Infidelity and Jealousy

INTRODUCTION

When Trust Wavers

There are pains that leave no visible trace. No cast, no scar that you could show to explain why you are staggering. Infidelity is one of those invisible wounds that nonetheless upend the entire existence of those who endure it — and, more than we think, of those who commit it.

As a cognitive-behavioral psychotherapist, I have supported hundreds of people confronting the issue of infidelity. Men and women of all ages, all backgrounds, all histories. Some came after discovering their partner's betrayal. Others came to confess their own, consumed by guilt or confusion. Still others came for a jealousy that was devouring them, even though no act of infidelity had been committed.

What I have learned over the course of these encounters is that infidelity is almost never what it appears to be at first glance. It is not simply the story of a body that strays or a heart that splits in two. It is the revealer of deep dynamics — individual, relational, sometimes transgenerational — that deserve to be explored with rigor and compassion.

The numbers speak for themselves. According to the most recent epidemiological studies, between 20 and 40 percent of people in committed relationships acknowledge having been unfaithful at least once in their lives. In France, IFOP estimates that roughly one-third of men and one-quarter of women have experienced an extramarital relationship. These figures, as striking as they may be, say nothing about the scale of suffering that each percentage represents. Behind every statistic, there is a couple, a family, sometimes children, and always human beings trying to make sense of something that seems to have none.

Jealousy, for its part, is an almost universal companion of love. Who has never felt that pang in the stomach imagining the beloved in someone else's arms? But between a passing jealousy, a sign of attachment, and a devouring jealousy that poisons daily life, there exists a broad spectrum that we will explore in these pages. Neuroscience teaches us that jealousy activates the same brain circuits as physical pain — so it is not "all in your head," as people sometimes say dismissively. It is in the body, in the gut, in every fiber of one's being.

This book was born from a conviction: that knowledge is the first step toward liberation. Understanding why infidelity occurs does not mean excusing it. Analyzing the mechanisms of jealousy does not mean trivializing it. On the contrary, the aim is to give each person the tools to navigate these ordeals consciously, to make informed choices rather than choices dictated by panic, anger, or shame.

You will find in these pages an approach firmly grounded in scientific research. Every claim is supported by studies published in peer-reviewed journals. I have drawn on the work of internationally renowned researchers — John Gottman, Shirley Glass, Esther Perel, Janis Abrahms Spring, to name but a few — as well as the most recent publications in clinical psychology and affective neuroscience.

However, this book is not an academic treatise. It is above all a practical guide, written to be useful. Each chapter contains fictional clinical cases — inspired by real situations but entirely anonymized and recomposed — that put a face and a story to the concepts presented. Each chapter also offers practical exercises that you can complete alone or as a couple, at your own pace.

The journey I propose follows a deliberate logic. We will begin by exploring the psychological reasons for infidelity, then address the new forms of betrayal born of the digital world. We will delve into the mechanisms of jealousy before pausing on the shock of discovery. The central chapters will guide you through the stages of recovery and the delicate question of forgiveness. We will also address, without taboo, the decision to leave when it is necessary. Finally, we will conclude with the rebuilding of trust — whether with the same partner or with a new one.

Whatever point you find yourself at in your own story — whether you have just discovered infidelity, are in the midst of questioning, are trying to rebuild, or are considering leaving — this book is for you. It will not tell you what to do, but it will give you the means to understand, to feel with greater clarity, and ultimately, to decide for yourself.

For this is perhaps the most important lesson that infidelity and jealousy teach us: no one can choose in our place. But we can choose to no longer be passive victims.

Happy reading.

Gildas Garrec

CBT Therapist

Nantes, October 2025

CHAPTER 1 — Why People Cheat: The Psychological Factors

The Enigma of Infidelity

"I don't understand what happened to me."

This is probably the sentence I hear most often in consultations when someone comes to talk about their infidelity. No bravado, no justification — just a deep incomprehension of their own actions. And it is this incomprehension, more than moral judgment, that constitutes the most fertile starting point for therapeutic work.

