1 Département de Psychopédagogie et d'andragogie, Université de Montréal, Montréal, Canada.
2 Substantive-Methodological Synergy Research Laboratory, Department of Psychology, Concordia University, Montréal, Canada. alexandre.morin@concordia.ca.
3 School of Education, Western Sydney University, Sydney, Australia.
4 Substantive-Methodological Synergy Research Laboratory, Department of Psychology, Concordia University, Montréal, Canada.
5 Département de Psychoéducation et de Psychologie, Université du Québec en Outaouais, Saint-Jérome, Canada.
6 Institute for Positive Psychology and Education, Australian Catholic University, Sydney, Australia.
7 Cyberpsychology Laboratory and Département de Psychoéducation et de Psychologie, Université du Québec en Outaouais, Saint-Jérome, Canada.

Youth with intellectual disability (ID) are at an increased risk of displaying fewer prosocial behaviors and more numerous aggressive behaviors in various environments. This study proposes a new multi-informant (youth, teachers, and parents) measure of social behaviors for youth with ID. The sample includes 348 youth with mild (51.41%) and moderate (48.59%) levels of ID, aged 11-22 years old (M = 15.73, SD = 2.14; including 138 females), enrolled in secondary schools in Canada (French-speaking; N = 116; 33.33%) and Australia (English-speaking; N = 232; 66.67%). Measures were completed by the participants, their teachers, and their parents. Results support the reliability, factor validity, discriminant validity (in relation to sex, ID level, and country), concurrent validity (with measures of victimization, depression, hyperactivity-inattention), and one-year test-retest stability of the measure. Youth, teachers, and parents all provided a complementary perspective on youth social behaviors, consistent with youth adjusting their behaviors to the various environments in which they share social interactions.