Gender differences in negotiation behavior—for instance, men's vs. women's likelihood to make (assertive) first offers—contribute to the globally prevalent gender pay gap (GPG). In an attempt to mitigate the social and economic consequences of this gender disparity, we first empirically validated two pre-negotiation message interventions in a pilot study (N = 203). In the main experimental intervention study (N = 585), male versus female participants randomly received this (1) informative message about the GPG, or (2) gender-specific empowering message, or (3) no message in the control condition. In a subsequent negotiation task on the starting salary for a new job, we assessed participants’ (a) likelihood-to-initiate a first offer and (b) first-offer assertiveness. Results showed a remarkably robust behavioral gender disparity: across all conditions, men were more likely to make the first offer (d = 0.178) and made them more assertively (d = 0.339). Importantly, compared to the control condition, the informative (d_inform = 0.304) and the empowering (d_empower = 0.255) pre-negotiation interventions increased women's first-offer assertiveness. Similar intervention benefits emerged for men (d_inform = 0.259; d_empower = 0.284), however, yielding an overall remarkably robust gender difference. To explore the underlying reasons for this gender disparity, we tested four competing psychological mechanisms (i.e., self-esteem, positive and negative affect, GPG awareness, and self-efficacy). Our results highlight the impact that even short, minimal interventions can have on gender differences in negotiation behavior and illustrate which psychological mechanisms explain the emergence of gender disparity in the first place.