Five studies investigated the interaction between language acquisition abilities and environmental factors. Subjects aged 5 to 20 imitated synthetic speech stimuli representing English and novel categories. All except the 5-year olds imitated better than was predicted from studies of categorical perception. The 12-year olds performed optimally. Children were studied who were becoming bilingual. When languages were learned simultaneously, children showed fused systems. Children who had already acquired native languages experienced a "silent" period in the new language during which learning occurred. Accent is discussed in terms of motor-theory and psychological factors. Effects of reduced language input were observed in a hearing child of deaf parents. The language was quantitatively and qualitatively different from normal children's. Absence of signing suggested that language must be directed to the child to permit acquisition. Characteristics of the language model presented to children was investigated by analyzing the speech of adults to a 2-year old child or to an adult. When speaking to the child, adults used shorter, less complex sentences, and more questions. The speech of male and female children could reliably be identified as to sex by judges. Differences in formant patterns suggested that children acquire cultural patterns for marking sex-identification in voice. (Author)