A baby who once smiled at everyone may begin hiding against a parent when an unfamiliar person approaches. This change, often noticeable during the second half of the first year, is called stranger wariness.
One evolutionary explanation is that fear develops when mobility creates new danger. A newborn cannot wander away, but a crawling infant can approach unfamiliar people and environments. Wariness may act as a protective brake, keeping an increasingly independent child near trusted caregivers.
The strongest evidence comes from a longitudinal study of 1,285 children, including identical and fraternal twins, followed from six to thirty-six months. Stranger fear did not follow one universal timetable: researchers identified four developmental trajectories. Most children changed substantially, while about 12% maintained unusually high fear. Twin comparisons suggested that differences between children were partly heritable. Persistently high or rapidly increasing fear also predicted greater behavioral inhibition later. Longitudinal study
Well supported: stranger wariness commonly increases during infancy, but its intensity and timing vary greatly. Temperament, caregiver behavior, the stranger’s approach and the surrounding situation all influence a baby’s reaction.
Plausible but debated: natural selection timed stranger fear to coincide specifically with crawling and exploration. The timing makes adaptive sense, but direct evidence that wary ancestral infants survived more often is unavailable.
Improved memory provides another explanation. As babies form detailed representations of familiar caregivers, they become better able to notice who does not match them. Fear may therefore arise partly as a consequence of cognitive development rather than as a specialized anti-stranger adaptation.
Stranger anxiety is usually not evidence that something is wrong. It may reflect a developing mind learning one of social life’s most important distinctions: familiar does not mean the same thing as unknown.
