Investigated were similarities and differences in the ability of 26 normally developing and 26 conduct-disordered children and adolescents to comprehend psychologically defensive behavior and the cognitive processes underlying differences due to age. Matched by cognitive level, subjects viewed vignettes depicting another child behaving defensively. Their answers to standard questions were evaluated against Chandler and his colleagues' Piagetian logical/structural analysis of eight defense mechanisms in three categories of increasing logical complexity: inversion (repression and denial); reciprocity (rationalization, displacement, turning against the self, reaction formation); and negations of joint propositional statements (projection and introjection). It was hypothesized that (1) if logical/analytical reasoning is the necessary prerequisite for comprehending irrational/defensive behavior, then normal and disturbed youngsters matched for logical/analytical ability ought to perform equivalently and (2) the ability to comprehend psychological defenses of increasing logical complexity ought to be systematically related to cognitive stage level for both normal and disturbed subjects. Contrary to Chandler and colleagues (1978), results indicated that although both normally developing and disturbed formal operational subjects found reciprocals easiest, the disturbed found negations of joint propositions easier than inversions, and the normally developing found inversions easier than negations of joint propositions. Implications are discussed. Two pages of references conclude the document. (RH)