Researchers are studying a pervasive psychological phenomenon in which an individual's mind wanders.
Mainstream psychology has not paid much attention to this common mental habit. But a spate of new studies is chipping away at its mysteries, and scientists say the topic is beginning to gain visibility.
Someday, such research may turn up ways to help students keep their focus on textbooks and lectures and drivers to keep their minds on the road. It may reveal ways to reap payoffs from the habit.
And it might shed light on attention deficit hyperactivity disorder, which can include an unusually severe inability to focus that causes trouble in multiple areas of life.
More generally, scientists say, mind-wandering is worth studying because it is just too common to ignore.
Michael Kane, a psychologist at the University of North Carolina at Greensboro, sampled the thoughts of students at eight random times a day for a week. He found that on average, they were not thinking about what they were doing 30 percent of the time.
For some students it was between 80 percent and 90 percent of the time. Out of the 126 participants, only one denied any mind-wandering at the sampled moments.
Prior work has also turned up average rates of 30 percent to 40 percent in everyday life.
"If you want to understand people's mental lives, this is a phenomenon we ought to be thinking about," Kane said.
A lot of mind-wandering is harmless, of course, as when you think about a work problem while munching a cheeseburger. The problem comes when it distracts you from something you should be paying attention to.
The result of that can be tragic. Kane noted the 2003 case of a college professor who drove to work in Irvine, California, one hot August day, parked and went to his office. Whatever was going through his mind, he had lost track of the fact that his 10-month-old son was in the back seat. The boy died in the heat. In 2004, virtually the same thing happened in Santa Ana, California.
A more common task that demands concentration is reading. Even here, people's minds wander 15 percent to 20 percent of the time, said Jonathan Schooler of the University of British Columbia in Vancouver, Canada. And they often don't realize it, he said.
He and colleagues had college students read passages from War and Peace and other books. The volunteers pushed a button every time they noticed their thoughts straying, and that happened regularly, Schooler said.
But more surprisingly in such experiments, when the volunteers are interrupted at random times and asked what they are thinking, "we regularly catch people's minds wandering before they've noticed it themselves," Schooler said.
And these stealth episodes appear to hamper reading comprehension, he said.
In Kane's study, scheduled for publication later this year, volunteers carried devices that beeped at random times and asked questions about their thoughts. Most of the time when caught mind-wandering, the students said they had deliberately stopped focusing on what they were doing.
Their wandering thoughts trained more on everyday things than on fantasies, and much more than on worries. That's similar to what previous studies have found.
"A lot of what they're reporting is ... mental to-do lists," Kane said.
But what leads to this?
"The mind is always trying to wander, every chance it gets," Schooler said.
In his view, the mind has not only the goal of achieving whatever task we are focused on, but also personal goals simmering outside of our immediate awareness. These are things like making plans for the future, working out everyday problems and better understanding oneself. Sometimes, one of these goals hijacks our attention. And so our mind wanders.
Brain-scanning evidence links mind-wandering to basic operation of the brain. Malia Mason of Harvard's Massachusetts General Hospital and colleagues recently reported that mind-wandering taps into the same circuitry that people use when they're told to do nothing -- when their brains are on "idle."
Schooler, who is studying brain-wave activity associated with mind wandering, welcomes what he sees as a surge of interest in the topic. He and others say there's plenty to learn.
One goal is finding ways to help people realize when their mind is wandering and bring it under control, Schooler said. He plans to test whether meditation training might help.
But there's even a more basic question, he said. Why is the brain wired to wander? What could possibly be good about that?
"Mind-wandering is probably more often helpful than harmful," Kane said.
For one thing, the cost is low: Despite notable exceptions, life usually doesn't demand our full attention.
"A lot of human daily life is autopilot," he said. "There's a whole lot of what we need to do that we can do without thinking about it, from driving to eating ... We do occasionally miss that turn on the way home, but we get through the day pretty well."
Given that, a mechanism that encourages us to devote some idle brain capacity to planning and solving problems "seems like a pretty good use of time," he said.
Schooler is exploring the idea that mind-wandering promotes creativity.
"It's unconstrained, it can go anywhere, which is sort of the perfect situation for creative thought," he said.
Mason points out that just because the human brain wanders doesn't necessarily mean there's a good reason for it. Maybe, she said, the mind wanders simply because it can.
But even she sees an upside.
"I can be stuck in my car in traffic and not go absolutely crazy because I'm not stuck in the here and now," she said. "I can think about what happened last night. And that's great."