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Separation anxiety occurs as "a normal stage of development for healthy, secure babies. It occurs as babies begin to understand their own selfhood—or understand that they are a separate person from their primary caregiver. At the same time, the concept of object permanence emerges—which is when children learn that something still exists when it is not seen or heard. As babies begin to understand that they can be separated from their primary caregiver, they do not understand that their caregiver will return, nor do they have a concept of time. This, in turn, causes a normal and healthy anxious reaction. Separation anxiety typically onsets around 8 months of age and increased until 13-15 months, when it begins to decline[1].
โSiegler, Robert (2006). How Childred Develop, Exploring Child Develop Student Media Tool Kit & Scientific American Reader to Accompany How Children Develop. New York: Worth Publishers. ISBN 0716761130.