In this photo, Maya Ramirez, a single mother helps her seven-year-old daughter Sherhar with homework while holding her five-month-old baby Alma on March 13, 2009 in El Centro, California. Getty Images/David McNew

Like millions of soon-to-be mothers, Charlotte Brock, 35, faced an agonizing decision before the birth of her child: Stay at work and pay for professional care or quit her job and devote time to the unpaid labor of child rearing. The stakes were especially gut-wrenching since Brock, a Marine veteran who spent two years in Iraq, is a single mother.

“They’re only babies for such a short period of time,” she says. “I really wanted to do the best I could do for him -- being there, bonding with him, being attentive to his needs, helping him develop in the best way possible.”

It was also a decision she didn’t think she’d have to make. Brock had a good job at a defense policy think tank in Washington, D.C., run by ex-military people and was earning upwards of $65,000 a year. The firm offered generous paid sick time and vacation benefits. It only seemed natural to Brock that those benefits would extend to paid parental leave.

“I guess I had just misunderstood,” she says.

Eventually, Brock's bosses offered her 12 weeks of unpaid leave -- the maximum amount provided under the Family and Medical Leave Act-- even though the federal law only covers firms with 50 or more workers and did not apply to Brock’s employer. Either way, she decided against it. The amount of time off was too short, and most importantly, it was unpaid. So she quit her job and moved in with her parents in Ottawa, Canada, where she spent the next two years.

When Brock dropped out of the labor force five years ago, she followed in the footsteps of millions of other Americans who are overlooked by the government's official count of the unemployed. As this month’s jobs report shows, a smaller share of the civilian population is officially in the labor force -- either employed or unemployed and looking for work -- than at any time since the late 1970s. It’s one of the big reasons why bosses across a wide array of sectors aren’t feeling pressure to hike pay, even as the official unemployment rate slides downward.

It’s a hot political talking point today as the White House touts the strength of the economic recovery. But the steady decline in so-called labor force participation predates the Obama administration by nearly a decade. It’s driven in large part by demographics: The Baby Boomer generation, whose careers extend from the waning years of the post-World War II boom to the recent recession, is gradually retiring. Economists deem that mass exodus responsible for about half of the total decline since 1999.

The Great Recession fueled the drop, too. People who can’t find work sometimes give up the search for a new job and drop out of the labor market altogether -- a trend that notably marked the recent downturn. On top of that, working-age men have gradually been leaving the labor force for decades -- in large part as decent-paying blue-collar jobs have either disappeared or left American shores, replaced by low-paying service sector jobs.

“It’s about the attractiveness of jobs that are available,” says Harry Holzer, a professor of public policy at Georgetown University. “If you were a high school graduate that had a good paying job in a field like construction or manufacturing, what you have left to choose from now is going to be a huge step down. It’s hard to take a job that’s paying 30 to 40 percent less than before.”

An often overlooked part of the story, though, is the decline in working-age female participation -- people like Brock.

For decades, women entered the U.S. labor force at a rapid rate, growing from near 40 percent participation at the dawn of the 1960s to 70 percent by the turn of the century. Since then, the trend has tapered off. From 2000 to 2014, labor force participation for women aged 25 to 54 fell nearly 3 points to 73.9 percent, according to the Organization for Economic Cooperation and Development (OECD). On this measure, the U.S. ranks behind the vast majority of its fellow OECD nations -- 17th out of 22. Others, meanwhile, are seeing steady gains.

A 2013 National Bureau of Economic Research paper from Cornell economists Francine Blau and Lawrence Kahn shed light on a major reason behind the discrepancy between the U.S. and other OECD countries: the dramatic difference in paid parental leave policies. Most of the world guarantees workers this benefit; only the United States and Papua New Guinea do not.

Paid parental leave makes re-entry into the labor market easier, since workers are entitled to their old jobs back, says Kahn. “But it also actually encourages women to get into the labor market even before they have children, because it’s a benefit you can only qualify for you if you have a job. The benefit actually serves to bring people into the labor market so they can qualify for the benefit in the first place.”

The system under the Family and Medical Leave Act is far more limited, Kahn says. Not only is it unpaid, but it doesn’t cover small employers or workers who’ve worked less than a certain number of hours in a year. The amount of actual guaranteed time off too is dramatically less.

“If you have a less than a 50 percent chance of qualifying for the benefit, and it’s a 12-week period. That’s much less encouragement than a country where it’s universal and you have a 57-week entitlement,” says Kahn. (Norway offers this.)

Instead the stateside system leads to plights like Charlotte Brock’s, whose re-entry into the labor force turned out to be as grueling as the decision to leave. After two years out of work, Brock she says she struggled to find a job when she was ready to work again. “I’ve never really recovered professionally from that,” she says.

Brock is now studying for a master’s degree in industrial and organizational psychology. She was also working part time as an editor before losing her job this week. In the meantime, she’s become an unabashed supporter of paid parental leave.

“It’s a question of human rights for the mother and the child,” she says. “We’re mammals, we’re meant to have this bonding. To not include it is inhumane.”