The literature on business ethics, corporate social responsibility and sustainability includes many studies on gender differences, however the results are often contrasting. In particular, there has not yet been full agreement on the role and significance of gender differences in customer expectations and perceptions of responsible corporate conduct. An extensive review of the research on the subject reveals that the published studies have not dealt with the substantive significance of gender differences, and as a result, the size of such differences is unknown. The current study analyses both the statistical and the substantive significance of gender differences in customer expectations and perceptions of corporate responsibility, also examining the influence of age and education. The analysis is carried out on a remarkably large sample of 908 clients, pertaining to 12 of the largest Italian banks. The overall resultsshow that there is a small substantive difference between women's and men's average expectations, with women showing higher average values than men. This result holds generally true with variation of education levels and age groups. Young females show the highest average values of expectations, and the significance of gender differences decreases with age. In contrast, the perception of corporate social responsibility does not show significant gender differences, either at the general level or when the analyses are repeated by education levels and age groups. The paper introduces the use of a standardized measure of effect size for analysing the substantive significance of gender differences in customer expectations and perceptions of corporate social responsibility. This novel approach increases results reliability and, accordingly, the effectiveness of company or policy maker strategies in designing, planning, implementing and assessing sustainability initiatives.