Ever since President Obama’s health care program was signed into law, there have been anecdotal reports of business executives pledging to shift more workers to part-time to get around the law’s mandate that larger employers provide full-time workers acceptable insurance coverage, or face costly taxes.

But now, there is empirical evidence that there has been a broader shift to increased part-time work in the U.S. economy.

Jed Graham of Investor’s Business Daily, who reviewed the latest findings from the Current Population Survey, a joint report from the Census Bureau and Bureau of Labor Statistics, concluded:

Even as the number of people working has grown by 2.2 million, or 1.6%, over the past year, the number clocking 30 to 34 hours a week has shrunk.

In the second quarter, the number of workers putting in 30 to 34 hours at their primary job fell by a monthly average of 146,500, or 1.4%, from a year earlier.

By comparison, the number working 25-29 hours per week in their primary job rose by 119,000, or 2.7%.

This oddity has an obvious explanation: ObamaCare’s employer mandate applies only to full-time workers, which the law defines as 30 hours per week.

The health care law imposes fines of $2,000 or $3,000 per full-time worker for businesses with at least 50 employees, meaning that some businesses could be on the hook for hundreds of thousands or even millions of dollars in fines.

Obama decided to delay the implementation of the mandate by a year, until January 2015. So it should be interesting to see if that will slow the part-timing trend, or whether businesses will continue to adjust their workforces in anticipation that the mandate will eventually go into effect.