Watson believed that all babies were born a blank slate and their behaviour can be conditioned (determined) from birth so that the individual’s choices and behaviour have actually been determined by external factors (upbringing, rewarded or punished at school). Biological psychologists believe that our behaviour is determined internally by our genes and brain chemistry. Schizophrenia is thought by biological psychologists to be caused by a problem with the levels of dopamine in the brain. This suggests that these individuals have no control at all over their illness.