Additional trade schools, and not four-year college degrees, may be a better bet for U.S. workers, according to new economic research.

The amount of vocational training available relative to the size of a country's manufacturing sector may reduce income inequality, and improve the fortunes of workers earning below the top 10 percent of household incomes, the data show.

"Pushing more students to B.A. granting colleges may no longer be the most efficient way to deal with the challenges caused by the decline in manufacturing employment," wrote Joshua Aizenman, the economics chair at University of Southern California. He did the research with academics at New Zealand's Victoria University of Wellington.

And declined it certainly has.

In September 1977, about 18.3 million people, or more than 18 percent of the U.S. labor force, worked in U.S. manufacturing. Forty years later, that number has since slipped to 12.4 million, or less than 8 percent, even as the general U.S. population has surged to 326 million, U.S. Census figures show.

US workers in manufacturing (in thousands)

Source: U.S. Bureau of Labor Statistics

But while one might suspect that fewer workers would mean decreased output, real gross domestic product (GDP) manufacturing has actually risen over the past two decades, leading to a popular conclusion that machines have simply replaced labor in the workplace.

Real GDP: Manufacturing (millions of dollars [chained to 2009])

Source: U.S. Bureau of Economic Analysis, retrieved from FRED

Also of note: Manufacturing's share of GDP trickled downward, as automation spread and the economy shifted toward service.

Manufacturing share of total GDP

*The value added of an industry, also referred to as gross domestic product (GDP)-by-industry, is the contribution of a private industry or government sector to overall GDP.

Source: U.S. Bureau of Economic Analysis, retrieved from FRED