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This paper empirically evaluates whether Environmental Impact Statements (EISs) for oil and natural gas field development projects lead to a significant reduction in environmental impacts. Based on our statistical analysis of projects within a four-state region, we conclude that EIS preparation does appear to produce final decisions that are substantially less impactive on the environment when compared to initially proposed projects. Impact reductions occur primarily between the Draft EIS and Final EIS, with minor reductions occurring between the Final EIS and Record of Decision. While reductions may be partially attributable to other legal requirements (such as Clean Air Act, Clean Water Act, or Endangered Species Act compliance or intervening economic and technological factors), external factors alone do not adequately explain impact reductions. We also found that the number of alternatives considered within an EIS affects the magnitude of impact reduction. EISs that consider a broader range of alternatives are more effective at reducing environmental impacts.