The theory of self-determination considers that both environmental issues (such as the opportunity of choice) and intra-individual factors are correlated or predictive of self-determination. With regard to within-individual factors, some studies have explored the links between intellectual functioning and self-determination in people with Down Syndrome (DS). But the links between self-determination and certain cognitive (memory and language) and motor skills have not been clearly established. In this study, we compared three groups of people with DS ranked according to their self-determination scores with a group of adult controls. The comparison covered various areas of cognitive and motor skills. The results show that the most self-determined individuals with DS have better skills in many areas such as memory, motor and language skills, compared to the least self-determined individuals. The theory of self-determination allows us to interpret our results in terms of the role of the environment (and appropriate environmental supports) in the development of self-determination and through the more frequent achievement of daily activities, in the development of cognitive and motor skills.