The key assumption of this approach is that all behaviour can be explained in terms of the functioning of biological systems.
Physiological psychology explains behaviour in terms of bodily activity, making reference to brain activity (e.g., some theories of dreaming, Dement and Kleitman), neurotransmitters (e.g., explaining depression), and hormones (e.g., stress). The nativist approach offers explanations in terms of genes and heredity. The medical model of mental illness assumes that all psychological illnesses can be explained in terms of physical causes and that diseases can be diagnosed from symptoms, and suitable treatments prescribed.
The strengths of the approach include its objective, reductionist nature, which facilitates experimental research, and a host of practical applications such as drug therapy for mental illness. The determinist, reductionist, and mechanistic nature of biological explanations can also be seen as a weakness because they oversimplify complex behaviours and de-emphasise personal responsibility. The biological approach overlooks the experiential aspect of behaviour and ignores past experience. Biological explanations are more appropriate for some kinds of behaviour (see Raine, Schacter & Singer).
Michael W.Eysenck & Cara Flanagan, 2001, Psychology for A2 level, Psychology Press, ISBN 1-84169-251-4 (Highly recommended text for broad overview of psychology, written in an easy to understand style)