It is a part of natural human behavior regulated by the brain. They do not learn it.Neonates(0-1 month) may smile from the day they are born to about one month, but at that time they show no emotion. At the age of 1-2 month babies gradually respond to stimulation. Their first smile when they are awake usually appears at 6-10 weeks. This is because their vision improves and they start to recognize faces. They respond to sounds, like music and parents’ voices. Finally, they will smile because of visual stimulation.