IASSIDD was born out of three international congresses held in Europe between 1960 and 1963. The first, held in London in 1960, was titled Observation of World Mental Health Year. This event was sponsored by the Royal Medico-Psychological Association, The Royal Society of Medicine, the British Psychological Society, in cooperation with the National Association of Mental Health and (the former) American Association on Mental Deficiency. A second congress was held in Vienna in 1961 and a third in Copenhagen in 1964. The International Association was duly formalized at the 1964 Congress, with a constitution published in the American Journal of Mental Deficiency (1965, 69 599) and the Journal of Mental Deficiency Research (1965, 9, 150).
As an international organization, IASSIDD promotes the scientific study of intellectual disabilities and related developmental disabilities and of conditions of persons with these disabilities and their families. IASSIDD defines Intellectual disability as a significant intellectual deficit present from birth, or which originates at an early age or during the developmental period.