We review the literature on the gender gap on concept inventories in physics. Across studies of the most commonly used mechanics concept inventories, the FCI and FMCE, men's average pretest scores are always higher than women's, and in most cases men's posttest scores are higher as well. There is sometimes a gender gap on commonly used E&M concept inventories, the BEMA and CSEM, but it is usually much smaller and sometimes favors women. However, these tests have far fewer gender gap studies and more variation in the existing studies.

Based on our analysis of 26 published articles comparing the impact of 30 factors that could potentially influence the gender gap, no single factor is sufficient to explain the gap. Several high-profile studies that have claimed to account for or reduce the gender gap have failed to be replicated, suggesting isolated claims of explanations should be interpreted with caution. For example, claims that the gap could be eliminated through interactive engagement teaching methods or through a "values affirmation writing exercise" were not supported by subsequent studies. Suggestions that the gap might be reduced by changing the wording of "male-oriented" questions or refraining from asking demographic questions before administering the test are not supported by the evidence. Other factors, such as differences in background preparation, scores on different assessments, and splits between responses to questions when answering for themselves or for a "scientist" do contribute to the gap, but their size is smaller than the size of the overall gender gap, suggesting that the gap is likely due to the combination of many small factors rather than one factor that can easily be modified.