Previous research has suggested that infants' early listening experience affects their ability to perceive speech. Many psychologists and linguists have also suggested an early impact of listening experience on prelinguistic production. This belief was formalized in the babbling drift hypothesis (see, e.g., Brown, 1958), which predicts that babbling begins to approximate characteristics of the mother tongue as infants approach meaningful speech. In order to investigate this hypothesis, four experiments were conducted in which adult listeners' perception of the babbling of infants from different language backgrounds was tested. In the first two experiments monolingual English and bilingual English-Spanish adults judged the babbling of fourteen 7–10 month-old English- and Spanish-learning infants. The third and fourth experiments investigated the babbling drift hypothesis with older infants (11–14 months of age).
For all experiments conducted during both the beginning and the end of the babbling period, adult judges were unable to identify language background significantly above chance level. Therefore, the findings do not support the babbling drift hypothesis. However, bilingual and monolingual judges showed consistently different patterns of judgment with regard to particular infants and utterances. Thus, it appears that judges are influenced by their language background even if the influence fails to improve their success in determining infant language environment.