If you want to know when do babies laugh, you must refer to the developmental millstones for two to six months old babies. According to the pediatricians, a baby may start smiling as soon as he is born, but the smile maybe more of a reflex action rather than in response to external stimulus.
By the time the baby is three month old, he will be smiling or at least appearing to do so. Later the baby may also develop a social smile which can be clearly made out. But the right answers to the question when do babies laugh would be six months because this is when the baby will be laughing like the adults.
According to research it has been found that when a baby is born, he does not know how to laugh but they learn through imitation. If you want to know when do babies laugh, you must watch the behavior of the baby and show him how to laugh.
You can also create a stimulating environment where the kids in the family are laughing and playing and the baby will easily learn from them. There are many books that will give you useful information like when do babies laugh, when do babies start crawling and mush more. You can also take the advice of friend and other families who have brought up children and find out when babies laugh.