A review of the relevant research indicates that parents of both normal and developmentally delayed children adjust their speech according to their child's age and communicative abilities. Therefore, the cause of handicapped children's language delay is unlikely to be found in their linguistic environment. Other people including unfamiliar adults may not be as effective in adjusting their speech to optimize communication with a develop-mentally delayed child. Handicapped children themselves appear to be less likely than normal peers to adjust their speech in interacting with other developmentally delayed children, which may be taken as one argument in favor of mainstreaming the handicapped child.