Final answer:
The biggest factor in the slow growth of average incomes in the US during the 1970s and 1980s was the significant slowdown in productivity growth.
Step-by-step explanation:
The slow growth of average incomes in the United States during the 1970s and 1980s can be attributed to several factors. However, the most significant aspect was the slow growth in productivity. During this period, there was an unexpected slowdown in productivity growth; the output per hour of U.S. workers in the business sector increased at an annual rate of 3.3% from 1960 to 1973 but dropped to only 0.8% from 1973 to 1982. This slump in productivity led to a deceleration of wage increases and a rising natural rate of unemployment, as the equilibrium wages did not adjust to the slower productivity gains immediately.
While there was increased competition from economies such as Japan, and stagflation, characterized by high inflation and unemployment, the fundamental issue that underpinned these economic symptoms was the lagging rate of productivity growth. Over time, wage increases needed to realign with productivity advances, and this realignment process contributed to the prolonged period during which average incomes grew more slowly.