Background: Persons with intellectual disability often omit acting on intentions in the future, e.g., to take medicine at lunch. Despite research in different clinical groups, prospective memory has received little attention in relation to intellectual disability. A prospective part (when to act/timing) and a retrospective part of prospective memory (what to do/plan)are identified. Retrospective memory and vigilance are important for execution in prospective memory in persons with intellectual disability.

Method: A group with intellectual disability (IQ < 70, n=58) was defined together with a control group matched on age, sex, level of education and years of education (n=116)in the Swedish Betula database.

Results: Prospective memory performance was lower in the intellectual disability than the control group. About half of the participants with intellectual disability remembered the retrospective part of the prospective memory task if the experimenter provided a cue. Persons with intellectual disability were able to perform on the prospective memory task, despite performance at floor level for verbal prospective memory tasks in previous studies. Episodic memory was related to recognition, recall and semantic memory in both groups. Contrary to previous results, prospective memory correlated with episodic memory, semantic memory, short-term memory, recall, recognition, and knowledge only int he control group.

Discussion: Persons with intellectual disability remembered the retrospective aspect of the prospective memory task despite lower performance than the control group on other memory tasks. The intellectual disability group performed lower than the control group although performance preceded the ongoing task. Finishing all tasks may have formed a context to associate with prospective memory performance in particular for the control group with a higher memory capacity to engage in memory processing compared to the intellectual disability group. We suggest matching on education as a reason for this result.