Whether spanking is detrimental for social-emotional (SE) development remains controversial, mostly due to disputes around the internal and external validity of existing evidence. This study examined the effect of spanking on the SE development of Bhutanese children, using a national, longitudinal sample (N = 1377; Mage = 50.5 months old; 50% girls). Following best-practice recommendations for mitigating issues of selection bias in observational developmental research, the study employed conservative methods (i.e., child fixed-effects and lagged-dependent variables) and robustness checks to assess the internal validity of estimates. Across approaches, spanking predicted reductions in SE skills of .09–.17 SD, even after controlling for all time-invariant confounders and baseline levels of SE skills. These findings strengthen the argument that spanking might be harmful to young children's SE development.