“…For instance, subjects of experimental research on language acquisition in which some form of reinforcement has been manipulated have been of a different species (e.g., Savage-Rumbaugh & Rumbaugh, 1980), or intellectually retarded (e.g., Guess, Keogh, & Sailor, 1978), or adults (e.g., Krasner, 1965), or have been studied in constrained laboratory settings (Whitehurst & Novak, 1973), or have been exposed to variables that might be unlike those found in the everyday environment (e.g., Nelson, Carskaddon, & Bonvillian, 1973;Ribes, 1979). Experiments have been performed rarely on questions of language acquisition, and then issues of external validity have loomed large.…”