Washington ? Your parents were right: Don’t study with the TV on.

Multitasking may be a necessity in today’s fast-paced world, but new research shows distractions affect the way people learn, making the knowledge they gain harder to use later on.

The study, in Monday’s Proceedings of the National Academy of Sciences, also provides a clue as to why it happens.

“What’s new is that even if you can learn while distracted, it changes how you learn to make it less efficient and useful,” said Russell A. Poldrack, a psychology professor at the University of California, Los Angeles.

That could affect a lot of young people. A study by the Kaiser Family Foundation last year found third-graders through 12th-graders devoted, on average, nearly 6 1/2 hours per day to TV and videos, music, video games and computers.

As Poldrack explains it, the brain learns in two different ways. One, called declarative learning, involves the medial temporal lobe and deals with learning active facts that can be recalled and used with great flexibility. The second, involving the striatum, is called habit learning.

For instance, in learning a phone number you can simply memorize it, using declarative learning, and can then recall it whenever needed, Poldrack explained.

A second way to learn it is by habit, “punch it in 1,000 times, then even if you don’t remember it consciously, you can go to the phone and punch it in,” he said.

Memorizing is a lot more useful, he pointed out. “If you use the habit system, you have to be at a phone to recreate the movements.”

The problem, Poldrack said, is that the two types of learning seem to be competing with each other, and when someone is distracted, habit learning seems to take over from declarative learning.