Matthew Cancian & Michael Klein:
96% of this is explained by the widening (and weakening) of the applicant pool due to increased college participation. Affirmative action played no role.
This paper analyzed the intelligence of the officers of the Marine Corps, and found a relevant and steady decline in intelligence, as measured by General Classification Test (GCT) scores, since 1980. This decline was closely associated with an expansion of the pool of young college graduates during the same time period, which potentially diminished the overall intellectual quality of that pool. If the pool of those wanting to become Marine officers is representative of the pool of college attendees, and if there is a positive, monotonic relationship between intelligence and GCT scores, then the expansion of the pool of college students will be associated with lower average GCT scores. The coefficient [of GCT scores] on college participation is negative and significant at better than the 99 percent level, and explains 96% of the actual percentage change in the GCT over this period. Our results also support those who argue that affirmative action has not had a negative impact on the quality of the officer corps. We find, in fact, a positive association between African American officers and mean GCT score, perhaps because recruitment efforts by the Marine Corps have attracted minority officers who are more qualified than the typical college graduate.