Researchers have suggested that kids who are involved in organized activities outside of school - extracurricular activities including art, music, and sports, after-school programs, and youth organizations - do better academically, socially, emotionally, and physically compared to "latchkey" children left alone after school. This fact, along with the fact that those activities also provide adult supervision for youth with working parents, is encouraging policy makers to expand opportunities for all youth to participate in after-school activities.
The opposite concern exists in other researchers concerned about the "over-scheduling" of youth motivated by parents wanting to give a competitive edge to their children by involving them in sports, music, community service, and pushing them to the point of stressing them out and compromising their psychosocial development.
To evaluate these two different perspectives, in the 4th issue of the 2006 Social Policy Report, Mahoney, Harris, and Eccles review evidence that help us understand intrinsic motivation and extrinsic pressure from parents to participate in activities. American youth average about 5 hours a week participating in organized activities, including about 40% do not participate in any organized activities and about 6% who spend 20 or more hours a week participating.
The consensus is that those youth who participate in activities outside school do better academically, complete high school and college more often, are psychologically healthier, have better interactions with parents, and are less likely to smoke or use drugs. The level of involvement however can be a negative influence on development if driven by parents, stressful to the child, or precludes family time (e.g., meals together or time to talk).
The entire report can be found at: http://www.srcd.org/press/mahoney.pdf