Introduction:
Deciding whether to stay in or leave a romantic relationship after infidelity is a complex and stressful challenge. Research shows that, besides individual emotions and values, social factors - including the opinions of close others - strongly shape this process. Existing measures capture only general social support, ignoring its direction, quality, and strength.


Purpose:
This study aimed to examine how post-infidelity relationship decisions relate to social influence by developing and validating the Perceived Social Influence after Infidelity Questionnaire (KPWZ).


Method:
Guided by theories of normative and informational influence, an item pool was created to measure three dimensions: pressure to leave, pressure to stay, and autonomy-supportive neutrality, plus perceived influence strength across five sources (family, friends, mental health professionals, online groups, spiritual advisors). Content validity was assessed by expert judges, then the questionnaire was tested on adults with infidelity experience (N = 180).


Results:
Analyses showed high reliability (Cronbach's α = 0.78) and satisfactory validity. Convergent validity revealed positive correlations with the Multidimensional Scale of Perceived Social Support (r = 0.36, p < 0.001), while discriminant validity with the Short Familism Scale confirmed the independence of constructs (r = 0.09, p > 0.05). The tool differentiates direction and strength of influence effectively, and participants rated it as clear and relevant.


Conclusions:
The KPWZ is a tool that enables quantitative measurement of perceived social influence in the context of romantic infidelity, taking into account both the quality and intensity of influence. It has strong potential for use in psychological research on post-infidelity decision-making as well as in clinical practice and family counseling.