Genetically informative data can be used to address fundamental questions concerning the measurement of behavior in children. The authors illustrate this with longitudinal multiple-rater data on internalizing problems in twins. Valid information on the behavior of a child is obtained for behavior that multiple raters agree upon and for rater-specific perception of the child's behavior. Rater-disagreement variance varsigma2(rd) accounted for 35% of the individual differences in internalizing behavior. Up to 17% of this varsigma2(rd) was accounted for by rater-specific additive genetic variance varsigma2(Au). Thus, the disagreement should not be considered only to be bias/error but also as representing the unique feature of the relationships between that parent and the child. The longitudinal extension of this model helps to make a distinction between measurement error and the raters' unique perception of the child's behavior. For internalizing behavior, the results show large stability across time, which is accounted for by common additive genetic and common shared environmental factors. Rater-specific shared environmental factors show substantial influence on stability. This could mean that rater bias may be persistent and affect longitudinal studies. Copyright (c) 2008 APA.