Using archival data, we examine the effects of the Hubble Space Telescope Time Allocation Committee (HST TAC)’s decision to adopt a dual- rather than single-anonymous review process. The change involved removing, to varying degrees, information about the Principal Investigator (PI) with the goal of reducing bias against women. Proposals led by female PIs were significantly more likely to be accepted in the five cycles following the changes compared to the 11 cycles using a single-anonymous review system. Taking a closer look at why these changes emerged, we examined data at the reviewer-level in the cycle immediately preceding the change compared to three of the cycles after the change. We found that male reviewers rated female PIs significantly worse than they rated male PIs before, but not after, dual-anonymization was adopted.