While considerable research has been carried out on the topic of infidelity, little agreement exists regarding its definition. Drigotas opined that infidelity occurs when a person feels that their partner has violated the relationship norm by interacting with someone who is not a part of their relationship [4]. However, Blow and Hartnett defined infidelity as “…a sexual and/or emotional act engaged in by one person within a committed relationship, where such an act occurs outside of the primary relationship, and constitutes a breach of trust and/or violation of agreed upon norms (overt and covert) by one or both individuals in that relationship in relation to romantic, emotional or sexual exclusivity” [5]. Reviewing both definitions, a distinction needs to be made between sexual and emotional infidelity and its newer concepts related to inappropriate online and offline behavior, which we will address later [6].

Generally, infidelity is defined as any type of secret emotional, sexual, or romantic behavior that violates the exclusivity that romantic relationships have by definition. However, there are varied definitions of infidelity, which can be divided into subtypes sexual, emotional, combined (sexual and emotional), and internet infidelity [7]. Examples of the various (and sometimes contradictory) definitions, can be gleaned from Bernard who believed that partners who failed to love, honor and support their partners were engaging in infidelity since they did not honor their vows to remain with their romantic partner [8]. In contrast, Pittman and Wagers held a different position and maintained that the hallmark of infidelity involves the secrecy and concealment of behaviors with an individual outside of the committed relationship [9]. Thompson had a more comprehensive view of infidelity and postulated that infidelity occurs if: (a) the extradyadic behavior is not condoned by one’s romantic partner, (b) the behavior occurs outside of the primary relationship, and (c) the behavior can be described, such as intercourse, flirting, etc. [10].

Sexual infidelity was defined by Leeker and Carlozzi as “the occurrence of sexual involvement with a third party that violates the ground rules established by the couple (e.g., kissing, fondling, oral sex, vaginal sex, anal sex)” [11].

Emotional infidelity was seen as “the occurrence of emotional involvement with a third party that violates the ground rules established by the couple (e.g., trusting another, sharing your deepest thoughts with another, falling in love in another, being vulnerable with another, being more committed to another, spending more money on another)” [11].

Research comparing the lasting effects of sexual and emotional infidelity found that men were more distressed by sexual infidelity, while women were more upset by emotional infidelity [12,13] Research that addressed the reactions of lesbian, gay, and heterosexual men and women to infidelity found that for all four groups, emotional infidelity was more distressing than sexual infidelity [11]. Cramer et al., found that women perceived emotional infidelity as more upsetting than men did, and the explanation provided by them was that women believe that men are not able to maintain sexual faithfulness in their relationships, but will still remain emotionally loyal to their spouses regardless [14].

Leeker and Carluzzi explored how sexual orientation, love, and infidelity expectations might affect the reaction toward emotional and sexual infidelity [11]. Their study involved 296 individuals: 72 lesbians, 114 heterosexual women, 53 gay men, and 57 heterosexual men, who were older than 18 years of age and who indicated that they were currently involved in a committed romantic relationship. They found that sex and sexual orientation were significant predictors of general distress, anger, anxiety, jealousy, and humiliation, in response to both emotional and sexual infidelity. Commitment was predictive of distress and anger in response to emotional infidelity, while sexual infidelity aroused distress and anxiety.

Addressing the various types of infidelity, emotional infidelity includes the development of deep, intimate feelings for an extradyadic partner, while sexual infidelity refers to engaging in sexual behavior with that person. Those who engage in both emotional and sexual behavior are said to be involved with composite infidelity, while internet infidelity is carried out (at least initially) virtually/online [7]. Other researchers have employed even narrower definitions of infidelity by focusing on specific behaviors such as spending time with another individual and going on romantic dates, engaging in kissing, fondling, or even sexual intercourse, suggesting that they all constitute unfaithful behaviour [5,15].

Differences between the various types of infidelity were also observed in the work of Guitar et al., who reported that emotional infidelity is more complex than sexual infidelity [16]. Three hundred and seventy-nine undergraduate students provided their interpretations of emotional and sexual infidelity, which were later categorized into themes for content analysis. Participants’ responses indicated that emotional infidelity included themes such as love and betrayal along with sexual infidelity and/or intentions to have sexual relations with someone outside the pair bond. Particularly, women saw emotional infidelity as carrying the potential of later sexual betrayal in such partnerships. This suggests that the nuances involved with conceptualizing emotional infidelity may surpass the conditions needed to fulfill sexual infidelity and that these differences may be most salient when observing differences across genders.

In fact, research has shown that men appear to hold more permissive attitudes toward extramarital sex than women do [17]. They also reported experiencing greater levels of stress related to the sexual infidelity of their partner, whereas women react more negatively to emotional infidelity than men [3]. However, women also seem to consider more behaviors as infidelity compared to men in both offline and online spaces [3].

Moreover, shared opinions regarding what specific behaviors are considered unfaithful in nature have also been identified in the literature. For example, work by Bozoyan and Schmiedeberg found that extradyadic intercourse was regarded as infidelity [3]. Kissing someone who is not one’s partner was also reported as infidelity, especially if emotional involvement was part of it. The results of their research point to a perception of sexual infidelity as more distressing than emotional infidelity. However, women tended to judge behaviors as being unfaithful slightly more strictly than men, which is in line with other research in the existing literature [17]. Despite this, it appears that overall gender differences regarding the prevalence of infidelity have been shrinking over the past few decades [18].

Whitty and Quigley constructed a survey that aimed to explore what would upset participants from a list of several described situations [19]. Next, drawing from Harris and Christenfeld’s work, participants were then asked how they would feel if their partner was unfaithful and was in love with someone else [20]. Sabini and Green relied on Buss et al.’s much-utilized approach and described a situation where the partner of the participant was having deep emotional or sexual involvement with someone else [21,22]. Participants were asked to describe how they would feel in such a situation. They found that both, men and women, saw a partner’s emotional involvement as a more threatening sign of their partner's leaving than when there was only sexual involvement.

In their study on infidelity, Leeker and Carlozzi utilized Cramer et al.’s questionnaire, in which participants rated the likelihood of their partner engaging in each item with a third party, on a seven-point Likert scale, aiming to identify their reactions to emotional vs. sexual intimacy [11,14]. Another measure that was utilized in Leeker and Carlozzi’s study was continuous emotion ratings [13]. These ratings served as the dependent variable in their study, assessing how angry, anxious, jealous, and humiliated each participant felt in response to infidelity in their romantic relationship.

Symons and Buss were the first to view infidelity from an evolutionary perspective [23]. They opined that women are more likely to be affected by emotional infidelity rather than by sexual infidelity due to the fact that women carry the fetus and give birth. Thus, they are more threatened by the emotional betrayal of their romantic partner, whom they count on to provide for the developing family unit. Men on the other hand are more threatened by sexual infidelity. Fisher et al., observed that for thousands of years, women depended on men to provide their food, shelter, and safety, and this is the reason why they are more hurt by emotional infidelity which may threaten the partner’s commitment [24]. Men, however, are more threatened by sexual infidelity, as evolutionarily, they were not sure whether the child was theirs (versus the mother who carries her own child) and did not want to protect, feed and care for someone else’s offspring. This was referred to as jealousy as a specific innate module (JSIM). While the evolutionary perspective is the most accepted one, the social-cognitive perspective was proposed as an alternative to JSIM, and maintained that jealousy is not a simple module but includes several different feelings, each triggered by a different aspect of the jealousy-provoking situation. Anger was identified as a major component of the response to infidelity [25,26].