to gather 250,000 data points on subjects’ thoughts, feelings, and actions as they went about their lives.

The research, by psychologists Matthew A. Killingsworth and Daniel T. Gilbert of Harvard University, is described this week in the journal Science.

‘A human mind is a wandering mind, and a wandering mind is an unhappy mind,’ Killingsworth and Gilbert write. ‘The ability to think about what is not happening is a cognitive achievement that comes at an emotional cost.’

Unlike other animals, humans spend a lot of time thinking about what isn’t going on around them: contemplating events that happened in the past, might happen in the future, or may never happen at all. Indeed, mind-wandering appears to be the human brain’s default mode of operation.

To track this behavior, Killingsworth developed an iPhone web app that contacted 2,250 volunteers at random intervals to ask how happy they were, what they were currently doing, and whether they were thinking about their current activity or about something else that was pleasant, […]