Sources of heterogeneity within the process of R&D investment, such as international differences in the maximum per-period rate of investment and level of regulatory uncertainty, offer a plausible explanation for US comparative advantage in biotechnology. Using dynamic stochastic simulation, the results presented in this paper suggest US biotechnology firms may initiate more R&D projects, innovate earlier and more rapidly, persevere longer in the face of mounting R&D costs, and successfully complete more R&D projects than European firms.