The Diagnostic and Statistic Manual (DSM IV- TR) is the most recent released manual by the APA which will list and describe in full every mental disorder and condition that is currently under study, or has been included throughout history, to this day.
According to that manual, and to the American standards of diagnosis, here are the cited sources of the condition:
A. Developmentally inappropriate and excessive anxiety concerning separation from home or from those to whom the individual is attached, as evidenced by three (or more) of the following:
(1) recurrent excessive distress when separation from home or major attachment figures occurs or is anticipated
(2) persistent and excessive worry about losing, or about possible harm befalling, major attachment figures
(3) persistent and excessive worry that an untoward event will lead to separation from a major attachment figure (e.g., getting lost or being kidnapped)
(4) persistent reluctance or refusal to go to school or elsewhere because of fear of separation
(5) persistently and excessively fearful or reluctant to be alone or without major attachment figures at home or without significant adults in other settings
(6) persistent reluctance or refusal to go to sleep without being near a major attachment figure or to sleep away from home
(7) repeated nightmares involving the theme of separation
(8) repeated complaints of physical symptoms (such as headaches, stomachaches, nausea, or vomiting) when separation from major attachment figures occurs or is anticipated
When an infant is born, it 'imprints' on it's mother first, then others closest to it (father,siblings,other family members,grandparents,carers, friends.) This is essential for survival - it learns to identify its mother through its five senses - touch,scent,sight,sound etc.) It can recognize its mothers scent and if parted from the thing that brings it comfort,security food and warmth - it will 'raise the alarm' (cry!)
However, at this stage it does not know it is dependent on its mother and cannot feel anxiety about what would happen if she were not there - its distress is instant and 'of the moment.' This presage of danger is built of experience and as time passes the child begins to associate the absence of its mother/prime carer with anxiety over potential unpleasant experiences - separation anxiety is developing and peaks between 1 - 5 yrs depending on the maturity/positive experiences and education of the child. With careful and sensitive handling it passes usually before the child reaches kindergarten - or even toddler daycare if it has been handled well. One of the forremost researchers into this subject is still the late Jean Piaget:
Separation anxiety is the term used for unhappiness on the part of children when they are going to be separated from their parents. Examples of this would be when children "throw tantrums" when their parents leave them with a babysitter.
Separation anxiety is typically believed to develop during the second half of a child's first year of life. Before that, children are not believed to have developed enough (psychologically speaking) to feel this anxiety.
Separation anxiety develops gradually after children are old enough to have the idea of object permanance -- the idea that objects and people still exist even when they are out of sight. Before this concept develops, separation anxiety is not possible.
Separation anxiety is a normal pattern of development that a child or infant experiences. It occurs when the child becomes fearful that the parent or caregiver will leave and not return. It is most common in babies who have not yet become cognitively aware that the absence of an object does not mean permanent disappearance of the object. For example, an eight month old having a ball removed from his grasp and hidden in a bag may not have the understanding of object permanence. Separation anxiety usually develops when a child is 6 to 8 months old and can continue through his/her toddler years. To experience a reduction in separation anxiety a child must go through three developmental phases; attachment to a caregiver, parent; realization of object permanence; and development of trust. Children who are not able to comfortably establish these developments may have long term separation anxiety which is unhealthy.