A degree of functional lateralization is characteristic of various aspects of human cognition, including aspects of language processing, which show left hemisphere dominance in most people. Left-right asymmetries of the human brain and behaviour are likely to arise from lateralized genetic-developmental programs that originate in the early embryo. In adults, a recent study of gene expression data from superior temporal and auditory cortex found subtle, quantitative lateralization of genes involved in synaptic transmission and neuronal electrophysiology. These observations are consistent with functional lateralization of this cortical region for language. Genetic polymorphisms that may have small, modifying effects on brain and behavioral asymmetries are starting to be identified through association studies, although core genetic mechanisms of asymmetrical brain development are not known. A major challenge will be to understand how neuronal circuits of the left and right hemisphere become differently fine-tuned, at the molecular level, to preferentially support particular cognitive functions. Through analyzing the inter-hemispheric genetic contrast, powerful insights may be gained into the exact properties of the left hemisphere's architecture which are particularly supportive of language-related functions