However much jealousy encourages an outsize response, a little bit is good for relationships, especially in the early stages, before trust has a chance to develop. "The paradox of jealousy is that we all want some of it," says Stosny. It's a measure of commitment. "In small doses it's an expression of caring. Jealousy is like a way of testing whether it's safe to invest more emotion. It's safe when a person cares enough to be uncomfortable. Jealousy is a fear of losing something you perceive you have—the affection, the fidelity of another person. The threat of losing it is a test of how much you value it.