PDF Summary:Not Just Friends, by Shirley P. Glass

Infidelity can devastate a marriage, but not all affairs look the same—and understanding the distinctions matters. In Not Just Friends, Shirley P. Glass explores the different forms of infidelity, from emotional betrayal to sexual affairs to combined infidelity that involves both. She examines why people cheat, looking at individual personality traits, relationship vulnerabilities, and environmental factors that create pathways to betrayal.

Glass also addresses what comes after the affair: the trauma, the symptoms, and the long road to recovery. She explains how couples can rebuild trust and emotional connection through honest communication, accountability, and a willingness to examine the vulnerabilities that led to the infidelity. This guide offers insight into both preventing affairs and healing from them.

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Type-T personalities are thrill-seekers who are drawn to the excitement of risking danger and the challenge of staying undetected. Alexithymic individuals have difficulty naming emotions and describing what they feel. They can't feel the calm satisfaction of a steady partnership. They enjoy passionate sexual experiences and the feeling of love, prioritizing intense connections over intimacy. Glass also notes that certain individuals are sexually addicted. They feel worthless and can't control their urges, even if it means potential humiliation or danger to their personal or professional lives.

(Shortform note: While some mental health researchers have argued that sexual addiction should be included in the Diagnostic and Statistical Manual of Mental Disorders (DSM), the American Psychiatric Association has not yet recognized it as a clinical diagnosis. The World Health Organization’s International Classification of Diseases (ICD-11) does include “compulsive sexual behaviour disorder” (CSBD), but one academic paper notes that the ICD-11 doesn’t classify CSBD as an addiction. This is because some clinicians and psychiatrists argue that the term “addiction” is stigmatizing and that the evidence for CSBD as an addiction is inconclusive.)

Narcissists are overly fixated on their own suffering and can't empathize with the hurt they inflict by being unfaithful. They possess an inflated perception of their own significance and demand that others give them particular notice. They feel entitled and lack remorse about being unfaithful. People diagnosed with ASPD are deceitful and manipulative. They find it hard to remain monogamous and might exploit their sexual partners. They possess a defective moral sense and often shift responsibility to their partners if their unfaithfulness is discovered. They disregard others and don't acknowledge legal limitations and social norms. Lastly, Glass describes chronic liars as people who commit tax fraud, fail to fulfill promises, and shift responsibility for their errors onto others. They easily come up with schemes to avoid being caught and don't feel guilty about betraying others.

The Dark Triad and Infidelity

Research on the “Dark Triad” personality traits—narcissism, Machiavellianism, and psychopathy—shows that people who score high on these traits are more likely to adopt short-term mating strategies. This means they’re more likely to seek out casual sexual encounters and have a lower commitment to long-term relationships. Narcissists, for example, tend to have an inflated sense of self-importance and a strong need for admiration. This makes them more likely to seek out multiple sexual partners to boost their ego. People with antisocial personality disorder (ASPD) often lack empathy and have a disregard for social norms, which can lead them to engage in risky sexual behaviors without considering the consequences for themselves or their partners. Chronic liars, who often exhibit traits of both narcissism and ASPD, may use deception to manipulate others into sexual relationships.

Relational and Environmental Risk Factors

Glass explains that emotional intimacy with someone outside the marriage can result in infidelity. This occurs when you share personal information with someone that you would normally only tell your spouse. It can start as a non-romantic bond and gradually become a romantic attachment, causing someone to live a sexual and deceitful double life, which can threaten a marriage.

(Shortform note: When you repeatedly share personal information with someone, your brain starts to associate that person with positive feelings and emotional rewards. This can make you feel closer to them and more comfortable opening up, even if the relationship starts out as non-romantic. Over time, this can lead to a shift in your brain's attachment and reward systems, making you feel like this new person is your primary bond.)

She also notes that modern settings help conceal affairs. People of different genders can freely spend time together in public, and it’s hard to tell if they’re just friends or not.

