Final answer:
The primary source responsible for the slowdown in U.S. economic growth after 1973 is the growth of technology, as it is the most significant factor influencing economic expansion according to growth accounting studies.
Step-by-step explanation:
The slowdown in U.S. economic growth after 1973 can be attributed primarily to a decline in the growth of technology, which is identified as the most important contributor to economic growth through growth accounting studies. The other two sources of growth as per these studies are human capital and physical capital. However, technology, which encompasses new ways of doing things and innovation, generally accounts for more than half of the economic growth, with human capital and physical capital contributing less significantly.