Cheating is not a mental illness. No major diagnostic manual classifies infidelity itself as a psychiatric disorder, and no credible mental health organization treats it as one. That said, the question isn’t as simple as it sounds. Certain mental health conditions can increase the likelihood of infidelity, and some people who cheat repeatedly do have identifiable psychological patterns worth understanding.
Why Cheating Isn’t a Diagnosis
The two systems clinicians use to diagnose mental health conditions are the DSM-5-TR (used primarily in the United States) and the ICD-11 (used internationally). Neither one lists infidelity as a mental disorder. Cheating is a behavior, not a condition, and behaviors on their own don’t qualify as disorders unless they meet specific criteria: they need to cause significant distress, impair daily functioning, and fall outside the person’s voluntary control in a meaningful way.
Most infidelity doesn’t meet that threshold. People who cheat typically make a series of conscious decisions, even if those decisions are influenced by emotional dissatisfaction, opportunity, poor boundaries, or selfishness. The distinction matters because labeling cheating as a mental illness would remove personal responsibility from a choice that causes real harm to others.
Conditions That Can Drive Infidelity
While cheating itself isn’t a disorder, several recognized conditions can make infidelity more likely. These don’t excuse the behavior, but they help explain patterns that might otherwise seem baffling.
Bipolar Disorder
During manic or hypomanic episodes, people with bipolar disorder often experience impaired judgment, heightened impulsivity, and increased sex drive. The Mayo Clinic lists “taking sexual risks” as one of the hallmark symptoms of a manic episode, alongside reckless spending and poor decision-making. Someone in a full manic state may pursue sexual encounters they would never consider when stable. This isn’t a character flaw; it’s a symptom of a brain state that genuinely distorts risk assessment and impulse control. That said, effective treatment significantly reduces these episodes, and many people with bipolar disorder never cheat.
Narcissistic Personality Traits
Research on 135 couples in long-term relationships found that both grandiose narcissism (the inflated sense of self-importance most people associate with narcissism) and vulnerable narcissism (a more fragile, insecure version) were positively linked to favorable attitudes toward infidelity. The connection played out differently by gender: women’s openness to infidelity tracked with their own narcissistic traits, while men’s tracked with both their own and their partner’s vulnerable narcissism. People high in narcissistic traits tend to feel entitled to sexual novelty, struggle with empathy for a partner’s pain, and prioritize their own gratification.
Compulsive Sexual Behavior
The World Health Organization added compulsive sexual behavior disorder to the ICD-11 as an impulse control disorder, describing a pattern where someone repeatedly fails to control intense sexual urges despite negative consequences. This is the closest the clinical world comes to recognizing a condition that could directly fuel repeated infidelity. However, the DSM-5-TR does not include it as a standalone diagnosis, and there’s still significant debate about whether it belongs in the impulse control category or the addiction category, or whether it’s better understood as a symptom of another condition like depression or anxiety.
The “Sex Addiction” Debate
When someone cheats repeatedly, the term “sex addiction” often comes up, either as a self-diagnosis or something suggested by a therapist. But the largest professional organization for sexuality professionals in the U.S., the American Association of Sexuality Educators, Counselors and Therapists (AASECT), has taken a clear stance: it does not find sufficient evidence to support classifying sex addiction as a mental health disorder. AASECT specifically warns against treatment approaches built around the addiction model, arguing they aren’t grounded in accurate knowledge of human sexuality and can pathologize normal sexual behavior.
This doesn’t mean that people can’t develop genuinely problematic sexual patterns. It means the “addiction” framing may not be the most accurate or helpful lens. Some researchers argue that compulsive sexual behavior is better explained by poor impulse control, untreated mood disorders, trauma responses, or attachment problems rather than a distinct addictive process in the brain.
Genetics and the Thrill-Seeking Connection
There is a biological thread worth knowing about. A study of 181 young adults found that people carrying a specific variation of a dopamine receptor gene (called the 7R+ allele) were almost twice as likely to engage in promiscuous sex: 45% compared to 24% among those without the variant. Among those who reported infidelity, carriers of this gene variant also reported more partners outside their relationship.
This gene variant affects how the brain processes dopamine, the chemical tied to reward and motivation. People who carry it tend to score higher on sensation-seeking measures across the board, not just sexually. They’re drawn to novelty and stimulation in general. But carrying a gene that nudges you toward thrill-seeking is very different from having a mental illness. Genes influence tendencies; they don’t dictate choices.
How Attachment Patterns Play a Role
One of the strongest psychological predictors of infidelity isn’t a disorder at all. It’s attachment style, the deeply ingrained way you relate to intimacy and closeness that forms in early childhood. A Purdue University analysis found that avoidant and anxious attachment tendencies together accounted for about 24% of the variation in people’s intentions to cheat. That’s a substantial chunk of the picture.
People with avoidant attachment tend to feel uncomfortable with deep emotional closeness and may use outside relationships to maintain distance from a partner. People with anxious attachment crave reassurance and may seek it from someone new when they feel insecure in their current relationship. Neither pattern is a mental illness, but both are well-documented risk factors that respond well to therapy, particularly approaches focused on recognizing and reshaping relational patterns.
The Real Harm: What Happens to the Person Cheated On
Ironically, while cheating itself isn’t a mental illness, being cheated on can trigger symptoms that closely resemble one. Betrayal trauma, the psychological fallout of discovering a partner’s infidelity, can produce intrusive thoughts about the affair, hypervigilance and suspicion, emotional numbness, difficulty sleeping, and physical symptoms like stomach distress and chronic pain. In some cases, the experience meets the clinical criteria for post-traumatic stress disorder.
The betrayed partner may also experience a deep loss of self-worth, difficulty trusting others in future relationships, and persistent anxiety or depression. These aren’t just emotional reactions that fade in a few weeks. For some people, they represent a genuine mental health crisis that benefits from professional support. The person who cheated may not have a disorder, but the damage they cause can create one.
What This Means in Practice
If you’re trying to understand why someone cheated, whether it was you or a partner, the answer almost certainly isn’t “because of a mental illness.” It’s more likely a combination of personality traits, attachment patterns, relationship dissatisfaction, opportunity, and in some cases, untreated conditions like bipolar disorder or compulsive sexual behavior that lowered the threshold for impulsive decisions.
The distinction is important for two reasons. First, framing cheating as a disease can become a way to avoid accountability. If it’s an illness, it’s something that happened to the cheater rather than something they did. Second, when a genuine condition like bipolar disorder is involved, recognizing it opens the door to treatment that can actually prevent the pattern from repeating. The goal isn’t to excuse or pathologize but to understand the specific factors at play and address the ones that are addressable.