Nonetheless, some researchers continue to argue that no clear dividing line can be drawn between the affective psychoses (psychotic depression and bipolar disorder) and schizophrenia. Consistent with this unitary psychosis hypothesis, many patients present with a mixture of schizophrenia and bipolar symptoms; in the DSM system the term schizo-affective disorder is used to describe this combination of symptoms.

In the DSM system, a distinction is now made between bipolar-1 disorder (in which there is a history of both depression and mania) and bipolar-2 disorder (in which there is a history of depression and hypomania but an absence of full manic episodes). Contrary to common thinking, mood during mania is more often irritable than euphoric, and manic patients often evidence high levels of dysphoria (depression) (Goodwin and Jamison, 1990). Often manic episodes begin with feelings of euphoria, but proceed to panic, irritability and psychotic symptoms (hallucinations and delusions).

Bipolar disorder affects approximately 1.5 per cent of the adult population (American Psychiatric Association 1994), males and females equally, and often begins in adolescence, with a mean age of onset of 21 years (Smith and Weissman 1992). Symptoms can begin much earlier in childhood and, as a result, can go undiagnosed for many years. The course of the disorder is most likely to be severe, with people suffering multiple episodes that usually have longer duration as the individual gets older. These episodes of severe mood disturbance can result in extreme alterations to the sufferer’s behaviours and thinking, often with undesirable consequences. The risk of attempted and completed suicide is high.

The personal and social costs of bipolar disorder are immense, having a particularly detrimental impact on areas such as interpersonal relationships (see s. 5), social and occupational functioning. People often start to become unwell at a time in their life when they would usually be establishing careers and developing meaningful adult relationships. Although most sufferers experience periods of relatively normal functioning between episodes of mood disturbance, they often have decreased social functioning and are never entirely symptom-free. Caregivers of those affected often experience substantial burden and distress (Woods 2000).

There is evidence from twin and family studies of a substantial genetic contribution to bipolar disorder (Torrey et al. 1994). However, the accidental discovery that lithium carbonate stabilises mood in bipolar patients, reducing the risk of relapse, has yet to lead to a clear understanding of the brain biochemistry of the disorder. Investigation of neurocognitive functioning in bipolar patients has consistently revealed evidence of mild neurocognitive deficits, but it is not yet known whether these precede the onset of illness.

Several psychological theories of bipolar disorder have been proposed. The manic defence hypothesis, first proposed by the psychoanalyst Karl Abraham (1911/1927), hypothesises that mania arises from extreme attempts to avoid depression. This can be restated as the hypothesis that, in an attempt to avoid feelings of depression, bipolar patients engage in dysfunctional coping strategies such as indulging in high-risk and stimulating activities, leading to mania. Recent psychological research, showing depression-like performance on cognitive tests in both remitted and currently manic bipolar patients, is consistent with this hypothesis (Lyon et al. 1999; Scott et al. 2000). It has also been argued that mania is triggered by sleep (see s. 2, sleep and biological rhythms) disturbance. Consistent with this idea, the onset of mania sometimes occurs following life events that disrupt social rhythms, for example, international travel or the birth of a child (Malkoff-Schwartz et al. 2000). It has also been argued that over-sensitivity of the behavioural activation system (the brain system that mediates response to reinforcement – see s. 3) confers vulnerability to depression, and consistent with this idea, manic episodes are sometimes triggered by positive life events involving goal attainment (Johnson et al. 2000).

The treatment of bipolar disorder has usually involved the prescription of medications such as mood stabilisers and antipsychotics. However, greater understanding of the psychological mechanisms involved in mood disturbance has resulted in a wide range of psychological therapies proving to be successful in the management of bipolar disorder. Cognitive behavioural therapy [see cognitive (behaviour) therapy], interpersonal and social rhythm therapy and family therapy have recently been studied as psychological interventions for bipolar patients, and there is some preliminary evidence that these can be effective.