Lead
Most parents remember the first time their child said "that's not fair." It might have been directed at a younger sibling getting a parent's attention, or at a friend who had a toy the child didn't. The phrase carries a weight that surprises — it sounds both deeply felt and somehow adult.
Is it an emotional expression, a moral complaint, or both? The psychology of jealousy and envy — how these two distinct emotions develop in children, and how children eventually find words for them — offers a more specific answer than we might expect, and it changes how the phrase "that's not fair" sounds to a parent who hears it.