It is a widely held belief that if infant gets stranger anxiety at around 5 months, it is a typical healthy development. Even, some parents think that the development of their child's anxiety over strangers or the separation from parents are a sign that their child can perform complex cognitive tasks by distinguish between familiar and unfamiliar adults. However, in recent research on child development and stranger anxiety demonstrates that extreme stranger anxiety may be a sign that the child is lacking emotional security from her parents, which then could pose as a potential developmental problem if the anxiety is serious. According to Stanley Greenspan, a psychiatrist who specialized in treating infants says, "Many infants who are quite healthy emotional don't have stranger anxiety at all" (Goleman).
Richard Davidson, a psychologist at University of Wisconsin, and Nathan Fox, a psychologist at University of Maryland did a research on stranger anxiety. They analyzed the brainwave pattern from 13 infants around the age of 10 months and found that the infants that cried after their mother left the room and leave them with a stranger had brain wave in the right side of their brain while those that did not cry tended to have the reverse brainwave pattern. Not only that in several studies, they found that the right side of infants' brain associated with negative emotion while the activity in the left side of the brain associated with positive emotion like joy. From these data, they believed that the stranger anxiety symptoms came largely from the child's temperament as well as the intimate relationship between the child and the parent. As soon as the parent returns to their child, the secure infants are able to find consolation from their mother while the insecure children get upset or resist their mother's embrace. Although stranger anxiety are from the child's innate change, it also shows weather the relationship between the parent and the child is causing the insecurity in the child because it showed that even fussy children can sometimes feel secure and comfortable to stranger.
Because of this, extreme signs of stranger anxiety may not be show as a normal behavior in infant. There are certain interactions between mothers and their child that will make the child more secure or insecure toward strangers, which then later may cause problem toward emotional development for the children. There were several studies showed that the parent can increase their child a sense of emotional security by respond to the sound their child made. Interestingly, these studies also showed the parent that constantly talks to their child regardless what their child is doing may consider as intrusive in the child's perspective and this can lead to emotional insecurity and a greater chance that the child may develop stranger anxiety.
Goleman, Daniel. New Research Overturns a Milestone of Infancy. The New York Time. 6 June. http://www.nytimes.com/1989/06/06/science/new-research-overturns-a-milestone-of-infancy.html?pagewanted=all&src=pm