The main tenet of the GBG model relates to the prenatal action of testosterone. Anomalies in cerebral dominance, immune functioning, abilities, and neural crest development are hypothesized to correlate with each other because all result from high levels of prenatal testosterone. Studies directly evaluating the effect of testosterone on these traits do not validate the model: sex ratios and animal studies suggest that testosterone has a protective, rather than facilitatory, effect on autoimmune diseases; individuals with high levels of early testosterone do not have elevated rates of left-handedness or learning disabilities. These findings reinforce Bryden et al.'s conclusions that there is little empirical support for the GBG model, and that it is wise to consider other theories in evaluating data derived from the model.