The study from Rutgers, called A Tale of Two Workforces: The Benefits and Burdens of Working Part Time, seems to be “the first national random sample of this population,” according to co-author Carl Van Horn, a Rutgers professor of public policy and the director of the university’s John J. Heldrich Center for Workforce Development, who spoke with Fortune.

BLS says people work part-time (less than 35 hours a week) for “economic” or “noneconomic” reasons. Economic reasons include poor business conditions, seasonal declines in labor demand, or another inability to find full-time work, and affect 6.5 million workers. Noneconomic reasons could mean caring for children, attending school, Social Security income limits, or other reasons, and include 19.6 million.

Van Horn and co-author Cliff Zukin, a Rutgers professor of public policy and political science, say the split really means involuntary and voluntary. The study found some critical differences between the two groups:

• 79% of voluntary part-time workers were either somewhat or very satisfied with their jobs, compared to half of involuntary part-time workers.
• Median monthly income of involuntary part-time workers was $1,200 for hourly and $1,000 for salaried employees. Median monthly income for voluntary was $971 for hourly and $850 for salaried.
• 20% of voluntary workers were at or below minimum wage, compared to 28% of involuntary.
• 37% of involuntary and 25% of voluntary part-time workers said full-time workers make more for the same amount of work.
• Top disadvantages of part-time work for part-time workers were difficulty in saving for retirement, difficulty in planning for the future, and earning less money than needed to support their family or get by.
• 27% of involuntary workers needed food stamps, 20% got help from food pantries, 11% received low-income energy assistance, 12% had help from religious or non-profit agencies, and 17% received other government social services.

Two thirds of part-time workers had only one job, while 26% had two, 4% had three, and 2% had four or more. And 15% held both a full-time and a part-time job. Someone was more likely to be involuntarily unemployed if they were:

• male
• black or Hispanic
• have at most a high school degree
• live in a household with an income of less than $30,000 a year

Prior to the Great Recession, there were 4.5 million involuntary part-time workers. At the height of the recession, there were 9.4 million. The current level is still significantly higher than pre-recession, suggesting that at least part of the post-recession jobs recovery has actually been a splitting of full-time positions into multiple part-time ones, meaning that the government essentially counts the positions multiple times, as anyone with part-time work is considered employed.

Employers may be doing so to avoid providing benefits and to pay less.

“The dividing line of voluntary and involuntary is very stark,” Van Horn said. “We’re talking about a larger number of people potentially who will struggle economically, who will not be able to support themselves, and will eventually become more dependent on public support of one form or another.”