Little is known about the behavioral characteristics of adults with Down syndrome (DS) without dementia. The main purpose of this study was to investigate the psychopathology and social behavior among adults with DS compared to adults with nonspecific intellectual disability (NSID). Thirty-four adults with DS were individually matched with 34 adults with NSID for raw scores on a task that measured nonverbal reasoning ability. Participants' caregivers completed 3 questionnaires assessing mental illness, behavior problems, and social behavior. The group with DS presented fewer difficulties associated with mental illness and behavior problems, as well as better adapted social behavior, than the group with NSID despite a similar level of nonverbal reasoning ability. Aging in the group with DS was found to be positively correlated with behavioral signs of depression and autistic symptoms. These findings highlight the specificity of the behavioral profile of adults with DS without dementia.