The biological approach focuses on both the physiological and evolutionary aspects which explain human behaviour.
The causal level of analysis incorporates physiological explanations, such as the effect of nerves and hormones on behaviour. According to biological psychologists, behaviour is controlled by the nervous system, which consists of the central nervous system (the brain and the spinal cord) and the peripheral nervous system (the surrounding nerves), which itself includes the autonomic nervous system that controls automatic processes such as heart rate and the fight or flight syndrome. Within the central nervous system, neurons communicate with each other via sending chemical impulses, neurotransmitters, across synapses. Biopsychologists believe that these chemical processes in the brain directly influence human behaviour. Too much or too little of these chemicals can result in over-activity or under-activity in various parts of the brain; this alters thoughts, emotions and behaviour. For example, a link has been made between excessive dopaminergic activity in the brain and the incidence of schizophrenia. Pearlson et al (1993) used Positron Emission Tomography (PET) scans and found a substantial increase in D2 receptors in patients with schizophrenia. Seeman et al (1993) also used PET scans, finding six times the density of D4 receptors in the brains of schizophrenic individuals. A limitation of such studies is the idea of cause and effect; for example, it is unclear whether the increase in dopamine receptors causes schizophrenia or is a result of the neuroleptic drugs taken. Yet, Pearlson’s study was carried out on individuals who had not been exposed to neuroleptic drugs, which therefore rules out cause and effect. Neuroimaging studies are able to study the structure and functioning of the brain, and have the advantage of being non-invasive.