Assessing social skills is one of the most complex and challenging areas to study because behavioral repertoires vary depending on an individual's culture and context. However, researchers have conclusively demonstrated that individuals with intellectual disabilities (ID) have impaired social skills as well as those with co-morbid autism spectrum disorders (ASD) and epilepsy. However, it is unknown how these groups differ. Assessment of social skills was made with the Matson Evaluation of Social Skills for Individuals with Severe Retardation. One hundred participants with ID were matched and compared across four equal groups comprising 25 participants with ID, 25 participants with epilepsy, 25 participants with ASD, and 25 participants with combined ASD and epilepsy. When controlling for age, gender, race, level of ID, and hearing and visual impairments, significant differences were found among the four groups on the MESSIER, Wilks's lambda = 0.58, F(18, 257) = 3.05, p less than 0.01. The multivariate eta[superscript 2] based on Wilks's lambda was 0.17. Significant differences were found on the Positive Verbal subscale, F(3, 96) = 3.70, p less than 0.01, eta[superscript 2] = 0.10, Positive Non-verbal subscale, F(3, 96) = 8.95, p less than 0.01, eta[superscript 2] = 0.22, General Positive subscale, F(3, 96) = 7.30, p less than 0.01, eta[superscript 2] = 0.19, Negative Non-verbal subscale, F(3, 96) = 5.30, p less than 0.01, eta[superscript 2] = 0.14, and General Negative subscale, F(3, 96) = 3.16, p less than 0.05, eta[superscript 2] = 0.09. Based on these results, individuals with ID expressing combined co-morbid ASD and epilepsy had significantly more impaired social skills than the ID only or groups containing only a single co-morbid factor with ID (ASD or epilepsy only). Implications of these findings are discussed. (Contains 3 tables.)