Geschwind's theory of cerebral lateralization: developing a formal, causal model

Geschwind and Galaburda (1987) have proposed a complex and influential model of cerebral lateralization that is based on the argument that increased fetal testosterone levels modify neural development, immune development, and neural crest development. The theory can explain many aspects of cerebral lateralization and its relation to learning disorders, giftedness, and immune deficits. This article clarifies the structure of the theory by presenting it as a causal-path model. The internal coherence of the model is then evaluated by assessing the central concept of anomalous dominance, the role of timing in the articulation of the model, and the invocation of nonlinear processes. Finally, the article considers the problems implicit in testing a "grand" theoretical model and derives some principles for assessing the testability of various predictions, given the practical constraints of sample size and the problems of measurement error.