Low completion rates and increased time to degree at U.S. colleges are a widespread concern for policymakers and academic leaders. Many "full time" undergraduates currently enroll at 12 credits per semester despite the fact that a bachelor's degree cannot be completed within 4 years at that credit-load. The "academic momentum" perspective holds that if, at the beginning of their first year in college, undergraduates attempted more course credits per semester, then overall graduation rates could rise. Using nationally-representative data and propensity-score matching methods to reduce selection bias, we find that academically and socially similar students who initially attempt 15 rather than 12 credits do graduate at significantly higher rates within 6 years of initial enrollment. We also find that students who "increase" their credit load from below fifteen to fifteen or more credits in their second semester are more likely to complete a degree within 6 years than similar students who stay below this threshold. Our evidence suggests that stressing a norm that full time enrollment should be 15 credits per semester would improve graduation rates for most kinds of students. However, an important caveat is that those undergraduates whose paid work exceeds 30 h per week do not appear to benefit from taking a higher course load.