Background: This study focused on attitudes of workers and family members towards self-determination of individuals with intellectual disability. First, we compare their self-determination attitudes. Second, we test the link from mutual communication (workers and family members perceive that the other party openly dialogues about self-determination) to individual attitudes. Finally, we examine the relationship between mutual communication and affinity in attitudes. Method: We conducted a survey study with 111 organisations (914 workers and 845 family members). Results: Workers have more positive attitudes than family members. In addition, high mutual communication was associated with individual positive attitudes towards self-determination and shared attitudes between workers and family members. Conclusions: When both workers and family members perceive that the other party is accessible and willing to engage in open dialogue about self-determination issues, their attitudes converge.