Theories should compete. It keeps them fit and trim. Left unchallenged, theories, like people, grow fat and lazy, and they eventually decay into shapeless blobs. Unchallenged theories petrify as “common wisdom,” immobilizing critical faculties with intellectually paralyzing assumptions. A challenged theory is a creative network of conscious and considered ideas, casting its nomological net over new observations and predicting where to look for the next catch.

Psychological theories are rarely made to compete, but they can, and they should, be exercised in this way. Competing theories about determinants of intellectual differences offer a challenging opportunity to test theoretically required predictions with available observations. Predictions about crucial research results are generated by Socialization Theory and by Behavior-Genetic Theory, and those predictions are quite different. The critical observations to test the adequacy of predictions from these competing theories have been made. Yet, there has been little, direct theoretical confrontation. That the theories have not been tested with existing data in a systematic fashion is either a sign of mutual ignorance or an aversion to being challenged.

This chapter presents Socialization and Behavior-Genetic theories of intelligence and evaluates the adequacy of the theories' predictions to account for existing observations. The adequacy of the theories to generate new research is also assessed. By posing a stark contrast between theories, I hope to sharpen the debate and to modify developmental theory to fit existing observations about intellectual resemblance in families and to be more productive in future research.