Studies have yielded mixed findings on the helpfulness determinants of online reviews in tourism and hospitality. To address this issue and unveil the overall sizes of helpfulness determinants, this research presents a systematic bare-bones meta-analysis of the six most investigated helpfulness determinants based on 86 effect sizes from 27 primary studies. Results reveal that the corrected mean effect sizes of four significant determinants – review length, reviewer expertise, review age, and profile disclosure – are 0.218, 0.064, 0.053, and 0.036, respectively; the effect sizes of two other determinants, review valence and readability, appear insignificant. Subgroup analyses also highlight several moderating factors related to effect sizes: service type, year of data collection, and helpfulness measurement. Overall, reviewer expertise and profile disclosure show larger effect sizes for restaurants (vs. hotels). The effect sizes of review valence and reviewer expertise are also found to decline over time while that of review profile increases.