Why Affairs Happen: What the Research Says

This module is not about blame. It is not about finding a reason that makes the affair acceptable, because no reason does. It is about replacing the chaos of “why?” with what decades of infidelity research have actually found. Understanding the mechanisms behind affairs is clinically useful for both partners: it helps the betrayed partner move from personalization (“What is wrong with me?”) to comprehension, and it helps the unfaithful partner move from defensiveness (“I don’t know why I did it”) to genuine accountability.

The research on infidelity is extensive. What follows synthesizes findings from John Gottman, Shirley Glass, Frank Pittman, Esther Perel, and large-scale epidemiological studies into a framework that accounts for most cases without reducing them to a single cause.

How Common Is Infidelity?

The General Social Survey (GSS), the most reliable longitudinal dataset on American sexual behavior, consistently finds that approximately 20 percent of married men and 13 percent of married women report having had sex with someone other than their spouse while married. These numbers have remained relatively stable over decades, with one notable exception: rates among women over 60 have been rising, likely reflecting increased financial independence and workforce participation.

These figures almost certainly undercount the real prevalence. They rely on self-report, they define infidelity narrowly as intercourse, and they do not capture emotional affairs, online relationships, or other forms of sexual behavior that many couples would consider betrayals. When researchers broaden the definition, lifetime prevalence estimates for some form of infidelity in committed relationships range from 25 to 40 percent.

The point of these numbers is not to normalize infidelity. It is to counter the isolation that both partners often feel. If you are dealing with this, you are not in a rare or exotic situation. You are facing one of the most common and most painful relational crises that exists.

The Three Conditions: Opportunity, Narrative, and Erosion

John Gottman’s research at the University of Washington identified three conditions that, in combination, predict the occurrence of an affair. No single condition is sufficient on its own. All three typically operate together.

Opportunity. Affairs require access to a potential partner in a context that allows escalation. The workplace is the most common setting, accounting for an estimated 31 to 44 percent of affairs, because it provides regular proximity, shared goals, emotional intensity, and plausible reasons for private contact. Travel, social media, and reunion events create similar opportunity structures. Opportunity does not cause affairs, but without it, they do not occur.

Narrative justification. Before crossing a boundary, the unfaithful partner constructs an internal story that makes the crossing feel permissible. “My partner doesn’t appreciate me.” “We’ve grown apart.” “This person understands me in a way my partner never has.” “It’s just a friendship.” These narratives are not necessarily false. The relationship may genuinely have problems. But the narrative serves a specific psychological function: it reframes a choice as an inevitability, reducing the cognitive dissonance that would otherwise make the behavior intolerable to the person’s self-concept.

Gradual boundary erosion. Affairs rarely begin with a decision to be unfaithful. They begin with small boundary violations that individually seem insignificant. A lunch that runs long. A text conversation that shifts from professional to personal. A disclosure of marital frustration to someone who listens with warmth and interest. Each step is small enough to justify (“We’re just friends”), and each step makes the next step easier. By the time the line is crossed, the person has been moving toward it for weeks or months through increments so small that they did not register as choices at the time.

Gottman’s framework is important because it explains how people who never intended to have an affair end up having one. It does not excuse the behavior. Every incremental step involved a choice, and the accumulation of those choices produced a devastating outcome. But understanding the mechanism helps both partners make sense of a process that, from the outside, can look sudden and inexplicable.

Pittman’s Four Types of Affairs

Frank Pittman, a psychiatrist who spent his career treating infidelity, proposed a typology that remains clinically useful. He identified four patterns, each with different dynamics, different risk factors, and different implications for recovery.

Accidental affairs. These occur in a specific set of circumstances: alcohol, proximity, emotional vulnerability, a moment of poor judgment. They are typically one-time events that the unfaithful partner immediately regrets. The person did not seek out the affair and does not want to continue it. Accidental affairs are not trivial. They still constitute a betrayal and still produce trauma in the partner who learns of them. But they carry a different prognosis than the other types because they do not reflect an ongoing pattern or a sustained deception.

Habitual affairs. Some individuals engage in repeated infidelity across multiple relationships. This pattern often reflects deeper psychological dynamics: attachment avoidance, compulsive sexual behavior, personality pathology, or an internalized model of relationships in which fidelity is not a core value. Habitual affairs are the most difficult to treat because the behavior is ego-syntonic, meaning the person does not experience it as fundamentally at odds with who they are.

Romantic affairs. The unfaithful partner falls in love (or believes they have fallen in love) with someone else. These affairs are often long-term, emotionally intense, and accompanied by a narrative in which the affair partner represents the person’s “true” match. Romantic affairs pose a particular challenge to recovery because the unfaithful partner may genuinely grieve the loss of the affair relationship, creating a situation in which the betrayed partner must witness their spouse mourning someone else.

