ABSTRACT
The asymmetries of chief interest for this book are handedness and cerebral speech laterality but there are many other asymmetries of human brain and behaviour. The theory developed in previous chapters suggests that there is no intrinsic relationship between hand preference and cerebral laterality for speech (CD). Both are influenced by a third variable, a gene that induces an asymmetry in favour of the left hemisphere during early growth. Left brain advantage leads to a typical pattern of hemisphere specialisation in the course of normal development and incidentally raises the probability of right-handedness. In the absence of the gene for left brain advantage, handedness and speech lateralise by chance. But what of all the other associated asymmetries? What is the pattern in the typical case, and in the absence of the typical pattern, do other lateralities assort together or do all lateralise by chance?
