the apparent slope for the United States can be readily predicted based on the relationship observed in other high-income countries with nothing more than time series for health expenditure and life expectancy. Truth be told, the average marginal effect of health expenditure for high-income countries in recent years is likely pretty close to zero (particularly as pertains macro-level indicators like life expectancy).
The US intercept, meanwhile, can be explained quite well by obesity. Obesity is likely to have very large negative effects on all manner of health outcomes. It is actually much more predictive of outcomes in the developed world than health spending.
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