A range of models is used to explain the concepts of normal and abnormal behaviour alongside difficulties related to diagnosis of mental health illnesses. This evaluation will discuss how behaviour is explained by the biological and cognitive models.

The concepts of normal and abnormal behaviour are social constructs whose definitions differ over time and across cultures. Social norms change over time, impacting upon which category behaviour is placed. For instance, homosexuality was previously viewed as abnormal, unlawful behaviour throughout our western society yet today it is recognised as normal, lawful behaviour. Most definitions of psychological abnormality are ethnocentric leading to a disproportionate number of certain diagnoses amongst specific ethnic groups or socioeconomic statuses. Another perspective suggests that a person's behaviour is abnormal if they are unable to cope with the demands of everyday life (Rosenham and Seligman, 1989). A diagnosis for a mental illness may depend on one's culture, the services they access and the psychiatrist who assesses and possibly diagnoses them. One's mental illness may be undiagnosed for some time. This is problematic in terms of receiving treatment as for example, antipsychotics commonly used to treat schizophrenia show a better response when started earlier. The label of having a mental illness may lead to discrimination.

The biological model views abnormal behaviour much as it views physical illness. It assumes mental illnesses to have an organic cause such as genetics, biochemical imbalances or changes in the brain. For instance, low levels of serotonin are used to explain the symptoms of depression such as disturbed sleep, appetite and periods of very low mood. The cognitive model assumes abnormal behaviour is a result of maladaptive thinking processes such as faulty or irrational thoughts or perceptions and faulty processing of information. For example, a depressed individual may interpret siuations as being their fault when in reality these situations occur accidentally or are beyond their control. Faulty thinking can result from polarised thinking, over-generalising or catastrophic thinking (Kxantz and Hammen, 1979). Beck's (1967) cognitive triad regards negative schemas and cognitive biases affecting one's negative views about their world, themselves and their future. These views lead on to each other, possibly leading into or perpetuating depression. Evidence of maladaptive thinking in a large proportion of those suffering from mental illnesses such as depression and anxiety exists (Clark and Watson, 1991). The biological model argues there is a genetic predisposition to developing mental illnesses. Gottesman (1991) found a 48% concordance rate of schizophrenia in monozygotic twins. It is vital to consider the role of environmental factors. Monozygotic twins are more likely to have similar experiences and be brought up in similar environments. Furthermore, elevated levels of dopamine are related to the symptoms of schizophrenia which include hallucinations, delusions, disordered thinking, emotions and behaviour.

The biological model favours physical interventions such as drug therapy, electro-convulsive therapy to treat mental illness. Commonly SSRIs (selective-serotinin-reuptake-inhibitors) are prescribed for treatment as they are believed to reduce symptoms of depression. Unfortunately, drug treatment can be problematic as side effects may be intolerable for individuals, for instance, excessive weight gain from antipsychotics. This reduces compliance, increasing the likelihood for relapse. Some drugs may be addictive, leading to substance misuse. Ethical implications need to be considered as arguably drugs may be used to control individuals. On the other hand, drug treatment is more convenient, cheaper and relatively quicker acting in comparison to talking therapies, such as