Identified groups of economically disadvantaged children showing high externalizing behavior in first grade that persisted (persistent) or decreased (improver) in third grade, or low first- grade levels that were stable (unproblematic) or increased (new problem) in third grade. Found that verbal ability, behavioral impulsivity, parent maladjustment, and harsh parenting distinguished persistent problem and unproblematic groups. Family instability related to change for improver and new problem groups. (Author/KB)