MRI brain scans of 5 to 25 year olds show that brain density decreases over that time period. Seems backwards, right?  Think of the trimming of the adolescent brain as the trimming of the monster HBO package that my parents bought when they first set up cable. Eventually they had to pick out which channels they didn’t use and get rid of them to reduce expenses. Similarly the developing brain contains many more brain cells (tv channels) than any person uses. So the unused brain cells are removed to reduce expenses on your body. The decrease in brain density seen in MRI scans is due to the death of unused brain cells.  It’s use ‘em or lose ‘em.
The pruning of unused neurons happens mainly in the brain’s gray matter during adolescence. The thought patterns used during adolescence are crucial to long-term personality, as the circuitry carries over into adulthood — it’s more than habits, it’s hard-wiring. This finding places an even greater weight on understanding and combating addiction, anti-social behavior, and depression in teens.  The most highly hit areas are in the forebrain, which is responsible for reasoning, logic, and decision-making. It’s not using this logic to get good SAT scores that concerns me, but rather having a solid foundation with which to make informed decisions and take solid risks. This is why we need to invest in research. Education is not like a car — it never depreciates.