We propose a novel approach to learn about sensitive periods in the formation of socioemotional skills in which returns to investments into skills are especially high. In particular, we implement the same investment in different school grades and study grade-specific treatment effects on children's self-control, patience, and prosociality. Based on a randomized controlled trial with about 3,200 children in grades 2 to 5 of 135 elementary schools in Bangladesh, our results indicate sensitive periods in the formation of self-control and patience around the ages 7 and 8, while prosociality appears to be similarly malleable throughout ages 7 to 11.
