The emotion of fear is fully developed by 9 months and is expressed in two ways: separation anxiety and stranger wariness. Stranger wariness is the distress that young children experience when they are exposed to people who are unfamiliar to them. When a stranger approaches a typical 6 month old, they would look away and begins to fuss. This begins somewhere between 8-9 months of age reaching its peak at 12-15 months. At this time infants begin to realize that all people are not the same and that the relationship they have with their primary caregivers is special. How wary an infant feels around strangers depends on a number of factors. Infants tend to be less fearful of strangers when the environment is familiar. If infants are given time to warm up to the strangers who are female than those who are male. Stranger anxiety is adaptive because it emerges at the same time that the child is maturing to crawling.
Being wary of strangers provides a natural restraint and makes the infant less likely to wander away from familiar caregivers. Stranger anxiety gradually declines as infants learn to interpret facial expressions. Separation anxiety is the intense fear or anxiety that occurs when a parent or caregiver leaves the child.
Infants growling cognitive skills allow them to ask questions with no readily apparent answers: why is my mother leaving? Where is she going? Will she come back?