There is a reason that peek-a-boo reliably delights babies: for much of the first year, a face that disappears behind hands and reappears is genuinely surprising — the face has stopped existing and then come back. The development of object permanence — the understanding that things continue to exist when out of sight — is one of the most important cognitive milestones of infancy, and understanding it helps make sense of both the appeal of certain games and the origin of separation anxiety.
Healthbooq supports parents with accessible explanations of key cognitive developmental milestones in the first years, including how to observe these milestones in everyday play.
What Object Permanence Is
Object permanence, as described by developmental psychologist Jean Piaget, is the understanding that objects (and people) have a continuous existence independent of perception — that they do not cease to exist simply because they are not currently being seen, heard, or touched. To an adult, this is so obvious as to be unremarkable. To a young infant who has not yet developed this understanding, the world works very differently.
Piaget described the development of object permanence as unfolding through the sensorimotor stage of development (birth to approximately two years), with the full understanding consolidating gradually across the first eighteen months.
The Development Timeline
In the first four to six months, infants show limited evidence of tracking objects after they disappear. A toy dropped out of view is not searched for; the infant's gaze moves away and the object is, in effect, gone. This has been described as "out of sight, out of mind" — a phrase that accurately captures the functional situation even if the underlying cognitive processes are more complex than this simple description implies.
From around four to six months, early signs of object permanence begin to emerge. Infants will look towards where an object has gone, or anticipate its reappearance at the end of a trajectory — suggesting some expectation that it continues to exist.
The clearest behavioural marker of emerging object permanence is active search for a hidden object, which typically begins between eight and twelve months. An infant of this age will reach under a cloth or look behind a barrier to retrieve a hidden toy — behaviour that was absent in earlier months. This is the developmental stage at which peek-a-boo transitions from a mystifying sequence to a delightful game: the baby now understands that the face is still there under the hands, which makes the reveal genuinely satisfying.
Piaget described a characteristic error at this stage called the A-not-B error: when a toy is consistently hidden in location A and the baby retrieves it there successfully, then the toy is hidden in full view of the baby in location B, the baby will often continue to search in location A — where the object was previously, not where they saw it hidden. This error, typically observed between eight and twelve months, reflects an incomplete understanding of object permanence that is still tied to previous experience rather than fully updated by new information.
By twelve to eighteen months, object permanence is substantially complete. The baby can track an object through multiple sequential displacements (hidden in one place, then moved to another under a cloth while the baby watches), update their search accordingly, and generally operates with a reliable expectation that objects persist.
Object Permanence and Separation Anxiety
The development of object permanence has a direct and important relationship with separation anxiety, which typically emerges around seven to ten months. Understanding that a parent who has left the room still exists — but is not here — creates the experience of meaningful absence that did not exist when people, like objects, ceased to exist on leaving the visual field. This is why separation anxiety emerges when it does, and why it is a sign of healthy cognitive development, not a regression or problem.
As object permanence consolidates and memory develops, the baby also becomes able to hold the image of the absent parent in mind and to understand that they will return. This developing capacity for what developmental psychologists call "object constancy" is what eventually allows separation to become manageable.
Peek-a-Boo and Hide-and-Seek
Simple games that involve hiding and finding are perfectly calibrated to the stage of object permanence development. Peek-a-boo, initially with faces, then with toys, then with whole-body hiding, tracks the developmental progression. At each stage the game is calibrated to slightly exceed the baby's current certainty — familiar enough to be engaging, uncertain enough to be genuinely interesting. These games also support the development of anticipation, turn-taking, and shared attention.
Key Takeaways
Object permanence — the understanding that objects and people continue to exist when they are no longer visible — develops gradually across the first eighteen months of life. Jean Piaget described this as one of the key achievements of the sensorimotor stage of development. Before around four to eight months, infants typically do not search for a hidden object — out of sight is out of mind. Between eight and twelve months, infants begin actively searching for hidden objects. By twelve to eighteen months, they can track an object through multiple displacements. The development of object permanence has direct practical implications for separation anxiety, which emerges around the same developmental window.
