Final answer:
Stranger fear typically emerges in infants between 6 to 12 months of age as part of normal cognitive development, coinciding with the understanding of object permanence and difficulty assimilating strangers into known schemas.
Step-by-step explanation:
Stranger fear becomes evident in infants around the age of 6 to 12 months. This is a time when children are developing object permanence, a cognitive milestone where they understand that objects continue to exist even when they cannot be seen or heard. The occurrence of stranger fear is a normal part of child development and links to the child's inability to assimilate the stranger into an existing schema that includes familiar faces and people they trust. Infants may express their fear by crying, clinging to caregivers, or showing distress in the presence of unfamiliar individuals. This indicates a growing awareness of self and others, as well as the infant's attachment to primary caregivers.