This work tackles the measurement invariance of the social cognition construct when different observers, age and participant’s age are considered. This is a prior question that needs to be answered before attributing discrepancies in information coming from diverse sources just to the varying behavior occurring across setting, and mainly interpret the discrepancies as indicative of cross-contextual variability. The article also studies the link between discrepancies and source-specific information and the validity of that information to predict several outcomes. The measurement invariance across sex, time and informant of a social cognition measure applied to children’s parents and teachers was longitudinally tested in a Spanish general population sample, at ages 5 (N = 581) and 10 (N = 438). Full or partial metric and scalar equivalence were found across sex and over time within informants. Partial scalar invariance was not obtained across informants. Latent class analysis identified 2 classes of difficulties in social cognition for both informants at both ages: low social cognition and high social cognition. Comparison of classes resulting predicting outcomes yielded differential predictions due not only to varying context but also to a different concept of social cognition across informants. In general, significant differences between raters were informant dependent. We conclude that it is important to consider both teachers’ and parents’ observations to fully understand the construct of social cognition.