Many social science theories that examine the connection between beliefs and behaviors assume that belief constructs will predict behaviors similarly across development. Converging research implies that this assumption may not be tenable across all ages or all belief constructs. Thus, to test this implication, the relation between behavior and beliefs about the self was examined in 2 independent data sets with 2 different constructs: aggression and achievement. The respondents were 6-18 years of age and predominately Caucasian. Results using quasi-simplex structural equation models suggest that self-beliefs become more strongly related to behavior as children grow older independent of the reliability of the measures used. Possible limitations in the use of self-report methodology with young children are discussed.