Eli5: why do babies sit with perfect posture when they learn to sit, but the older you get, the worse it gets?

Question

Babies sit straight up naturally. But the rest of us have to put effort in to having good posture, why?

In: 1304

Answer 1

Their heads are enormous and weighty relative to their torso, so they MUST align their spine under it. As we age the size of the torso grows and we can adopt many more postures and retain head control, so we often adopt very low-energy positions and slump, to our detriment in the long run.