What Causes Bipolar Disorder?

Written by Nichole Owens
Medically Reviewed by Zilpah Sheikh, MD on July 09, 2025
7 min read

Doctors don’t completely understand the causes of bipolar disorder. But they’ve gained a greater understanding in recent years of the bipolar spectrum, which includes the elated highs of mania to the lows of major depression, along with various mood states between these two extremes.

Raphael Braga, MD, a psychiatrist at Northwell Health, New York, said the answer as to what causes bipolar disorder is “always tricky.”

“Among the conditions of psychiatry, bipolar is one of the most well known to have a more biological underpinning than probably any other condition in psychiatry, apart from maybe schizophrenia,” Braga says. “We know that the genetic components of bipolar are very, very high compared to the average psychiatric diagnosis. Like everything else in psychiatry, we can never say it’s just a genetic disease.”

Bipolar disorder seems to often run in families, and there is a genetic part to this mood disorder. There is also growing evidence that environment and lifestyle issues have an effect on how bad the disorder can be. Stressful life events — or alcohol or drug abuse — can make bipolar disorder more difficult to treat.

 

“We don’t have a clear-cut mechanism of action that we know why somebody is bipolar,” Braga says.

Experts believe bipolar disorder is partly caused by an underlying problem with specific brain circuits and the functioning of brain chemicals called neurotransmitters.

Three brain chemicals — norepinephrine (noradrenaline), serotonin, and dopamine — are involved in both brain and bodily functions. Norepinephrine and serotonin have been consistently linked to psychiatric mood disorders such as depression and bipolar disorder. 

Nerve pathways within areas of the brain that control pleasure and emotional reward are controlled by dopamine. Disruption of circuits that communicate using dopamine in other brain areas appears connected to psychosis and schizophrenia, a serious mental disorder defined by distortions in reality and illogical thought patterns and behaviors.

The brain chemical serotonin is connected to many body functions, such as sleep, wakefulness, eating, sexual activity, impulsivity, learning, and memory. Researchers believe that abnormal functioning of brain circuits that involve serotonin as a chemical messenger lead to mood disorders (depression and bipolar disorder).

Many studies of bipolar patients and their relatives have shown that bipolar disorder sometimes runs in families. Perhaps, the most convincing data come from twin studies. 

In studies of identical twins, scientists report that if one identical twin has bipolar disorder, the other twin has a greater chance of getting bipolar disorder than another sibling in the family. Researchers conclude that the lifetime chance of an identical twin (of a bipolar twin) to also get bipolar disorder is about 40%-70%.

Jeffrey J. Rakofsky, MD, a psychiatrist at Emory Healthcare, noted that in fraternal twins, the risk of the other twin developing bipolar disorder goes down to about 6%-11%.

“What this means is that genetics plays a big role,” Rakofsky says, “but it must not be the only factor since it’s not 100% in the identical twins as one would expect since they share all the same genes.”

Is bipolar disorder hereditary?

Rakofsky says bipolar disorder is considered “a highly heritable disorder, meaning that a large amount of variability (60%-80%) in the illness is due to genetics rather than other causes.”

But researchers haven’t found a specific gene for bipolar disorder.

“We have a group of very different genes that in one way or another could be involved in bipolar, but none of them explain having bipolar,” Braga says.

In studies at Johns Hopkins University, researchers interviewed all first-degree relatives of patients with bipolar I and bipolar II disorders and concluded that bipolar II disorder was the most common affective disorder in both family sets. 

Bipolar I is when a person goes through a full manic episode, including potentially going into psychosis. With bipolar II, a person goes through periods of depressive and hypomanic episodes but don’t have manic episodes.

The researchers found that 40% of the 47 first-degree relatives of the bipolar II patients also had bipolar II disorder, and 22% of the 219 first-degree relatives of the bipolar I patients had bipolar II disorder. But among patients with bipolar II, researchers found only one relative with bipolar I disorder. They concluded that bipolar II is the most prevalent diagnosis of relatives in both bipolar I and bipolar II families.

Family history and risk factors

“Risk factors are other family members with bipolar disorder,” Rakofsky says. “Having a first-degree relative with the illness is associated with a 7%-10% risk of having it, and having two parents with the disorder is associated with a 28.6% risk of having the illness.”

Studies at Stanford University that explored the genetic connection of bipolar disorder found that children with one biological parent with bipolar I or bipolar II disorder have a raised chance of getting bipolar disorder. 

