Students who take too long to earn bachelor's degrees are the frustration of parents, college leaders and policy makers alike -- who see the six-year bachelor's degree (or longer) as being more expensive for all involved, and particularly wasteful when many campuses are bulging due to increased enrollments.

A new study (abstract available here) from the National Bureau of Economic Research suggests that the growth in the length of time needed to earn bachelor's degrees is indeed real and cause for concern. But the study finds that the shift, over recent decades, to longer time-to-degree rates is not uniform across colleges, but is concentrated among students who enroll at less competitive four-year public institutions and at community colleges. Further, the analysis finds likely links between longer time-to-degree rates and resources, both of institutions and of students.

The implication of the study, the authors write, is that those who want students to graduate more promptly need to talk about money. "Our finding of increased stratification in resources among colleges and universities -- both between publics and privates and within the public sector -- suggests that the attenuation of resources at less-selective public universities in particular limits the rate of degree attainment," write the authors, John Bound of the University of Michigan, Michael F. Lovenheim of Cornell University, and Sarah Turner of the University of Virginia.

The authors start by verifying the widely-held view that too many students take longer than people expect (four years) to earn a bachelor's degree. Using databases that track students over time, they write that of those who graduated from high school in 1972, 58 percent of those who eventually earned a bachelor's degree did so within four years of finishing high school, which is what many consider to be "on time." For the class that graduated in 1992, only 44 percent did so. Then the authors examine time-to-degree rates by sector and find relatively little change among private colleges or among top public universities. But among public institutions not considered "top 50," the authors find a contrast.

Among those at the top-ranked publics, 55.5 percent finish in four years. At all other state and local institutions, the share is only 34.7 percent.

This raises the question of why, and the authors explore various options. One theory -- frequently advanced by those who question the goals of having more Americans earn college degrees -- is that those coming into higher education outside of competitive colleges are less well prepared, and so are unable to move ahead in college at expected rates. But the authors find no evidence for this (based either on the courses students have taken before college or on their performance in college), and reject this theory.

They do find evidence of links between various measures of resources and time to degree. For example, while acknowledging that any one resource measure may be imperfect, they examine student-to-faculty ratios. During the period studied, student-faculty ratios increased overall in public institutions from 25.5 to 29.8 to 1. But at the top 50 institutions (and at private colleges), the ratios decreased, meaning that the increase everywhere else was larger. Other measures as well, the authors write, suggest that the institutions that have preserved time-to-degree rates are those with relatively more resources.

As another illustration of the resource impact, the authors focus on states that experienced enrollment increases. Given that state appropriations frequently (and consistently in recent years) have lagged such increases, they speculate that enrollment increases decrease resources per student, and thus could increase time to degree. And that's what they find: For every 1 percent increase in a state's population of 18-year-olds, time to degree increases by 0.71 years. For those outside the top 50 institutions, the increase is greater -- 1.11 years -- again suggesting that states find ways to provide more to the more competitive institutions.

Resource gaps also extend to the students at the different types of institutions -- with those outside of the elite institutions more likely to work longer hours in jobs, limiting the time they can devote to their educations.

"The sum total of our evidence points strongly toward the central role of declines in both personal and institutional resources available to students in explaining the increases in time to baccalaureate degree in the U.S.," the authors conclude. "That these increases are concentrated among students attending public colleges and universities outside the most selective few suggests a need for more attention to how these institutions adjust to budget constraints and student demand and how students at these colleges finance higher education."