(Shortform note: This claim doesn’t apply to all cultures. In Sex and the Citadel, Shereen El Feki describes how, in many parts of the contemporary Arab world, social and legal norms strictly limit contact between unrelated men and women.)

Recovering and Repairing a Partnership Following Infidelity

Glass believes that restoring trust after infidelity involves understanding each other's perspectives and vulnerabilities. Restoring trust requires both partners to make a sustained effort for an extended time. The betrayed partner will be alert to any indications of dishonesty or further betrayal. The partner who was unfaithful should be open, honest, and accountable, ready to respond to questions about their plans, activities, and company. They must also significantly alter the behavior patterns that resulted in the infidelity. The couple must be willing to investigate the weak points in how they interact that contributed to the infidelity.

(Shortform note: In The Body Keeps the Score, Bessel van der Kolk writes, “Being able to feel safe with other people is probably the single most important aspect of mental health; safe connections are fundamental to meaningful and satisfying lives.” One way to begin restoring trust is to create a nightly “safety ritual.” Set aside 10 minutes each night to quietly review one small part of the day together while breathing slowly. This helps your body associate increased transparency with calmness rather than alarm.)

Next, we’ll look at healing actions and accountability, as well as trauma, symptoms, and therapeutic approaches.

Healing Actions & Accountability

Glass asserts that both partners must take responsibility for restoring the relationship. They need to be open to putting in the effort to heal, and the affair must be over for this process to begin. It becomes evident that partners are healing when they display greater resilience and cope with the symptoms of trauma collectively. They’re also open to each other's attempts to offer reassurance and rebuild trust.

(Shortform note: In Why Does He Do That?, Lundy Bancroft argues that in relationships where there’s ongoing coercive control or violence, the safety of the targeted partner should take precedence over preserving the relationship. He contends that meaningful change can only begin when the abusive partner accepts full responsibility for his actions. When the targeted partner is encouraged to “work on the relationship” or “share responsibility,” it can increase her emotional and physical risk.)

Next, we'll examine rebuilding trust and emotional connection.

Reestablishing Trust and Emotional Bonds

According to Glass, rebuilding trust and emotional connection requires communicating truthfully and shared meaning. The recovering partners must continue within a supportive, secure, and committed environment. You'll know the healing journey is progressing well when the infidelity has ended, the unfaithful partner is clearly returning to the marriage, and you're working through the wounded partner's unresolved issues. Betrayed spouses who are recovering respond positively to the efforts of unfaithful partners to provide reassurance and reestablish trust. The aim is to redefine the couple's identity and view the past more clearly, despite lingering hurt.

(Shortform note: In What Makes Love Last?, John Gottman and Nan Silver suggest a daily check-in to foster a supportive, secure, and committed environment. Set aside time each day to talk about a stressor outside the relationship. One partner shares while the other listens and reflects back what they hear. If either partner feels physically overwhelmed, pause and take a break. This practice helps lower stress hormones, strengthens your emotional bond, and builds a stable sense of trust and safety between you.)

Trauma, Symptoms, and Therapeutic Approaches

Unfaithfulness may cause trauma, especially if there are pre-existing weaknesses or stressors. Glass explains that people with low self-esteem, a history of neglect or abuse, or who witnessed parental infidelity may be more deeply affected by a partner’s betrayal. The timing of the infidelity can also intensify the trauma, such as when it occurs during a stressful life event like pregnancy, illness, or losing a loved one. The nature of the affair matters too: a long-term affair, one involving a close friend, or one with multiple betrayals can have a greater impact than a short-term infidelity with a stranger.

Betrayal Trauma Theory

The points above are all aspects of betrayal trauma theory, which is a framework for understanding how the brain processes and stores traumatic experiences. In Betrayal Trauma, Jennifer J. Freyd explains that betrayal trauma occurs when the people or institutions on which a person depends for survival significantly violate that person’s trust or well-being. Betrayal trauma theory suggests that the brain may process and store memories of betrayal differently than other types of trauma, particularly when the victim is dependent on the betrayer. This can affect how the traumatic events are perceived, encoded in memory, and later recalled.