In this study, researchers reported that 51% of the bipolar offspring had a psychiatric disorder, most commonly major depression, dysthymia (low-grade, chronic depression), bipolar disorder, or attention deficit hyperactivity disorder (ADHD). Interestingly, the bipolar parents in the study who had a childhood history of ADHD were more likely to have children with bipolar disorder rather than ADHD.

In other findings, researchers report that first-degree relatives of a person diagnosed with bipolar I or II disorder are at an increased risk for major depression than first-degree relatives of those with no history of bipolar disorder. Scientific findings also show that the lifetime risk of affective disorders in relatives with family members who have bipolar disorder increases, depending on the number of diagnosed relatives.

“Environment and lifestyle are thought to increase one’s risk if the person already has the biologic predisposition due to genetics,” Rakofsky says. “Those factors could include substance use, stress, intrauterine infection, birth trauma, et cetera.”

Along with a genetic link to bipolar disorder, research shows that children whose parents are bipolar are often surrounded by enough environmental stressors. That may include living with a parent who:

  • Has severe mood swings 
  • Misuses alcohol or other substances 
  • Is prone to financial and sexual indiscretions 
  • Is hospitalized often 

Although most children with a bipolar parent will not develop bipolar disorder, some could develop a different disorder such as: 

  • ADHD
  • Major depression
  • Schizophrenia
  • Substance abuse

Some findings show that people with bipolar disorder have a genetic predisposition (tendency) to sleep-wake cycle problems, which may trigger symptoms of depression and mania.

The problem for those with bipolar disorder is that sleep loss may lead to a mood episode such as mania (elation) in some patients. Worrying about losing sleep can raise anxiety, worsening bipolar mood disorder altogether. Once a sleep-deprived person with bipolar disorder goes into the manic state, the need for sleep drops even more.

In one study, researchers interviewed 39 bipolar patients with primarily manic or depressed episodes to figure out the presence of social rhythm disruptions during the two months before the onset of the mood. A social rhythm disruption is a disturbance in daily routines, such as sleeping, eating, exercising, or interacting with other people, which in turn could affect patterns of brain activity tied to mood regulation.

When comparing the results with volunteers in the control group, researchers concluded that most people with bipolar disorder go through at least one social rhythm disruption before a major mood episode. 

Plus, the researchers found that social rhythm disruption affected more bipolar patients with mania than the patients with depression. Their findings concluded that 65% of the patients with bipolar disorder had at least one disruption in their daily rhythm in the eight weeks before the onset of a manic episode.

Talk to your doctor if you have trouble falling asleep or maintaining sleep. There are many nonaddictive sleep medicines available that can help sleep problems go away. Also, cognitive behavioral therapy has been shown to be a helpful treatment for patients with bipolar disorder who have poor sleep or anxiety and fears about poor sleep.

Hormones can trigger episodes, especially in women who are pregnant. 

“It’s not unusual for pregnant women with bipolar disorder who are untreated to have mood episodes during this time and for postpartum mood episodes to occur as well,” Rakofsky says. “These are times when women experience dramatic shifts in their hormone levels. Additionally, some women respond poorly to hormonal birth control, while for others, it can stabilize their mood.”

The diathesis-stress model is the theory that the development of mental health conditions and psychological disorders can be the result of the interaction between genetic predispositions and environmental stressors.

In other words, if someone has an average risk of bipolar disorder, but they grew up in a more stable family without a history of trauma or substance abuse, then the onset might begin “at a later age and might be less severe presentation,” Braga says.

If someone predisposed goes through trauma, stress, or abuse, then those experiences might “push you over the threshold” and trigger the onset of bipolar disorder much earlier.

Bipolar disorder has a major genetic component, but evidence also shows that environment and lifestyle issues have an effect on how bad the disorder can be. Having a parent or sibling with bipolar disorder may raise the risk of having bipolar disorder. Environmental stressors, including trauma, stress, or abuse, play a role in triggering bipolar episodes in people who are genetically predisposed.

Can trauma cause bipolar disorder​?

Trauma does not cause bipolar disorder on its own, but it can affect those who are genetically predisposed.

Which hormone causes bipolar disorder​?

There’s no specific hormone that causes bipolar disorder. But hormonal issues can trigger mood episodes among individuals with bipolar disorder, especially pregnant women.

Can illicit drug use cause bipolar disorder​?

Illicit drug use can be a factor in bipolar disorder but not necessarily a cause.