Infidelity
What Is Infidelity?
Infidelity is one of the most painful experiences a relationship can go through, and one of the hardest to define. Ask ten people what “cheating” means and you may get ten different answers. That uncertainty matters, because what one partner sees as harmless, the other may experience as a profound betrayal.
A working definition
At its core: one review of the literature suggests infidelity covers any actions hidden from a partner, involving a third party, which breach the emotional, sexual or romantic exclusivity of the relationship.
Secrecy is central: it is rarely the act alone that wounds most deeply, but the deception around it.
It depends on the agreement: for couples who have chosen an open or non-monogamous relationship, infidelity is defined by breaking the agreed terms rather than by contact with someone else.
People don’t agree on what counts
Sexual infidelity is clearer: in a study of 379 people, both men and women were more consistent about what counted as sexual infidelity than emotional infidelity, suggesting emotional infidelity is a vaguer, more complex idea.
Definitions fall into distinct camps: a recent cross-cultural study grouped people’s definitions into five profiles: sex and intimacy-centred (around 48%), agreement-violation (around 22%), deception-focused (around 16%), health-risk (around 6%) and “nothing counts” (around 9%).
Men and women often differ: one study found women were more likely than men to see both sexual and emotional acts as infidelity.
Digital behaviour is especially contested: in one UK survey of gay and bisexual men, 55% considered using a hook-up app as infidelity, but only 43% thought sharing sexual images counted, and just 34% saw private messages that way.
Types of infidelity
Sexual: physical or sexual contact with someone outside the relationship
Emotional: a deep, intimate bond with someone else that takes the place of closeness with your partner
Digital or electronic: online relationships, sexting, dating or hook-up apps, secret messaging
“Micro-cheating”: smaller behaviours such as hidden flirtation, secret contact or minimising a relationship’s existence; often a grey area
Financial: hidden spending, secret accounts or debts; not always called infidelity, but often experienced as betrayal
How common is it?
Figures vary widely, mostly because studies define infidelity so differently.
Huge variation: a 2010 review of 31 studies found reported rates ranging from 1.2% to 85.5%, with narrow, sex-only definitions producing lower figures and broader definitions much higher ones.
The largest synthesis to date: a 2024 meta-analysis included 305 studies and 508,241 respondents across 47 countries.
Its estimates: among heterosexual people in married and dating relationships, rates were 17.5% for sexual infidelity, 27% for emotional infidelity and 18.4% for electronic infidelity.
Honesty depends on anonymity: people reported more sexual infidelity when their answers were anonymous, but this wasn’t the case for emotional infidelity, perhaps because it carries less stigma.
In the UK: a YouGov survey found one in five British adults admitted to an affair, and a third said they had thought about it; rates for men and women were almost identical (20% and 19%).
Affairs are rarely brief: 82% of those who’d had an affair said their longest lasted more than a week.
Why does it happen?
There is rarely a single cause. Common contributing factors include:
Emotional disconnection: distance, loneliness and unmet needs within the relationship
Opportunity and environment: Shirley Glass found that 62% of unfaithful husbands and 46% of unfaithful wives in her practice had affairs with someone from work
Attachment patterns: a meta-analysis of 13,666 participants found weaker attachment insecurity was linked to lower rates of infidelity
Personal history, transitions and stress: midlife, illness, bereavement, children, exhaustion
A wish to feel alive or different: sometimes the affair is less about the partner and more about a lost part of the self
Importantly, affairs don’t only happen in unhappy relationships. In Glass’s research, 56% of unfaithful men and 34% of unfaithful women described their marriages as happy or very happy before the affair.
Walls and windows
Shirley Glass’s metaphor remains one of the most useful ways of understanding how emotional affairs develop:
In a healthy relationship: partners keep windows of openness between them and walls that protect the relationship from outsiders.
In an affair: this is reversed, with walls of secrecy towards the partner and windows of intimacy with someone else.
A simple test: if a friend knows more about your relationship than your partner knows about that friendship, the walls and windows have already shifted.
It is often gradual: Glass argued affairs typically begin with emotional sharing that would normally belong to the partner.
The impact on the betrayed partner
Discovery of infidelity is increasingly understood as a form of trauma.
Clinically significant distress: in one study of 112 people whose partner had been unfaithful, most met or exceeded clinical thresholds for depression (71.6%), anxiety (66.4%) and trauma-like symptoms (73.9%).
PTSD-like symptoms: reactions can include flashbacks, nightmares, severe anxiety, hypervigilance and intense mood swings.
Suspicion doesn’t soften it: research with women in long-term marriages found that having suspected infidelity did not lessen the impact of discovery, which affected them emotionally, physically and cognitively.
Hypervigilance is a natural response: checking phones, bills and messages is common, and is better understood as a trauma response than a character flaw.
An IFS perspective
From an Internal Family Systems (IFS) perspective, infidelity often involves parts on all sides:
In the partner who strays: a part seeking aliveness, validation or escape, often alongside a part that compartmentalises so the behaviour can continue
In the betrayed partner: protectors that rage, investigate or shut down, guarding younger parts flooded with shame, fear and the old question Am I enough?
In the relationship: patterns of withdrawal and protection that may have been building long before the affair
Understanding these parts doesn’t excuse the betrayal. It can, however, help both people make sense of what happened, and decide what comes next from a steadier place.
What can help
Validation: the betrayed partner’s reactions are normal responses to a profound shock
Honesty and transparency: trust is rebuilt through consistent openness, not a single apology
Time: recovery is rarely linear
Clarity about the agreement: what does fidelity mean for this couple?
Professional support: individual or couples therapy can help whether the relationship is rebuilt or ends
A final thought
Infidelity is not simply about sex, and it is not always about an unhappy relationship. At its heart, it is a rupture of trust, a breach of the agreement two people believed they shared. Whether a couple rebuilds or parts, the work is the same: understanding what happened, honouring the pain, and finding a way to move forward with clarity.
If you are affected by infidelity, whether you have been betrayed or have strayed, you are welcome to get in touch to explore support, individually or as a couple.
References
Glass, S. P. (2003). Not “Just Friends”: Protect Your Relationship from Infidelity and Heal the Trauma of Betrayal. Free Press.
Guitar, A. E. et al. (2017). Defining and distinguishing sexual and emotional infidelity. Current Psychology.
Warach, B., Josephs, L. & Gorman, B. S. (2024). The current state of affairs in infidelity research: a systematic review and meta-analysis. Personal Relationships.
Ciaffoni, S. et al. (2026). Love the way you lie: a latent class approach to defining and experiencing infidelity. European Association of Social Psychology General Meeting.
The interplay of attachment styles and marital infidelity: a systematic review and meta-analysis (2023). Heliyon.
YouGov (2015). One in five British adults admit to affair.
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