Anguish over how a child will react upon a newborn’s arrival is quite unnecessary. The quandary stems from the popular, but erroneous, notion that jealousy is nonexistent until a sibling arrives. If we think carefully about this assumption, -we’ll see how absurd that is. If it were true that jealousy is nonexistent until the arrival of a sibling, then children who do not have siblings would never become jealous. Yet jealousy is readily apparent in only children and in children from large families alike. Studies on adult jealousy also show that having a phlegmatic or ercely jealous temperament is unrelated to having siblings. Knowing if an individual is a rstborn or a later-born child, whether he is from a small family or a large family, if he has sisters or brothers, close in age or many years apart, whether he even has siblings, tells you nothing about how jealous he might be.
The arrival of a sibling does not actually cause jealousy. Rather, it simply represents the rst occasion on which jealousy is typically displayed. In fact, the emotion of jealousy has been rmly formed since the infant’s rst birthday, usually well before the little rival’s appearance on the scene. In my laboratory studies, my colleagues and I induced jealousy by simulating a domestic situation in which mothers simply attended to another child. Twelve-month-old infants were disturbed and they protested. They did so even if they did not have siblings and despite never having seen their mothers behave in this manner toward another child.