Measuring commitment in paws: Adaptation of the Investment Model Scale to the context of companion animals with a Portuguese sample

Commitment has a crucial role in predicting intentions and behaviors toward relationship maintenance. The Investment Model Scale (IMS; Rusbult et al., 1998) offers an empirically supported framework to describe and quantify commitment in various contexts, particularly romantic relationships. Only a few studies have attempted to extend this theoretical framework to examine human and companion animals (H-CA) relationships and measurement instruments lack psychometric evidence. We adapted the IMS to H-CA relationships based on previous studies and discussions with various experts (Study 1a), to help us clarify some items and develop new ones. We then performed Exploratory (Study 1b, N = 248) and Confirmatory Factor Analyses (Study 2, N = 236), to assess the scale's psychometric capabilities in this new context. During study 2, we also investigated scale sensitivity and criterion-based validity. Exploratory factor analyses showed that variables assessed by the IMS are transferable to the H-CA context. The same structure was found in the Confirmatory Factor Analyses. Results showed this new version of the IMS is psychometrically sound to be used by researchers and professionals, with the added precaution that the "Quality of Alternatives" dimension does not translate completely to this new H-CA environment and warrants further study.