More than 30,000 union machinists have been on strike against the giant airplane maker Boeing for nearly six weeks. Boeing is hemorrhaging money from that and other problems, and it looks like the strike will go on a while longer now, with union members rejecting a second contract Wednesday by a two-thirds margin.

Union workers are holding out for better terms from the company on wages, health insurance and retirement plans.

In particular, the union has been adamant that Boeing to return to a defined-benefit pension plan, which it suspended 10 years ago in favor of defined-contribution, or 401(k), retirement plans. 

The shift in the private sector from pensions to 401(k)s has been going on for decades, but now there’s more pushback from workers and unions. 

Broadly speaking, defined-benefit, or pension plans, are funded by employers and pay retirement benefits for life. 

Defined-contribution, or 401(k) plans, are usually funded by the employer and employee, and the employee then manages the investments for as long as they last.

Pensions put financial pressure and risk on employers to fund and manage them. Because 401(k)s put that pressure mostly on workers, they’ve become more popular with employers. But now there’s pushback led by unions.

“I am not surprised that the Boeing workers rejected the contract,” said Teresa Ghilarducci, who studies retirement at The New School.

She said workers increasingly understand the risk inherent in 401(k)s.

“A 401(k) plan does not guarantee a stream of income for the rest of your life,” Ghilarducci said. “People are filled with stress having to manage their nest egg.”

But pensions have disadvantages too, said Alicia Munnell at the Center for Retirement Research at Boston College.

“I am surprised with the enthusiasm for defined-benefit plans,” said Munnell, who recently posted an article on the subject, titled “Don’t Bring Back Traditional Private Sector Defined Benefit Plans.”

Munnell said in the private sector, pension benefits typically aren’t indexed to inflation, which has soared in recent years.

“And the key loser was people who had defined-benefit plans that did not keep pace,” Munnell said.

Another ding against pensions is the fear the employer-sponsor will go belly up. Monique Morrissey at the Economic Policy Institute said that concern is overblown.

“In the private sector, benefits are guaranteed by the Pension Benefit Guaranty Corporation,” Morrissey said.

In the public sector, pension funds come from government coffers, and Morrissey said one way to balance the budget “is by not contributing the full amount to the pension.”

Many experts agreed that the biggest problem today is that (as documented by the Bureau of Labor Statistics), aside from Social Security, millions of workers don’t have any plan — 401(k) or pension — to help pay their bills in retirement.