Is it advantageous to make the first offer and to do so ambitiously? Although initial studies suggested clear advantages across cultures and contexts, recent findings have challenged the robustness of this first-mover advantage. A preregistered meta-analysis of 374 effects from 90 studies (Study 1; N = 16,334) revealed three beneficial effects of making the first offer: (a) a general first-mover advantage (g = 0.42, m = 80), (b) a positive correlation between first-offer magnitude and agreement value (r = 0.62, g = 1.56, m = 53), and (c) an advantage of ambitious (vs. moderate) first offers on agreement value (g = 1.14, m = 187). The meta-analysis also identified two detrimental outcomes of ambitious first offers: (d) fewer deals (i.e., more impasses; g = −0.42, m = 13) and (e) worse subjective value experienced by recipients (g = −0.40, m = 41). Two preregistered experiments (Study 2a-2b; N = 2,121) replicated both the beneficial and detrimental meta-analytic effects and simultaneously tested multiple psychological mechanisms driving these effects. Across the experiments, selective accessibility drove the effect of first-offer magnitude on counteroffers, while anger drove the effects on impasses and subjective value. Across both the meta-analysis and the experiments, negotiation complexity moderated both the beneficial and detrimental effects of first offers; as the number and type of issues (i.e., complexity) increased, the effects of first offers became smaller, and the mechanisms changed. Overall, the current meta-analysis and experiments collectively illuminate the direction, size, psychological pathways, and boundaries of first-offer effects in negotiations.