The process of medical research is influenced by current ideas and cultural issues. Medical research into the causes and origins of bipolar illness has been influenced by an idea known as the Diathesis-Stress Hypothesis. This model has been applied to many medical and mental health disorders.
The basic diathesis-stress hypothesis proposes that people have vulnerabilities (known as predispositions) for illnesses (known as diatheses). Some people have more of these vulnerabilities than others, for varying reasons having to do with their genetics, biology and experience. However, just having a vulnerability for an illness alone is not enough to trigger that illness into action. Instead, people's vulnerabilities must interact with life stresses to prompt the start of the illness. The greater a person's natural tendency for developing an illness, the less stress is necessary to get the illness started. Where there is a smaller vulnerability for developing an illness, a greater amount of stress is required to produce the illness. Until this critical amount of stress is reached (however much or little of it is necessary in a given case) people cannot be said to have an illness. Their vulnerabilities are said to be "latent" or hidden.
The current thinking regarding the causes of bipolar disorder are essentially that bipolar disorder results when bipolar diatheses (hidden vulnerabilities) meet a source of sufficient stress necessary to activate them and cause the disease to start.
It's not quite as simple as that last statement may sound, of course. For instance, various sources of stress may combine to produce the releasing effect. These stressors can be psychological, social, and biological. For example, trauma, family stress, depression, viruses, and birth complications can be forces powerful enough to raise the vulnerability towards bipolar disorder. Other stresses that can add to vulnerability include sleep deprivation (known to trigger mania), Major Depressive Disorder with a Seasonal Pattern (previously SAD), alcoholism and drug abuse, imbalances in brain chemicals, hormones, infections, and premenstrual syndrome in women. Although the relationship between menstrual cycles and bipolar disorder is still unclear, this connection may contribute to the fact that more women experience depressive episodes than men. It is important to remember that the events or stresses that set an illness in motion can be very different from those that keep it going once it has started.
Evidence that bipolar disorder is biological in nature comes from a wide range of studies in the genetics, neurochemistry, endocrinology and immunology fields.