Comorbidity in psychology refers to the co-occurrence of two or more mental health disorders in the same person at the same time. Someone diagnosed with major depression who also meets the criteria for generalized anxiety disorder, for example, has comorbid conditions. About one in three adults with a mental health disorder have at least one additional co-occurring disorder, making comorbidity remarkably common rather than the exception it might seem.
The term was originally introduced in general medicine in 1970 by epidemiologist Alvin Feinstein, who defined it as “any distinct additional clinical entity that has existed or that may occur during the clinical course of a patient who has the index disease under study.” Psychiatry later adopted the concept to describe the overlap between mental health conditions. But there’s an important nuance: comorbidity in psychiatry doesn’t necessarily mean multiple separate diseases are present. It can simply mean a person’s symptoms meet the diagnostic thresholds for more than one condition simultaneously.
Why Comorbidity Is So Common
Mental health conditions don’t exist in neat, isolated categories. Researchers have identified what they call the “p-factor,” a general psychopathology factor that captures a shared vulnerability running across different psychiatric diagnoses. Think of it as a common thread of psychological distress that can express itself in multiple ways. Studies using statistical modeling have found this single factor accounts for roughly 23% of the variation in psychiatric symptoms across the population.
This shared vulnerability has deep biological roots. Twin and family studies have long shown that genetic risk for psychiatric disorders crosses diagnostic boundaries. More recent genetic research confirms this: conditions like ADHD, anxiety disorders, depression, bipolar disorder, and schizophrenia all share significant genetic overlap, with pairwise genetic correlations ranging from about 0.20 to as high as 0.75. A single genetic factor captures 20% to 43% of the genetic effects across these disorders. In other words, many of the same genes that increase your risk for one condition also raise your risk for others, which helps explain why comorbidity is the rule rather than the exception.
Environmental factors play a role too, and they’re often intertwined with genetics. People with a genetic predisposition toward anxiety may gravitate toward or create environments that also fuel depression, a pattern researchers describe as gene-environment correlation. The result is a dense web where biology and life experience reinforce each other across multiple conditions.
The Most Common Overlaps
Depression and anxiety are the most frequently co-occurring pair. In clinical samples, rates of comorbid anxiety in people with depression have been reported as high as 85%, and up to 90% of people with anxiety also have depression. In one large study using electronic health records, 43.6% of individuals with either depression or anxiety had both conditions, compared to 35% with depression alone and 21.4% with anxiety alone. When the two conditions overlap, rates of insomnia, chronic pain, acid reflux, and muscle pain are all significantly higher than in people with either condition on its own.
ADHD and autism spectrum disorder frequently co-occur as well. Approximately one in eight children diagnosed with ADHD also receive an autism diagnosis. Before 2013, clinicians were actually prevented from diagnosing both conditions in the same person, since earlier editions of the diagnostic manual treated them as mutually exclusive. That rule has since been dropped, reflecting the reality that these conditions share symptoms like difficulty with social cues, sensory sensitivities, and challenges with executive function.
Substance use disorders overlap heavily with other mental health conditions. People with depression, anxiety, PTSD, or bipolar disorder are all at elevated risk for developing problems with alcohol or drugs, and the reverse is also true. This particular form of comorbidity is sometimes called “dual diagnosis.”
True, Artifactual, and Spurious Comorbidity
Not all comorbidity reflects genuinely separate conditions. Researchers distinguish three types. True comorbidity occurs when a person has two distinct disorders with different underlying causes that happen to coexist. Artifactual comorbidity is a byproduct of how diagnostic manuals carve up symptoms into narrowly defined categories. If the manual splits what might be a single broad syndrome into multiple diagnoses, a person can meet criteria for several of them without truly having separate conditions. Spurious comorbidity is diagnosed when the overlap is meaningless, such as when two diagnoses describe essentially the same thing under different names.
This distinction matters because it shapes how clinicians think about treatment. If two diagnoses are really just different faces of the same underlying problem, treating them as separate conditions could lead to fragmented, less effective care.
How Comorbidity Affects Treatment Outcomes
Having more than one mental health condition generally makes treatment harder. Research on early psychosis patients shows that comorbid conditions are associated with reduced symptom improvement, higher hospital readmission rates, poorer daily functioning, lower quality of life, and increased risk of suicidal thoughts. Comorbid anxiety in particular is linked to worse initial outcomes, while co-occurring depression is associated with more lifetime hospitalizations and lower chances of achieving functional recovery.
That said, the impact isn’t uniform. In one study using advanced modeling, researchers found that roughly 45% of patients with comorbid conditions experienced relatively limited effects on their chances of remission. The other 55% were more strongly affected. This suggests that how comorbidities interact with each other and with a person’s broader circumstances varies considerably from one individual to the next.
Diagnostic Overshadowing
One of the biggest practical problems with comorbidity is that it can be missed entirely. Diagnostic overshadowing occurs when a clinician attributes new symptoms to an existing diagnosis rather than recognizing a second condition. A person already diagnosed with depression who develops chronic fatigue, for example, might have that fatigue chalked up to depression when it actually signals a separate medical or psychological problem.
This bias is especially well documented in people with intellectual disabilities or serious mental illness, where physical health complaints are frequently dismissed as behavioral symptoms of the primary diagnosis. The result is underdiagnosis and delayed treatment that can worsen outcomes significantly.
Clinicians use structured interviews, like the Structured Clinical Interview for DSM-5 (SCID-5), to systematically screen across multiple diagnostic categories and reduce the chance of missing a co-occurring condition. These interviews walk through modules corresponding to different disorder categories, making it less likely that a secondary diagnosis will be overlooked.
Transdiagnostic Treatment Approaches
Because comorbidity is so common, some therapists use treatments designed to address the shared processes that cut across multiple disorders rather than targeting one diagnosis at a time. The most prominent example is the Unified Protocol for Transdiagnostic Treatment of Emotional Disorders, a form of cognitive behavioral therapy that focuses on emotional regulation skills applicable to depression, anxiety, and related conditions simultaneously.
Systematic reviews suggest these transdiagnostic approaches perform about as well as, or slightly better than, comparison treatments overall. For a single primary disorder, a diagnosis-specific treatment (like exposure therapy for a specific phobia) tends to be more effective. But for people with comorbid conditions, transdiagnostic approaches appear to have a modest advantage, likely because they address a broader range of symptoms rather than focusing narrowly on one diagnosis while leaving others untreated.
This trade-off reflects the central tension in treating comorbid conditions: specificity is powerful for a single problem, but flexibility matters when multiple conditions are in play. For many people, the most effective path involves recognizing that their symptoms may be interconnected rather than treating each diagnosis as a separate, unrelated issue.