The factors that influence response to neuroleptic drugs are poorly understood. These factors may include clinical variables such as length of illness before treatment, age at onset, and the presence of negative symptoms; and pharmacologic factors such as rates of drug metabolism. For example, pharmacodynamic differences in therapeutic response to haloperidol have been observed between Chinese and Caucasian patients. Response rates may also reflect clinical heterogeneity, although familiality, history of obstetric trauma, or ventricular enlargement have been not shown to be significant factors. It is also possible that genetic differences in receptors that are targets for these drugs may influence response.