Around 6 to 8 months, most children begin to show fearfulness. Fear develops later than other “basic” emotions because certain steps in cognitive development need to take place first. Before they express fear, children need to be capable of comparing one event (one that causes fearfulness) with some other event they remember. For example, fear of a stranger occurs only when a child can compare the face of a stranger with other faces she knows and remembers. Fear expressions may include eyes open wide, with the mouth pulled back to the sides a little bit, along with crying and seeking people or things that are safe and comforting.
Lewis, M. (2000). The emergence of human emotions. In M. Lewis and J. M. Haviland Jones (Eds.), Handbook of Emotions, Second Edition, 265-280). New York: Guilford Press.