Bipolar disorder, formerly known as manic depression, is a mental health condition characterized by extreme mood swings. People with bipolar disorder experience episodes of mania (elevated moods and energy) and depression (feelings of sadness and hopelessness). Bipolar disorder is complex and likely caused by a combination of genetic, biological, and environmental factors.
What is bipolar disorder?
Bipolar disorder is a mental illness characterized by extreme shifts in mood and energy levels. There are three main types of bipolar disorder:
- Bipolar I Disorder – Defined by manic episodes that last at least 7 days or by manic symptoms that are so severe that the person needs immediate hospital care. Depressive episodes usually occur as well, typically lasting at least 2 weeks.
- Bipolar II Disorder – Characterized by a pattern of depressive episodes shifting back and forth with hypomanic episodes, but no full-blown manic or mixed episodes.
- Cyclothymic Disorder (also called cyclothymia) – Characterized by numerous periods of hypomanic symptoms as well numerous periods of depressive symptoms lasting for at least 2 years.
During a manic episode, a person with bipolar disorder may feel extremely energetic and euphoric. They may exhibit risky behaviors like overspending, making impulsive decisions, and increased substance use. On the other end of the spectrum, depressive episodes may involve feelings of extreme sadness, hopelessness, lack of energy, and even suicidal thoughts.
These mood episodes can last for days, weeks, or even months. In between episodes, people with bipolar disorder may experience stable moods and an absence of symptoms.
What causes bipolar disorder?
The exact causes of bipolar disorder are not fully understood, but research suggests there are likely multiple contributing factors, including:
- Genetics – Bipolar disorder tends to run in families, indicating a genetic component. If one identical twin has bipolar disorder, the other twin has a 40-70% chance of developing it as well. Children with a bipolar parent have a 15-30% chance of developing the disorder.
- Brain structure and functioning – Imaging studies show differences in the brains of people with bipolar disorder compared to those without the condition. There appear to be abnormalities in emotion regulation and motivation pathways in the brain.
- Neurotransmitter imbalances – People with bipolar disorder often have imbalances in neurotransmitters like serotonin, norepinephrine and dopamine, which regulate mood.
- Hormonal imbalances – Abnormalities in cortisol, thyroid hormone and melatonin may play a role in bipolar disorder.
- Environmental factors – High levels of stress, childhood trauma, drug/alcohol abuse, and lack of social support are environmental factors that may trigger bipolar episodes in those with a genetic predisposition.
Most experts believe bipolar disorder arises from a complex interplay between biological vulnerabilities and life experiences. Genetics load the gun, but environment pulls the trigger, as the saying goes.
Is bipolar disorder primarily genetic or environmental?
There is ongoing debate around whether nature or nurture plays a bigger role in bipolar disorder. Research suggests both genes and environment are substantial contributors:
Evidence for a Strong Genetic Influence
- Twin studies show a 40-70% concordance rate in identical twins vs. a 5-10% concordance in fraternal twins.
- First-degree biological relatives of people with bipolar disorder have a 5-10 times higher risk of developing the disorder.
- Children with a bipolar parent have a 15-30% chance of developing bipolar disorder.
- Studies have identified possible gene variants associated with bipolar disorder, including ANK3, CACNA1C and ODZ4 genes.
Evidence for a Strong Environmental Influence
- Rates of bipolar disorder are similar globally, suggesting cultural/social factors play a role.
- Early life stressors like childhood abuse and neglect increase risk of developing bipolar disorder later in life.
- Substance abuse is associated with onset of bipolar symptoms in those genetically predisposed.
- Only 40-70% of identical twins share bipolar disorder, indicating non-genetic factors are also impactful.
Overall, genetics account for 60-80% of the risk of developing bipolar disorder, while environmental influences make up the remaining 20-40% of the risk.
Genetic Factors
Research has consistently shown that bipolar disorder runs strongly in families, indicating a genetic component. Some key evidence includes:
- Family studies show first-degree relatives of people with bipolar disorder have a 5-10 times higher lifetime risk of developing the disorder compared to the general population.
- The risk of developing bipolar disorder is 15–30% if one parent has the disorder.
- Identical (monozygotic) twins have a 40–70% concordance rate for bipolar disorder. This means if one twin has bipolar disorder, there is a 40–70% chance the other twin will develop it too.
- Fraternal (dizygotic) twins have only a 5–10% concordance rate for bipolar disorder.
This pattern of higher concordance rates in identical versus fraternal twins indicates there are strong genetic underpinnings of bipolar disorder. However, genetics are not the whole story since even identical twins do not share the disorder 100% of the time.
Researchers have conducted genome-wide association studies (GWAS) to identify specific gene variants associated with increased susceptibility to bipolar disorder. Some of the top candidate genes uncovered so far include:
- ANK3
- CACNA1C
- ODZ4
- NCAN
However, each of these gene variants only confers a small increase in risk. Experts believe bipolar disorder likely arises from the combined effects of variations in a large number of genes, each contributing a small but additive amount of risk.
Environmental Factors
While genetics play a clear role, environmental influences also substantially impact whether or not someone develops bipolar disorder. Some environmental risk factors include:
- Childhood trauma – Experiencing abuse, neglect or other trauma as a child increases risk of developing bipolar disorder later in life by 2-4 times.
- Substance abuse – Alcohol and drug abuse are strongly linked to onset of bipolar symptoms in those with a genetic predisposition.
- Severe life stressors – High levels of stress and adverse life events can trigger onset of mood episodes.
- Lack of social support – Social isolation and lack of social/family support networks have been associated with worse bipolar disorder symptoms.
Experts theorize environmental influences like these may trigger onset of bipolar disorder in those who are already genetically vulnerable. This “two-hit” model helps explain why even identical twins do not share bipolar disorder 100% of the time.
Interplay Between Genes and Environment
Today, most researchers believe bipolar disorder arises from an intricate interplay between genetic and environmental factors over the course of development. A simplified model of how this might occur is:
- A person is born with certain genetic vulnerabilities that predispose them to bipolar disorder to some degree.
- Exposure to environmental stressors and adversities early in life further increase susceptibility and “load the gun”.
- Additional triggers later in life like substance abuse or life stress “pull the trigger” and tip the balance towards full blown onset of bipolar illness.
So while someone may be born with bipolar disorder risk genes, these genes alone are usually not enough to cause the disorder outright. Environmental influences throughout the lifespan modulate gene expression and work together with genetic factors to shape overall risk.
Nature vs. Nurture: The Verdict
Based on current evidence, most experts believe both genes and environment play crucial and complementary roles the development of bipolar disorder:
- Genetics account for approximately 60-80% of the risk for bipolar disorder.
- Environmental influences make up the remaining 20-40% of the risk.
So while genetics may tip the scales more heavily, the environment is still an important piece of the puzzle. Moving forward, research that aims to elucidate the complex interplay between nature and nurture will be key to unraveling bipolar disorder.
Conclusion
In summary, bipolar disorder is best conceptualized as arising from a “two-hit” model, with both genetic and environmental risk factors playing substantial and complementary roles. Genetics load the gun, but life experiences and adversities pull the trigger. Twin studies demonstrate the strong hereditary nature of bipolar disorder, while adoption studies confirm environmental experiences also help shape risk. While the debate continues, the current consensus is that this disorder – like most human health conditions – stems from a complex interaction between biology and life experiences.