Final answer:
Infants lacking object permanence do not realize that objects continue to exist when out of sight, leading to confusion and potential stranger anxiety. Developmental studies indicate that children may understand object properties earlier than Piaget proposed, but the development of object permanence is still a critical cognitive milestone.
Step-by-step explanation:
Understanding Object Permanence
Infants who lack object permanence do not understand that objects continue to exist when they cannot see them. This concept, which develops during the sensorimotor stage of cognitive development, is typically established by the age of 8 months. The development of object permanence is an essential cognitive milestone, seen as foundational for the understanding of the world. Without it, infants may appear confused when an object is hidden from their view, indicating that they may believe it no longer exists. This can also lead to increased stranger anxiety, where infants are unable to assimilate unfamiliar people into their existing understanding of the world.
Research has challenged the views of development proposed by Piaget. Studies by Baillargeon and others have shown that even very young infants possess an understanding of objects and their properties beyond what Piaget predicted. Nevertheless, object permanence remains a key developmental milestone, as mastering it leads to engaging in games like hide and seek, understanding that people return after leaving a room, and seeking hidden objects when prompted.