Bank holdings

FRED provides
1. U.S. Government Securities at All Commercial Banks (USGSEC), and
2. Total Investments at All Commercial Banks (INVEST)

This graph looks at commercial bank holdings of U.S. government securities as a percent of total commercial bank investments:

High but falling before 1960, probably a result of post-WWII conditions in general.

From 1960 to the present, commercial bank holdings of U.S. government securities ranged between 50% and 80% of their total investments.

Decline during the golden age, until 1974, as private-sector growth created investment opportunities apart from U.S. government securities.

Increase from 1974 to 1994 or so.

Decline again, beginning with the improved economic growth of the mid-1990s.

Increase again, since the crisis.