We add households with heterogeneous discount factors and constrained credit to a research and development (R&D)-based endogenous growth model. Borrowers' access to credit has profound implications for growth. The direction and magnitude of this effect depend on preferences over labor supply. If labor supply is highly elastic and households do not smooth their labor supply between labor that produces output and R&D, annual growth decreases from 11.6% to approximately zero as the debt-to-capital ratio rises from 0 to 1.38. If households instead have a strong preference for smoothing their labor supply, then growth increases from 2.91% to 3.83% as the debt-to-capital ratio rises from 0 to 1.55. In both cases, less elastic labor supply weakens these effects. The results are similar if existing ideas do not affect the creation of new ideas. Now, when households do not smooth their labor supply, less debt results in faster growth, and productivity and output converge to much higher values.