1. The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reverse sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. 2. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this discrepant body of evidence in the primary literature. 3. Using pre-registered methods and hypotheses, we performed a systematic review and meta-analysis combining published (n = 19 studies, k = 106 effect sizes) and unpublished data (n = 17, k = 242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand sources of heterogeneity across effects. 4. Meta-analysis revealed an overall positive correlation between male size and fitness proxies (r = 0.23, 95% confidence interval: 0.10 – 0.35, n = 36, k = 348, 4514 males, three Gambusia species). Despite high heterogeneity, the large male advantage appeared robust across all fitness proxies studied (i.e. female choice, mating success, paternity, sperm quantity and quality), but was considerably larger when the fitness proxy was female choice (r = 0.43, 95% confidence interval: 0.28 – 0.59, n = 14, k = 43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status, and environmental conditions. We found evidence of publication bias; however, its effect on our estimates was attenuated by including a substantial amount of unpublished data, highlighting the importance of unpublished (open) data for accurate meta-analytic estimates. 5. Our study underscores the need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.

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