Answer will include that although experts do not yet agree on exactly how quickly emotions unfold, early emotional development also follows a pattern closely tied to maturation. Even the basic emotions of anger, fear, and joy, which appear to be unlearned, take time to develop. General excitement is the only emotion newborn infants clearly express. One researcher, Bridges, observed that all the basic human emotions appear before age two. Bridges found that emotions appear in a consistent order and that the first basic split is between pleasant and unpleasant emotions. Psychologist Carroll Izard thinks that infants can express several basic emotions as early as 10 weeks of age. When Izard looks carefully at the faces of babies, he sees abundant signs of emotion. The most common infant expression, he found, is not excitement, but interest, followed by joy, anger, and sadness. If Izard is right, then emotions are "hard-wired" by heredity and related to evolution. Smiling is one of a baby's most common reactions. Smiling probably helps babies survive by inviting parents to care for them. At first, a baby's smiling is haphazard. By the age of eight to 12 months, however, infants smile more frequently when another person is nearby. This social smile is especially rewarding to parents. Infants can even use their social smile to communicate interest in an object.