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Many infants around the world display stranger wariness (also known as stranger anxiety), a fear of unfamiliar people. Whether infants show stranger wariness depends on the infants’ overall temperament, their past experience, and the situation in which they meet a stranger ( Goodvin, Thompson, & Winer, 2015). In many, but not all, cultures, stranger wariness emerges at about 6 months and increases throughout the first year of life, beginning to decrease after about 15 months of age (Bornstein et al., 2013; Sroufe, 1977).

Recent research has suggested that the pattern of stranger wariness varies among infants. Some show rapid increases and others show slow increases in stranger wariness; once wariness has been established, some infants show steady decline and others show more rapid changes. Twin studies suggest that these patterns are influenced by genetics, because the patterns of change are more similar among monozygotic twins (identical twins who share 100% of their genes) than dizygotic twins (fraternal twins who share 50% of their genes) twins (Brooker et al., 2013).

As attachments form, infants become more wary and display “stranger anxiety” when in the presence of unfamilar people. In many, but not all cultures, stranger wariness emerges at about 6 months and increases throughout the first year of life.

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Among North American infants, stranger wariness is so common that parents and caregivers generally expect it. However, infants of the Efe people of Zaire, Africa, show little stranger wariness. This is likely related to the Efe collective caregiving system, in which Efe babies are passed from one adult to another, relatives and nonrelatives alike (Tronick, Morelli, & Ivey, 1992), and the infants form relationships with the many people who care for them (Meehan & Hawks, 2013). In contrast, babies reared in Israeli kibbutzim (cooperative agricultural settlements that tend to be isolated and subjected to terrorist attacks) tend to demonstrate widespread wariness of strangers. By the end of the first year, when infants look to others for cues about how to respond emotionally, kibbutz babies display far greater anxiety than babies reared in Israeli cities (Saarni, Mumme, & Campos, 1998). In this way, stranger wariness may be adaptive, modifying infants’ drive to explore in light of contextual circumstances (Easterbrooks, Bartlett, Beeghly, & Thompson, 2012).

Stranger wariness illustrates the dynamic interactions among the individual and context. The infant’s tendencies toward social interaction and past experience with strangers are important, of course, but so is the mother’s anxiety. Infants whose mothers report greater stress reactivity, who experience more anxiety and negative affect in response to stress, show higher rates of stranger wariness (Brooker et al., 2013; Waters et al., 2014). Characteristics of the stranger (e.g., his or her height), the familiarity of the setting, and how quickly the stranger approaches influence how the infant appraises the situation. Infants are more open when the stranger is sensitive to the infant’s signals and approaches at the infant’s pace (Mangelsdorf, 1992).