It shows a positive association consistent with mutually reinforcing health and income, not a uniquely identified causal effect.

It proves that extending life expectancy by 20 years causes a twelvefold increase in output.

It is based on a randomized controlled experiment.

It establishes that education has no effect once health is included.

They measure educational quality perfectly but are unavailable for rich countries.

They measure inputs or participation without adequately capturing the quality of learning.

They measure only physical capital accumulation.

They necessarily underestimate the number of male students.