My goodness that chart is misleading. The x-axis measures number of years of increase in schooling. Put another way, the x-axis will roughly be the inverse of development at the beginning of the period. This is because nations that were highly developed at the start of the period wouldn't have reasonably been able to add an additional 8 years of schooling on average. Nations adding 8 years will overwhelmingly have been starting from low bases.
So the chart essentially says that adding education is more valuable in developed economies. Which is good to demonstrate empirically, but is hardly shocking.
The other major flaw in this study is that the timeframe is short. Again, consider the x-axis. Adding 8 years of schooling on average to even the least developed nation takes the first cohort out to ~15 years of age. By the time that first cohort hits age 47, the study period is over. This is measuring improvement without capturing the a full decade or more of the first cohort (and even less for subsequent cohorts). And obviously the first highly educated cohort wouldn't have had time to mentor a productive generation yet (or completely infiltrate the political infrastructure, etc.). Perhaps there's more useful data in the linked papers, but the data in TFA is unconvincing.
Finally, on the larger point of the correlation between education and productivity/wealth creation, there are ample empirical correlations. Obviously these do not imply causation, but the onus is certainly to show why this is not the case given that the correlation of education and wealth exists in roughly 100% of developed economies. If a country could become developed without spending on education, why has nobody done it?
So the chart essentially says that adding education is more valuable in developed economies. Which is good to demonstrate empirically, but is hardly shocking.
The other major flaw in this study is that the timeframe is short. Again, consider the x-axis. Adding 8 years of schooling on average to even the least developed nation takes the first cohort out to ~15 years of age. By the time that first cohort hits age 47, the study period is over. This is measuring improvement without capturing the a full decade or more of the first cohort (and even less for subsequent cohorts). And obviously the first highly educated cohort wouldn't have had time to mentor a productive generation yet (or completely infiltrate the political infrastructure, etc.). Perhaps there's more useful data in the linked papers, but the data in TFA is unconvincing.
Finally, on the larger point of the correlation between education and productivity/wealth creation, there are ample empirical correlations. Obviously these do not imply causation, but the onus is certainly to show why this is not the case given that the correlation of education and wealth exists in roughly 100% of developed economies. If a country could become developed without spending on education, why has nobody done it?