Introduction
Infidelity is the most common cause of divorce and can have consequences, such as dissatisfaction, depression, blame, and hopelessness for couples. Infidelity is defined as engaging in emotional or sexual acts occurring outside of the primary relationship, such as flirting, emotional affair, sexual intercourse, or pornography use. With the increased access to the internet and smartphones, pornography use has become more common as online infidelity among couples. The individual characteristics of couples, such as attachment styles, personality traits, socio-demographic factors, sexual attitudes, and the quality of marital relationships (including sexual satisfaction and emotional relationship), can predict infidelity.  Regarding personality traits, the dark personality triad (Machiavellianism, narcissism, and psychopathy) has been linked to infidelity. Furthermore, the quality of marital relationships can also be related to infidelity. Social-emotional loneliness is also one of the factors that can be related to infidelity and its consequences. Loneliness caused by infidelity is associated with depression in couples. Therefore, it can be hypothesized that social-emotional loneliness can have a mediating role in the association between the quality of marital relationships and infidelity and between dark personality traits and the quality of marital relationships. However, to the best of our knowledge, these associations have not been fully studied. Therefore, this study aims to evaluate the mediating role of social-emotional loneliness in the relationship between the quality of marital relationships and dark personality traits and between the quality of marital relationships and online infidelity.

Methods
This descriptive survey study was conducted on married couples in Mashhad, Iran. The inclusion criteria were at least two years of marriage, age 20-45 years, less than 12 years of age difference between couples, at least a high school education, access to a smartphone and the internet, membership in social networks, no history of drug or alcohol abuse, and no history of receiving psychiatric medications. Exclusion criteria were a history of psychiatric diseases leading to hospitalization or medication administration, request for divorce or separation, unwillingness to participate in the study, and incomplete responses to the questionnaires. The minimum sample size of 410 was determined using the G*Power software, which was increased to 470 by considering a 15% sample dropout rate. 
A demographic form (age, gender, educational level, number of children, age difference between couples, and marriage duration), the dark triad scale (Jenison and Wister, 2010), the relationship quality scale (Chondoy et al., 2016), the internet infidelity scale, and the Social and Emotional Loneliness Scale for Adults (DiTommaso et al., 2004) were used to collect data. Structural equation modeling (SEM) was used to check the relationship between the study variables using Smart PLS software.

Results 
Among 434 participants, the majority were female (89.4%), and the mean age was 32.41±5.74 years. The goodness of fit index for the final SEM model was 0.409, indicating the high quality of the model, and the standardized root mean square residual (SRMR) was less than 0.1, indicating the good fit of the model. There was a direct relationship between dark personality traits and online infidelity (P<0.05). The marital relationship quality had an indirect relationship with online infidelity through social-emotional loneliness (P=0.023). No mediating effect was observed for social-emotional loneliness in the relationship of dark personality traits and marital relationship quality with online infidelity (P>0.05) (Table 1).



Conclusions
According to the results, there is a weak relationship between dark personality traits and online infidelity in Iranian married couples. There is also a significant relationship between social-emotional loneliness and the quality of marital relationships and between social-emotional loneliness and online infidelity. The social-emotional loneliness does not have a mediating role in the relationship of dark personality traits and the quality of marital relationships with online infidelity in Iranian married couples. 

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of Mashhad University of Medical Sciences (Code: IR.MUMS.REC.1402.205).

Funding
This study was funded by Mashhad University of Medical Sciences. 

Authors contributions
Conceptualization: Elham Didehban; Methodology: Elham Taheri; Investigation, original draft: Elham Didehban, Arezoo RanjbarMoqadam, and Ali Kanaghestani; Writing, review and editing: All authors; Funding acquisition: Elham Taheri.

Conflicts of interest
The authors declare no conflict of interest.

Acknowledgments
The authors would like to thank all participants and the Deputy for Research of Mashhad University of Medical Sciences for their cooperation and support.

 

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