Until a baby is eight or nine months old, he believes that objects and people exist only if he can see them. At six months, if you take a toy away from your baby and hide it behind your back as he watches, he’ll act as though there no longer is a toy. In the same way, when you leave his side to go into another room, he may believe you no longer exist. Your disappearance upsets him, which explains the anxiety and tears you see.
When you play peek-a-boo with your baby, you reenact the anxiety and relief he feels each time you leave and return. You hide behind your hands or a blanket, and he believes you’re no longer there. He may even become momentarily upset and whimper. When you suddenly reappear and say “peek-a-boo,” he laughs with delight to have you back.
By nine or ten months, your baby will have some idea that objects exist even when he can’t see them. At this age, he may look for a hidden toy if he sees you put it behind your back or under a pillow. But at times he may still react with fear and uncertainty when you leave him, because his understanding of people’s permanence is not fully developed and won’t be until he’s between eighteen and twenty-four months old.