Reducing climate change requires persuading people about climate change and climate solutions. The nearly universal way of evaluating which climate and environmental messages are most effective is to calculate and compare average treatment effects (ATEs). The problem with the ATE, however, is that it does not describe the underlying pattern of persuasion: whether effect size is better predicted by how many people are persuaded (i.e., breadth), or how much they are persuaded (i.e., depth). Here, we investigate this in a preregistered meta-analysis of our experimental data archive, avoiding publication bias (14 experiments; 94 treatments; total N = 41,265). We find that as treatment effects increase, the breadth and depth of persuasion both increase. However, the predictive power of breadth (β = 0.47, 95 % CI [0.29, 0.65]) is far stronger than that of depth (β = 0.10, 95 % CI [0.04, 0.15]). We also find that breadth and depth are only weakly correlated (β = 0.23, 95 % CI [0.003, 0.46]), confirming they are distinct. Additionally, as average treatment effects increase, we also find that negative (i.e., backlash) effects decrease (β = −0.30, 95 % CI [-0.46, −0.15]), suggesting that larger ATEs are a function of increased persuasion and decreased backlash. Through three case studies, we show how these results generalize and when they can be audience-dependent. These results have important implications for both scholars and practitioners of climate and environmental communication.
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