Research has shown that differences among ordinary people in intelligence and personality depend equally on individual genetic variability and on differences in the environments that siblings experience within the same family, not differences in the neighborhood, school, and community environments. As of yet, there are no adequate theories to explain how the environment works to affect personality and intellectual differences among people. Present interventions are based on naive environmentalism. If it is known what variations in environment make a difference, intervention programs can be better designed and implemented. In planning intelligent interventions, lumping human talents into a single domain may be useful, especially if focused on the practical concept of social competence. Theories on the role of intelligence and personality can be tested by the usefulness of the interventions they imply. Specific behaviors at specific times in people's lives can be changed, but the outlook for more pervasive aspects of people's functioning is pessimistic. A modest theory of general intellectual and personality development and of robust individual differences that underlie important human affairs, with the pessimism that implies about the effects of situationally-bound and time-bound interventions may be useful. (ABL)