This paper articulates a citation-based approach to science policy evaluation and employs that approach to investigate the impact of the United States' 2001 policy regarding the federal funding of human embryonic stem cell (hESC) research. We evaluate the impact of the policy on the level of U.S. hESC research, the U.S. position at the knowledge frontier, and the strategic response of U.S. scientists. Consistent with recent research on the science of science and innovation policy, we employ a difference-in-differences approach using bibliometric data with the aim of analyzing the causal impact of the policy on cumulative research. Our estimates suggest that in the aftermath of the 2001 policy, U.S. production of hESC research lagged 35 to 40 percent behind anticipated levels. However, this relative decline was largely concentrated in the years 2001 to 2003 and ameliorated over time. The rebound in U.S. hESC research after 2003 was driven by contributions by researchers at elite U.S. institutions and U.S. researchers who collaborated with international partners. The results suggest that scientists respond strategically to research funding restrictions and that modest science policy shifts can have a significant influence on the within-country composition of research and the pattern of global research collaboration. (Contains 21 footnotes, 6 tables, and 5 figures.)