Babies treat the sounds of crying differently from other sounds. In one study with 34-hour-old infants, infants cried often when hearing the sound of another newborn’s cry. When these infants were in a quiet room or when they heard a cry-like sound (just as loud) made on a synthesizer, they cried less often. Some researchers believe that this ability is the foundation for empathy, which develops in the second year of life.
Brothers, L. (1989). A biological perspective on empathy. American Journal of Psychiatry, 146, 10
Sagi, A., & Hoffman, M. L. (1976). Empathetic distress in the newborn. Developmental Psychology, 12, 175