Separation anxiety and stranger anxiety both coincide with a new intellectual skill called object permanence. Your baby now remembers objects and specific people who are not present. He will search for toys that have dropped out of sight. He is able to call up a mental image of what (or who) he is missing. He doesn’t want a stranger, because the stranger is not you.
Babies understand about people leaving before they learn about people returning. They can tell from your actions that you are about to leave. Anxiety begins to build even before you go.
Babies can’t tell from your actions that you are about to return. They have no idea when — or even if — you will come back. And they miss you intensely. For them, each separation seems endless.
To us, peek-a-boo and bye-bye are fun ways for us to interact with kids. For babies, these are issues of great concern.
This is an excerpt from: From First Kicks to First Steps: Nurturing Your Babys Development from Pregnancy Through the First Year of Life, McGraw-Hill, 2004, p. 284