Right, I take this to be an implication of our best economic and demographic models (respectively).
Do you think we can trust the predictions of such models over more than a few decades? It looks like increasing population increases longterm income per capita, but not even this is clear (the conclusion relies on extrapolating historical trends, but it is unclear these will hold over long timeframes).
I don’t know what you mean by “a decreasing effect size of fertility- and income-boosting interventions”. Whether an intervention has a noticeable short-term effect on these targets? That would seem to address a different question.
Yes. I see it is a different question. No difference between the treatment and control group after a few decades could be explained by the benefits spilling over to people outside those groups. However, an increasing population or income gap between the treatment and control group would still be evidence for increasing effects, so a decreasing population or income gap is also evidence againt increasing effects.
Do you think we can trust the predictions of such models over more than a few decades? It looks like increasing population increases longterm income per capita, but not even this is clear (the conclusion relies on extrapolating historical trends, but it is unclear these will hold over long timeframes).
Yes. I see it is a different question. No difference between the treatment and control group after a few decades could be explained by the benefits spilling over to people outside those groups. However, an increasing population or income gap between the treatment and control group would still be evidence for increasing effects, so a decreasing population or income gap is also evidence againt increasing effects.