Three studies compared learning and problem-solving performances of normal and familially-retarded children on tasks differing in complexity, and one study investigated motivational-personality differences. Main purpose of the first three studies was to investigate the controversy between developmental and defect theorists in mental retardation. Study 1 revealed no normal-retarded differences on conservation tasks of unequal difficulty and offered support for the developmental theorists. Studies 2 and 3 also revealed no normal-retarded differences on tasks of low complexity but differences did become apparent on more complex tasks. Normals were better able to eliminate irrelevant dimensions and respond to the relevant dimension in the learning tasks of Study 2 and showed more efficient use of information for generating problem-solving strategies in Study 3. Interpretation of findings was difficult because appropriate equating for socio-economic backgrounds of subjects was possible only in Study 1. Study 4 showed that the personality variables Achievement Need and Locus of Control are significant determiners of probability learning performance. It was concluded that findings had value for any theory of intellectual and motivational systems, but did not resolve the developmental-defect controversy. (Author/KW)