Just a hypothesis, but often intelligence is a proxy for the mental and social stimulus a child is exposed to in their formative years. So you would expect that

Just a hypothesis, but often intelligence is a proxy for the mental and social stimulus a child is exposed to in their formative years. So you would expect that to grow and then taper off as the useful limit of childhood well-being is reached. Since childhood enrichment is heavily indexed to wealth in most couuntries...