Research in relational psychology teaches us that infidelity is rarely the product of a single cause. It most often results from a convergence of individual, relational, and situational factors that together create what researchers call a "window of vulnerability." Understanding these factors is not an endeavor aimed at exonerating anyone from their responsibility. It is an effort of lucidity indispensable for anyone who wishes to avoid repeating the same patterns or, conversely, for anyone who wishes to understand what happened in their relationship.

A meta-analysis published in the Journal of Sex Research (Fincham and May, 2017) identified three main categories of factors predisposing to infidelity: individual factors, relational factors, and contextual factors. We will explore each in turn, drawing on the most recent data from the scientific literature.

Insights from Evolutionary Psychology

Before examining individual and relational factors, it is illuminating to step back and consider what evolutionary psychology teaches us about human infidelity. The work of David Buss, professor at the University of Texas at Austin, and his collaborators — notably Joshua Duntley — have considerably enriched our understanding of the phenomenon.

According to Buss (2018), infidelity is not a behavioral aberration but a "strategic dilemma" that the human species has faced for hundreds of thousands of years. In his "Sexual Strategies Theory" model, Buss proposes that men and women have developed, over the course of evolution, different but complementary reproductive strategies. Men, whose minimal reproductive investment is biologically low (a few minutes compared to nine months of pregnancy for women), would have been selected to value sexual diversity — hence a statistical predisposition, not a destiny, to seek multiple partners. Women, whose reproductive investment is considerable, would have been selected to prioritize partner quality and resource security — hence a heightened sensitivity to emotional infidelity, perceived as a threat to paternal investment.

It is crucial to emphasize that this evolutionary perspective in no way constitutes a justification for infidelity. Understanding the origin of a tendency does not legitimize it. We are endowed with a prefrontal cortex capable of overcoming our ancestral impulses — that is, in fact, what defines psychological maturity. However, knowing these predispositions helps us better understand why fidelity requires an active and deliberate effort, and why temptation is such a universal experience.

Duntley and Buss (2012) also proposed the concept of "adaptive anti-fidelity": in certain ancestral contexts, infidelity could present adaptive advantages for both sexes — genetic diversification for men, access to better resources or better genes for women. This model explains why infidelity persists despite its considerable social costs: it is, in a sense, "inscribed in the repertoire" of our behavioral possibilities, even though our culture and morality condemn it.

A fascinating study by Buss and Haselton (2005) showed that women in the fertile phase of their menstrual cycle report more fantasies involving men other than their partner, particularly men displaying markers of "good genetic quality" (facial symmetry, deep voice, social dominance). This finding, replicated in several laboratories, illustrates the tension between our biological programs and our social commitments — a tension that each individual manages differently depending on their history, values, and psychological resources.

The Different Types of Infidelity

Before exploring risk factors in detail, it is essential to distinguish the different types of infidelity, as each rests on distinct psychological mechanisms and calls for different therapeutic responses.

Sexual Infidelity

This is the most "classic" and most easily identifiable form: physical contact of a sexual nature with a person other than one's partner. It can be one-time (a single night, an isolated lapse) or sustained (a maintained affair over time). Research by Blow and Hartnett (2005) shows that one-time sexual infidelity is often linked to situational factors (alcohol, opportunity, distance), while sustained sexual infidelity is more associated with relational factors (chronic dissatisfaction, intimacy deficit).

Emotional Infidelity

Less visible but potentially equally devastating, emotional infidelity consists of developing a deep bond of intimacy — confidences, emotional support, exclusive complicity — with a person outside the couple, while keeping this bond secret. Shirley Glass, a pioneer in research on this subject, showed that emotional infidelity follows an insidious progression: what begins as an innocent friendship gradually slides toward an increasingly intimate connection, with an inversion of "walls and windows" — the walls go up between the partners in the couple, while the windows open toward the outside person.