While for many infants, a thing out of sight is also out of mind, there is a developmental milestone, called object permanence, that a child reaches when he or she realizes that the object exists even when it can't be seen. The term was coined by child development expert and psychologist Jean Piaget, who believed most children reached this stage when they were about eight or nine months old. All children develop slightly differently, however, and may reach this point earlier or later than others.
Piaget studied this milestone by conducting relatively simple tests on infants. He would show an infant or young baby a toy and then cover it with a blanket. A child who had a clear concept of object permanence might reach for the toy or try to grab the blanket off the toy. A child who had not yet reached this stage might appear distressed that the toy had disappeared.
Parents, of course, have tested object permanence for years with young babies. Games of peek-a-boo with a three month old are quite delightful because the child will often be pleasantly surprised each time the parent covers her hands with her face and then reappears. According to Piaget's theory, the delight results in the sudden reappearance of the parent, who magically disappeared and came back. Children over five or six months may also hide under blankets and expect that their parents can't possibly find them, since the child cannot see the parent.
A lack of understanding of this concept might also explain why children tend not to fuss as much when they are younger and the parent leaves. This is not always the case, however, which may call into question some of Piaget's theories. For instance, studies testing breastfed week-old infants suggest they can easily differentiate between their mother's breast milk and that from another woman. Basing the concept on what can be only visually perceived discounts what can be heard, smelled, or touched.
In complete dark, for instance, a baby far too young to have developed object permanence may feel comforted by the touch and smell of a mom sleeping nearby or picking up the baby. The mother exists even before the sight of the mother can be determined, so vision cannot be the only factor guiding this concept.
It is clear, however, that as infants begin to expand their visual perception, they may seem quite surprised by the sudden visual disappearance of a beloved toy or person. They may be still able to smell, hear, or sense the missing object, however. This suggests that the infant has more ways of perceiving than were summarized by Piaget in his development and testing of the theory.