Bipolar I Disorder (BD-I) is a severe, chronic, and episodic mood disorder defined by the presence of at least one lifetime manic episode lasting at least one week, causing marked functional impairment or accompanied by psychotic features (APA, 2022). Depressive and hypomanic episodes frequently co-occur but are not required for the BD-I diagnosis. BD-I represents the most severe end of the bipolar spectrum and carries substantial morbidity, premature mortality, and socioeconomic burden. Recent comprehensive reviews have characterized BD-I as a multifactorial condition driven by genetic, neurobiological, and environmental factors, frequently complicated by psychiatric and medical comorbidities (Fico et al., 2025).

Epidemiology

Prevalence

According to the 2021 Global Burden of Disease (GBD) Study, an estimated 37 million people (approximately 0.5% of the global population) were living with bipolar disorder in 2021, the majority being working-age adults (World Health Organization [WHO], 2025). Global incidence has been rising, increasing from approximately 30.2 million cases in 1990 to 53.9 million in 2021, with projections indicating continued growth through 2035 (Hu et al., 2025). In the United States, the lifetime prevalence of BD-I and BD-II combined is estimated at 4.4%, with BD-I being most prevalent among young adults aged 18 to 25 years (Nierenberg et al., 2023, as cited in Hu et al., 2025). The condition is one of the 17 leading causes of disability globally and contributes substantially to disability-adjusted life-years (DALYs) (Fico et al., 2025).

Patterns
Bipolar I disorder onset typically occurs in late adolescence to early adulthood, with a median age of onset of approximately 20 years; more than 70% of individuals exhibit clinical features before age 25 (World Mental Health Surveys, as cited in SingleCare, 2024). BD-I affects men and women at roughly equal rates; worldwide, approximately 48% of people with bipolar disorder are male and 52% are female, though women are more likely to experience rapid cycling, mixed features, and depression-predominant illness (WHO, 2025). Seasonal patterns may occur, with manic episodes more frequent in spring and summer. The burden of disease shows regional variation, with high-sociodemographic-index (SDI) countries demonstrating elevated incidence and prevalence, while low-SDI regions face progressively significant burden with limited access to diagnosis and treatment (Hu et al., 2025).

Determinants

Etiology

BD-I arises from complex gene-environment interactions. No single causative mechanism has been identified, and the condition is best understood as a multifactorial disorder with biological, psychological, and social contributors (Fico et al., 2025; Scott & McClung, 2023).

Biological Factors

• Heritability is estimated at 60–80%; the largest genome-wide association study (GWAS) of bipolar disorder to date, comprising over 40,000 cases, identified 64 genomic risk loci, with risk alleles enriched in synaptic signaling pathways and brain-expressed genes, particularly in the prefrontal cortex and hippocampus (Mullins et al., 2021)

• Recent fine-mapping analyses have identified specific causal risk genes and implicated neurobiological and developmental pathways, underscoring the polygenic and neurodevelopmental nature of BD-I

• Neuroimaging meta-analyses report cortical thickness abnormalities and frontolimbic connectivity disruption in BD; structural changes in limbic and cortical regions serve as functional biomarkers associated with emotion dysregulation (Scott & McClung, 2023)

• Dysregulation of calcium homeostasis, circadian rhythm pathways, mitochondrial function, and neurodevelopment have been implicated via induced pluripotent stem cell (iPSC) research (Scott & McClung, 2023)

Psychological Factors

• Childhood trauma, adverse life events, and high expressed emotion in the family environment are associated with earlier onset and worse clinical prognosis

• Reward hypersensitivity and behavioural activation system (BAS) dysregulation are core psychological vulnerability mechanisms in BD-I

• Sleep disturbance and disrupted circadian regulation play a central role in episode precipitation and maintenance (Scott & McClung, 2023)

Social Factors

• Social rhythm disruption (e.g., irregular sleep-wake cycles, shift work, stressful life events) precipitates manic and depressive episodes

• Air pollution and climate change have been identified as emerging environmental risk factors for mental health outcomes including BD (Fico et al., 2025)

• Low socioeconomic status, social isolation, and substance use disorder are associated with worse outcomes and higher rates of non-adherence

Risk Factors

• Family history of BD-I or other mood disorders (strongest single risk factor)

• Substance use disorders (cannabis, stimulants, alcohol) — estimated comorbidity rate of 56% (Fico et al., 2025)

• Sleep deprivation, circadian disruption, and irregular daily routines

• Stressful life events, particularly interpersonal losses and goal-disruption

• Antidepressant use without mood stabilizer coverage carries risk of triggering mania or rapid cycling

Effects on Population

Individual

BD-I is associated with premature mortality, with all-cause mortality approximately twice that of the general population (relative risk [RR] = 2.02; 95% CI: 1.89–2.16). Suicide-specific mortality is dramatically elevated (RR = 11.69; 95% CI: 9.22–14.81), underscoring BD-I as a high-risk condition for suicidal death (Stubbs et al., 2023). Globally, 15–20% of individuals with bipolar disorder die by suicide, with 30–60% making at least one attempt; these rates are not declining (Stubbs et al., 2023). Individuals with BD-I also face substantially elevated cardiovascular mortality and significant cognitive and occupational impairment.

Family

Families experience high caregiver burden, financial strain, and interpersonal conflict. Children of a parent with BD-I have increased risk for mood, anxiety, and neurodevelopmental disorders. Family functioning is frequently disrupted during both manic and depressive episodes, and the impact on family wellbeing is underrecognized in clinical practice.

Community

BD-I is among the leading causes of disability worldwide, contributing disproportionately to productive years lost. High-SDI regions bear the greatest absolute incidence burden, while low-SDI countries face escalating disease burden with inadequate mental health system capacity. Stigma and discrimination remain widespread barriers to diagnosis, treatment, and social inclusion (WHO, 2025). Comorbid psychiatric disorders (anxiety disorders, 71%; substance use disorders, 56%; personality disorders, 36%; ADHD, 10–20%) compound the societal burden (Fico et al., 2025).