Final answer:
The single most important factor for rising real-wage rates in the long term is productivity growth, which links directly to the value workers produce and results in wage adjustments reflecting workers' efficiency and output.
Step-by-step explanation:
The single most important factor underlying the long-run increase in average real-wage rates in the United States is productivity growth. This is essentially the increase in output produced by workers per hour. Over time, as workers become more efficient and can produce more, employers are willing to pay higher wages that correspond to the value of the output produced. If workers are paid less than their productivity, they are likely to receive better wage offers from other employers, while overpayment without matching productivity can lead to losses for employers. Hence, productivity per hour becomes a crucial determinant of wages in the long term.
In economic terms, an increase in productivity shifts the Aggregate Supply (AS) curve. A common way to measure this growth is by observing the GDP per capita, which over time has shown an average increase, reflecting how an economy like that of the United States has become more productive.