Tolerance for Inclusion: Scale Development, Psychometric Evaluation, and Initial Examination of Cross-Contextual Applicability

The inclusion of people with diverse circumstances and needs is an important goal of contemporary society and building such an inclusive society requires mutual tolerance among its members. This study aimed to develop a scale measuring individuals’ attitudes and behaviours based on the concept of tolerance for inclusion and to evaluate its psychometric properties. The study comprised three major steps involving four surveys. First, we developed the Individual-level Tolerance for Inclusion Scale (ITIS) based on a conceptualization in previous qualitative research. Second, we conducted three online surveys in Japan. Survey 1 (n = 165) was conducted to explore the factor structure of the newly developed scale. Survey 2 (n = 2,526) and Survey 3 (n = 428) were conducted to evaluate its construct validity, internal consistency, test–retest reliability, and standard error of measurement. Third, we conducted Survey 4 (n = 1,114) in Australia, the United Kingdom, and the United States of America and examined the cross-cultural validity of the new scale and its concurrent validity with an existing measure of individual-level tolerance. The results of present study support the validity of the ITIS for assessing tolerance for inclusion and its reliabilities may also be acceptable. In addition, the findings provide initial support for the applicability of the concept of tolerance for inclusion in a combined sample from three English-speaking countries.