At birth, infants begin to show basic emotional behaviors. To demonstrate this, researchers carefully studied infants’ facial expressions as they interacted with their mothers. These studies have found that infants begin to display the basic emotional expressions of contentment, interest, and distress in their first days of life. Before they’re 3 months old, babies are already perceiving and responding to the emotional expressions they see in others. By 3 months, joy appears with smiles and other ways of showing excitement and happiness. These expressions often occur when babies experience familiar events, objects, and people. At this age, even strangers can bring big smiles and enthusiastic responses.
Izard, C. E., Fantauzzo, C. A., Castle, J. M., Haynes, O. M., Rayias, M. F. & Putnam, P. H. (1995). The ontogeny and significance of infants’ facial expressions in the first 9 months of life. Developmental Psychology, 31(6), 997
Lewis, M. (2000). The emergence of human emotions. In Lewis, M., & Haviland-Jones, J. M. (Eds.). Handbook of emotions (2nd ed.). New York: Guilford Press.