Read the recent testimony of Robert E. Hall (pdf):
Most of the decline in participation occurred among teenagers and young adults. The finding that these effects tend to be larger in more prosperous families points strongly away from much of a role for rising influence of benefit programs, because these programs, especially food stamps, are only available to families with incomes well below the median.
So what is going on here? Could it be “culture”? Hall cites, suggestively, time use surveys showing that sleep and personal consumption of video are up strongly.