The average length of time spent earning a bachelor’s degree has been steadily rising among American university students for the past 30 years, and the change is not entirely explained by a consideration of part-time and returning students, an economist says.

Extended graduation dates means higher costs to students, a fact that should provide most with an incentive to finish up early or on time. So what’s going on?

Among other reasons, Judith Scott-Clayton points to overcrowding brought on by circumstances in which university resources are not equal to student needs. Alongside nationwide budget cuts, a steady, predictable rise in enrollment has impaired colleges’ abilities to expand programs and make faculty hires when necessary. –ARK