1 Substantive-Methodological Synergy Research Laboratory, Department of Psychology, Concordia University, 7141 Sherbrooke W, Montreal, QC, H4B 1R6, Canada.
2 Département de psychopédagogie et d'andragogie, Université de Montréal, Montreal, Canada.
3 Substantive-Methodological Synergy Research Laboratory, Department of Psychology, Concordia University, 7141 Sherbrooke W, Montreal, QC, H4B 1R6, Canada. alexandre.morin@concordia.ca.
4 School of Education, Western Sydney University, Sydney, Australia.
5 Institute for Positive Psychology and Education, Australian Catholic University, Sydney, Australia.
6 Cyberpsychology Laboratory, Department of Psychoeducation and Psychology, Université du Québec en Outaouais (UQO|Campus de Saint-Jérôme), Saint-Jérôme, Canada.

This study proposes a multi-informant (youth, teachers, and parents) measure of relationship quality with adults for youth with intellectual disabilities (ID). A sample of 395 youth with mild (49.15%) and moderate (50.85%) ID, aged 11-22 (M = 15.82) was recruited in Canada (French-speaking, N = 142), and Australia (English-speaking, N = 253). Results support the reliability, factor validity, discriminant validity (in relation to sex, ID level, country, and comorbidity), convergent validity (depression, anxiety, aggressiveness, and prosocial behaviors), and one-year longitudinal stability of the measure. Youth self-reports provide a complementary perspective on relationship quality with adults relative to teachers' or parents' reports, whereas teachers and parents seem unable to differentiate their own perspective from that of the target youth.