Nobody knows absolutely what causes bipolar disorder. Studies suggest there is a genetic component present, but DNA isn't the only reason people develop bipolar disorder. Most researchers agree that there are likely physical and environmental factors that contribute as well.﻿﻿

Genetic Factors

When talking about biological causes, the first question is whether bipolar disorder can be inherited. This issue has been researched through multiple families, adoption, and twin studies.

In families of persons with bipolar disorder, first-degree relatives (parents, children, siblings) are more likely to have a mood disorder than the relatives of those who do not have bipolar disorder.

Studies of twins indicate that if one identical twin has bipolar disorder the likelihood that the other twin will have it is about 40 percent.

In fraternal twins, the occurrence in both is around 5 percent.

This is important for genetic theories because identical twins occur when one fertilized egg splits in two, meaning that they share the same genetic material. Fraternal twins, on the other hand, come from separate fertilized eggs, so their inherited genes can be different. There is overwhelming evidence that bipolar disorder can be inherited and that there is a genetic vulnerability to developing the illness.

Brain Structure

When it comes to figuring out exactly what is inherited, the neurotransmitter system has received a great deal of attention as a possible cause of bipolar disorder. A link exists between neurotransmitters and mood disorders because drugs that alter these transmitters also relieve mood disorders:﻿﻿

A low or high level of a specific neurotransmitter such as serotonin, norepinephrine, or dopamine is involved.

Other studies indicate that an imbalance of these substances is the problem, i.e., that a specific level of a neurotransmitter is not as important as its amount in relation to the other neurotransmitters.

Still, other studies have found evidence that a change in the sensitivity of the receptors on nerve cells may be the issue.

In short, researchers are quite certain that the neurotransmitter system is at least part of the cause of bipolar disorder, but further research is still needed to define its exact role.﻿﻿

Societal Factors

Mood episodes in bipolar disorder can both be set off by a stressful event or circumstances, but can and frequently do occur spontaneously.

How stress triggers a bipolar episode is not fully understood. But scientists do believe that the stress hormone cortisol plays a role. Stress increases the level of cortisol in the body, which causes alterations in how the brain functions and communicates. In fact, in people who have depression or bipolar disorder, cortisol levels may stay high even when stress isn't present.﻿﻿

Stressful life events can range from a death in the family to the loss of a job, and from the birth of a child to a move.

Stress may stem from a variety of experiences. It cannot be precisely defined, since one person may perceive an event as extremely stressful while another individual encountering the same event may not experience much stress.

Stressful life events can lead to the onset of symptoms in bipolar disorder. However, once the disorder is triggered and progresses, "it seems to develop a life of its own." Once the cycle begins, psychological and/or biological processes take over and keep the illness active.

Environmental Triggers

For Depressive Episodes

Once someone experiences bipolar disorder, small stresses may trigger depressive episodes.﻿﻿ Reading a sad book, talking to someone who's depressed, receiving a poor grade on an assignment, or even catching a cold might trigger a depressive episode.

Other examples of bipolar depressive episode triggers include:

sleep deprivation or disruption

physical injury or illness

menstruation

lack of exercise

travel

For Manic Episodes

While triggers for manic and depressive episodes can be the same, there are some that are specific to manic or hypomanic episodes. According to a 2012 study in the Journal of Affective Disorders, unique triggers of manic or hypomanic episodes include:﻿﻿

falling in love

recreational stimulant use

starting a creative project

late night partying

vacationing

loud music

In addition, the postpartum period and the use of an antidepressant, like an SSRI, may also trigger a manic or hypomanic episode.

The Diathesis-Stress Model

When we look for the cause of bipolar disorder, the best explanation according to research published in 2015 is what is termed the "Diathesis-Stress Model."﻿﻿

The word diathesis, in simplified terms, refers to a physical condition that makes a person more susceptible than usual to certain diseases. Thus the Diathesis-Stress Model says that each person inherits certain physical vulnerabilities to problems that may or may not appear depending on what stresses occur in his or her life.

So the bottom line is that if you have bipolar disorder, you were likely born with the possibility of developing this disorder and something in your life triggered it. However, scientists could refine that theory tomorrow. The one sure thing is that they won't give up looking for answers.