This study was conducted to meta-analytically investigate the influence of teaching vs. no teaching expectancy on the learning effects of teaching after preparatory learning. A meta-analysis of 39 studies revealed that a weighted mean effect size for the effect of teaching after studying with or without teaching expectancy vs. merely studying without teaching expectancy on one's learning was g = 0.27, 95% CI [0.15, 0.39]. Most importantly, teaching vs. no teaching expectancy significantly moderated the learning effect of teaching: The learning benefit of teaching after studying with teaching expectancy was nearly medium, g = 0.48, 95% CI [0.34, 0.63], whereas that of teaching after studying without teaching expectancy did not significantly differ from zero, g = - 0.02, 95% CI [- 0.14, 0.11]. This moderator effect was independent of the effects of two possible confounding factors: comparison treatment (the use of a sophisticated or unsophisticated learning strategy) and teaching mode (teaching in written or unwritten mode). An additional meta-analysis of 14 studies also found that the effect of teaching after studying with teaching expectancy vs. merely studying with teaching expectancy on one's learning was significantly greater than zero, g = 0.38, 95% CI [0.17, 0.60], ruling out the possibility that the effectiveness of learning by teaching after studying with teaching expectancy is entirely attributable to the learning effects of preparing to teach (i.e., merely studying with teaching expectancy). These findings suggest that preparing to teach catalyzes learning by teaching.