Returns to education and the decline of the Marines

Oh, folks will have fun working out explanations for this one.

Brookings suggests that increasing college graduation rates could be behind it, where a college degree is required to become an officer. The drop in the IQ threshold for getting a college degree then is reflected in the chart above. But that can't be all of it, as the Marines could increase the GCT requirement for admission if the parchment no longer signals ability and if ability really matters more than parchment. It seems more likely that they're just not able to draw as well from higher ability cohorts.

Other plausible candidate explanations, though without data. They're all testable though:

• Higher opportunity costs for high IQ people since the 80s.
• Greater cultural disparaging of the military in elite circles, so it is not aspired to by those of higher ability.
• Decreasing trust in that military is a force for good or really much needed (see the decline as the Soviets turned friendlier under Gorbachev, the levelling off and rise around Gulf War I, and the resumed decline after that).
• Fewer smart but lower income kids needing to use ROTC to afford college with expansions in student aid. 

I'd bet on the first and last being most important.