These also improved memory and learning, similar to improvements made by environmental stimulation.

BBC: NEWS | Health | Scientists 'reverse' memory loss

Early environmental enrichments that include better nutrition, more exercise and cognitive stimulation from ages 3 to 5 both improves brain functioning at age 11 and also reduces crime at age 23 by 35%.

CNN: Opinion: Unlocking crime using biological keys

Environmental toxins may play a role, watching too much television may play a role, and excessive stimulation may play a role.

NPR: Excerpt: 'Delivered from Distraction'