This study examined the effects of unsubsidized federal Stafford loans on six-year degree attainment at 4-year colleges and universities in the U.S., and how these differentially impact students across income groups. For this, nationally representative data from the Beginning Postsecondary Students (BPS:04/09) and the Integrated Postsecondary Education Data System (IPEDS) was merged to analyze N=6,561 students attending n=651 four-year institutions. To account for possible selection bias in loan borrowing and the nested data structure, this study employs a propensity score matching, multilevel modeling approach. In addition to financial aid measures, the analytic model draws from the heterogeneous research approach and incorporates students' college experiences and institutional context to estimate effects. Results reveal a comparatively large, negative association with the outcome measure for low-income students. For every $1,000 borrowed additionally in unsubsidized loans, low-income students are found to be 5.45% (p<0.000) less likely to graduate within six years. Beyond that, results show no significant effect for other income groups. Findings provide further support for the need to monitor student borrowing behavior, creating a more targeted approach to student aid that minimizes the use of unsubsidized loans for low-income students, and more closely examining differential effects of aid types across various student groups.