What explains trends in labor supply among U.S. undergraduates, 1970-2009?

"Recent cohorts of college enrollees are more likely to work, and work substantially more, than those of the past. October CPS data reveal that average labor supply among 18 to 22-year-old full-time undergraduates nearly doubled between 1970 and 2000, rising from 6 hours to 11 hours per week. In 2000 over half of these "traditional" college students were working for pay in the reference week, and the average working student worked 22 hours per week. After 2000, labor supply leveled off and then fell abruptly in the wake of the Great Recession to an average of 8 hours per week in 2009. This paper considers several explanations for the long-term trend of rising employment-including compositional change and rising tuition costs-and considers whether the upward trend is likely to resume when economic conditions improve"--National Bureau of Economic Research web site.