Infants' reactions to an approaching stranger: description, validation, and functional significance of wariness.

Reliable descriptions of infants' behavioral responses to an approaching stranger were made from video records. Subtle negative responses (wariness) were validated against heart rate acceleration and responses to mother approach and showed significant age changes. Behavioral aspects of wariness appear to serve a "cutoff" (coping) function for the infant, preventing all-or-none responses (crying) and facilitating subsequent reengagement of the stranger. Attention to both positive and negative responses, especially in the milder forms, helps clarify inconsistencies in reported age of onset and frequency of "stranger fear." The role of data on infants' responses to strangers in formulating an integrated picture of development in the second half-year of life is discussed.