Arrangement affairs. In some relationships, infidelity operates within an implicit or explicit understanding. “Don’t ask, don’t tell” agreements, open relationships that one partner did not fully consent to, or cultural contexts in which extramarital sex is expected for one gender. These arrangements become traumatic when the implicit rules are violated or when one partner realizes they never actually agreed to the arrangement they are living inside.

Glass’s Walls and Windows

Shirley Glass, a clinical psychologist whose research on infidelity spanned three decades, introduced a metaphor that captures the structural shift that occurs when an affair develops. In a healthy relationship, there is a “window” between partners: transparency, openness, shared access to each other’s inner lives. There is a “wall” between the relationship and the outside world: certain things are private to the couple, certain forms of intimacy are reserved for each other.

An affair reverses this architecture. A wall goes up between the partners (secrecy, lies, emotional withdrawal), and a window opens between the unfaithful partner and the affair partner (shared confidences, emotional intimacy, sexual access). The betrayed partner finds themselves on the outside of their own relationship, sensing that something has changed but unable to see through the wall that has been erected.

Glass’s framework is particularly useful for understanding emotional affairs, which often begin without any physical contact. The structural shift (wall between partners, window to outsider) can occur through purely emotional channels. By the time anyone recognizes what has happened, the architecture of the relationship has already been fundamentally altered.

Perel’s Meaning-Making Framework

Esther Perel, a couples therapist whose work on infidelity has reached a wide audience, offers a framework that focuses less on the mechanics of how affairs happen and more on what they mean to the person who had one. She argues that affairs, while always harmful to the betrayed partner, are not always about the relationship. Sometimes they are about the self.

Perel identifies three questions that the unfaithful partner needs to answer: What was the crisis that the affair was responding to? What meaning did the affair hold? What vision for the future does the person want to pursue?

The crisis may be relational (a marriage that has become emotionally barren), existential (a confrontation with mortality, aging, or unlived life), or psychological (depression, loss, identity disruption). The meaning may be escape, self-discovery, rebellion, compensation for perceived deprivation, or a misguided attempt to feel alive. The vision involves what the person actually wants going forward, stripped of the justification narratives that sustained the affair.

Perel’s framework is controversial among trauma-informed clinicians because it can feel like it centers the unfaithful partner’s experience. That concern is valid. The framework is most useful when applied after the betrayed partner’s trauma has been stabilized and acknowledged, not as a first-line response to discovery. In the right therapeutic context, it can help both partners understand the affair as something other than a simple verdict on their relationship.

Why “The Relationship Caused It” Is Insufficient

One of the most harmful narratives in popular culture is that affairs happen because the relationship was broken. This framing implies that the betrayed partner is partly responsible: if they had been more attentive, more sexually available, more interesting, the affair would not have occurred.

The research does not support this. Gottman’s data shows that people in satisfying relationships also have affairs, and that many people in deeply unsatisfying relationships never do. Relationship dissatisfaction is a risk factor, not a cause. The difference between a dissatisfied partner who has an affair and a dissatisfied partner who does not is a series of individual choices made in the presence of opportunity and justified by an internal narrative.

This distinction matters clinically. If the relationship “caused” the affair, then both partners share responsibility for the betrayal. If, instead, the relationship had real problems AND the unfaithful partner made a series of choices that violated their commitment, then both things can be true simultaneously without the second being caused by the first. Real problems in the relationship can be addressed honestly. The affair is a separate matter that requires separate accountability.

Understanding Is Not Excusing

Nothing in this module reduces the unfaithful partner’s responsibility. Understanding why affairs happen does not make them acceptable. Knowing that gradual boundary erosion is a common pathway does not mean the person who eroded those boundaries had no choice. Recognizing that opportunity plays a role does not mean affairs are inevitable.

What understanding does is replace confusion with clarity. If you are the betrayed partner, understanding these dynamics can help you stop personalizing the affair as evidence of your inadequacy. If you are the unfaithful partner, understanding these dynamics can help you move past “I don’t know why I did it” toward the kind of honest reckoning that recovery requires.

Reflection

If you are the betrayed partner, notice which explanatory framework resonates most with what you have observed. Does the gradual erosion model match what you now know about the timeline? Does one of Pittman’s types describe what happened? You are not looking for excuses. You are building a map of what occurred, because a map is more useful than a fog.

If you are the unfaithful partner, consider which of Gottman’s three conditions (opportunity, narrative, erosion) you recognize in your own experience. What story were you telling yourself that made each step feel permissible? Answering this honestly, without minimizing or self-flagellating, is the beginning of the accountability your partner needs from you.