You've received great responses. I'll just add that when a child finds academics to be routinely very easy, the child may become accustomed to coasting along, without valuing effort. In fact, some may even begin to despise effort.
Evidently too much coasting along can cause brain changes, making it difficult for a child to apply themselves and learn new things when they do meet a challenge worthy of their potential.
Single-subject acceleration allows the child to keep their chronological-age peers while also meeting more closely matched academic/intellectual peers.
