Your baby’s reflex smile will disappear by time she’s 2 months old, and her first real one will make an appearance somewhere between one-and-a-half to 3 months (or 6 and 12 weeks) of life. You can tell the difference between a reflex and real smile by the timing and duration. Generally, reflex smiles tend to be shorter and occur randomly, when the baby is sleeping or tired. Real smiles, on the other hand, occur in response to something, like seeing her mama’s face or hearing a sibling’s high-pitched voice, and they are consistent, Dr. Gettleman explains. When it’s the real deal, you will see the emotion expressed in your baby’s eyes.

At first, your little guy’s happy face will be in response to a mix of vocal and visual stimulation. Therefore, he may light up from watching you sing a favorite lullaby or talk him through a particularly yucky diaper change. Later, when his vision improves, simply seeing the face of his two favorite people (you and your partner!) will be enough to make him crack a smile. You’ll be the recipient of most of your baby’s smiles, but he’ll also exchange grins with others (that is, until stranger anxiety kicks in at around 6 months).

Article From parents.com