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** p≤0.10, * p≤0.05, Wilcoxon test. In experiment 1 (a), we used two-tailed statistics. Based on the directional results of experiment 1 and those of [12], we had a priori predictions bonobos are spontaneously and preferentially prosocial toward strangers. Therefore, in experiment 2 (b) and 3–4 (c) we were justified to use one-tailed statistics.
