Background: A growing number of services are focused on people with intellectual disabilities. Our study investigates interpersonal justice (interpersonal treatment families receive from service professionals) perceived by family members as a heuristic (cognitive shortcut) they can use to evaluate service performance directed towards improving the quality of life of their relatives with intellectual disability. Method: Using a sample of 86 family members, we tested a within-person model. Interpersonal justice and service performance were measured four times (T1-T4). Results: Over time, changes in interpersonal justice were positively related to changes in service performance. Family members use the treatment they receive from professionals to superficially draw inferences (heuristic) about service performance. Conclusions: Our study warns about the use of superficial justice information based on a heuristic to evaluate service performance. More active participation of families in the organizational life is suggested to avoid the use of these types of cognitive shortcuts.