Multitasking less efficient, easier for young people

Multitasking less efficient, easier for young people:

TIME magazine covers what they call “Generation M” – M for multitasking – made up of overstimulated teenagers who constantly split their time and attention between instant messaging, Facebook, Myspace, their iPods, cell phone and blogs.

Psychologists say this kind of multitasking is easier for young adults than children or older people because of development in a certain area of the brain, and is also a lot less efficient than focusing on one task at a time.

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if human brains adapt or can be habituated to certain mental conditions, which in my experience is clearly true, within certain frameworks of capacity, then it seems clear to me that if you train a brain to multitask efficiently, it could become less efficient at other things, that is… if there is a trajectory model of adaption which is implied by limited capacities. Thoughts?