This synaptic pruning, as it is called, causes the brain's cortex—the outer layer of gray matter where we do much of our conscious and complicated thinking—to become thinner but more efficient.
Taken together, these changes make the entire brain a much faster and more sophisticated organ.
This let the adults use a variety of brain resources and better resist temptation, while the teens used those areas less often and more readily gave in to the impulse to look at the flickering light—just as they're more likely to look away from the road to read a text message.
If offered an extra reward, however, teens showed they could push those executive regions to work harder, improving their scores.
To succeed, you must override both a normal impulse to attend to new information and curiosity about something forbidden. Ten-year-olds stink at it, failing about 45 percent of the time. In fact, by age 15 they can score as well as adults if they're motivated, resisting temptation about 70 to 80 percent of the time.
What Luna found most interesting, however, was not those scores.
It was the brain scans she took while people took the test.
Compared with adults, teens tended to make less use of brain regions that monitor performance, spot errors, plan, and stay focused—areas the adults seemed to bring online automatically.
Your instructions are to not look at the light and instead to look in the opposite direction. It's a tough assignment, since flickering lights naturally draw our attention.
Stress, fatigue, or challenges can cause a misfire.
Abigail Baird, a Vassar psychologist who studies teens, calls this neural gawkiness—an equivalent to the physical awkwardness teens sometimes display while mastering their growing bodies.
These studies help explain why teens behave with such vexing inconsistency: beguiling at breakfast, disgusting at dinner; masterful on Monday, sleepwalking on Saturday.
Along with lacking experience generally, they're still learning to use their brain's new networks.