Charles Dickens’ famous phrase opening “A Tale of Two Cities” — It was the best of times, it was the worst of times — lends itself to a new study by the John J. Heldrich Center for Workforce Development.

Professor Carl Van Horn and his colleagues at Rutgers University this spring did an in-depth survey of 944 workers, 504 of whom were involuntary part-time workers and 440 who were part-timers by choice. They were a sample of the 26 million Americans who hold part-time jobs.

The survey found that most of the part-timers were happy with their work lives. But there was a lot of unhappiness among the 6.5 million people who were working fewer than 35 hours a week because they couldn’t land a full-time job or because their employers cut their hours.

Van Horn can’t speak to individual motivation. He doesn’t know how hard any one person tries to get full-time hours. But, he says, “The fact that there are still more involuntary part-time workers than before the Great Recession” is a national economic problem.

Van Horn cautions against blaming a slacker workforce: “It is employers who decide how many hours people will work.”

Also, employers control part-time scheduling. Scheduling that often changes week to week can make it impossible to hold two part-time jobs.

Republican presidential candidate Jeb Bush recently got into part-timer politics when he talked about economic growth goals. He said, in part, “It means that people need to work longer hours and, through their productivity, gain more income for their families. That’s the only way we’re going to get out of this rut that we’re in.”

Political opponents said he unfairly put the onus on workers, so Bush later clarified, “If we’re going to grow the economy people need to stop being part-time workers; they need to be having access to greater opportunities to work.” And he specified 40 hours a week as the goal.

But many employers are reducing hours to under 30 a week to escape health insurance coverage mandates. Millions of relatively low-paid workers are getting only their basic hourly paychecks with no employer-subsidized health insurance, no paid vacations, no paid sick leave or other benefits tantamount to a pay raise.

Van Horn’s survey found that voluntary part-timers are most often supplementing full-time jobs or working while they go to school, care for family, or ease into retirements. The part-time deal works for them. But the involuntary part-timers, averaging just 25 hours a week, are suffering.

The Rutgers survey found 1 in 4 involuntary part-timers rely on food stamps, nearly 1 in 5 use other social service assistance, and 1 in 10 get low-income energy assistance. Most can’t pay their bills, can’t save money and can’t add another job.

Small wonder they’re unhappy. And no wonder customers sense it in poor customer service. Unhappy workers, no matter the cause, aren’t the best employees.