We assessed the extent to which feelings of sympathy and aggressive behaviors co-developed from 6 to 12 years of age in a representative sample of Swiss children (N = 1,273). Caregivers and teachers reported children's sympathy and overt aggression in three-year intervals. Second-order latent curve models indicated general mean-level declines in sympathy and overt aggression over time, although the decline in sympathy was relatively small. Importantly, both trajectories were characterized by significant inter-individual variability. A bivariate second-order latent curve model revealed a small–moderate negative correlation between the latent slopes of sympathy and overt aggression, suggesting an inverse co-developmental relationship between the constructs from middle childhood to early adolescence. In terms of predictive effects, an autoregressive cross-lagged model indicated a lack of bidirectional relations between sympathy and overt aggression, underscoring the primacy of the variables’ rank-order stability. We discuss the co-development and developmental relations of sympathy and aggression, their potential conjoint social-emotional mechanisms, and the practical implications thereof
