The heyday of U.S. employer-provided retirement plans, 1999 to 2002, feels like a bygone era. Americans were worried about Y2K and Richard B. Cheney, and the first millennials were only just entering the workforce. We didn’t even really call them millennials yet.

But in two states, stacked together atop the nation’s heartland, that era never really ended. At least as far as retirement plans are concerned. Private-sector employers in Iowa and Minnesota are still more likely to sponsor retirement plans, and employees there are more likely to take them up on the offer — as of 2019, the most recent year available.

What makes those states different? Everyone has a theory — from big businesses to weather to unions — but everything seems to come back to culture. The cliches “Minnesota nice” and “Iowa nice” arise again and again. And really, what’s nicer than helping your employees save for retirement?

Unfortunately, our data set doesn’t track “nice.” And ultimately, it’s an unsatisfying explanation for an economic phenomenon. So, we looked deeper, with the help of hundreds of thousands of responses to several decades of federal surveys.

We can’t distinguish between traditional defined-benefit pensions and contribution-based plans such as the 401(k), but other sources show that, in this era, the 401(k) is (k)ing. A methodology change in the federal Current Population Survey data we used for this analysis makes it hard to compare levels before and after 2014, but two facts remain constant: retirement plans have fallen everywhere, and Minnesota and Iowa remain on top.

There’s no single force at work. Many folks pointed to Minnesota’s fame as an incubator of Fortune 500 firms such as Best Buy, Target, U.S. Bancorp and 3M — a conglomerate once known as the Minnesota Mining and Manufacturing Company. After all, large corporations are much more likely to provide retirement benefits.

But when we called University of Minnesota professor Myles Shaver — known for his recent book “Headquarters Economy,” which explains how the Twin Cities became host to almost 20 Fortune 500 companies — he pointed out Iowa also tops our list yet has only two mid-tier entries in the Fortune 500. There must be more to it than just major corporate headquarters.

As Shaver ticked off the factors that made Minnesota and Iowa special, two stood out: skilled workers and women. The states have unusually high shares of workers with at least high school diplomas, and more women are working or looking for work there than anywhere else in the country.

“This region seems to be really amenable to finding dual professional careers,” Shaver said. It’s a diverse economy where both partners have a chance at securing fulfilling employment, no matter their specialization. That helps explain why, according to Shaver’s research, couples that move into the Twin Cities are much more likely to stick around.

“That’s the story of this region,” Shaver said. “It’s become a magnet for high-educated, professional human capital, and it seems to suck people in. Because they refuse to leave, they keep moving around companies here, and that’s what causes the region to grow and evolve.”

Indeed, women’s labor-force participation, especially in middle age, is the best indicator we found of whether a state’s employers are likely to offer retirement plans. The next highest? The share of the population with at least a high school education — in other words, a skilled blue collar workforce.

Why? We can’t disentangle cause from effect on this, but either way, it comes down to slack in the labor force. Thanks to working women, Iowa and Minnesota lead all states in 2019 and 2020 with labor-force participation rates of roughly 70 percent; the equivalent national rate was about seven points lower. Because everyone who wanted a job already had one — as the thinking went in a pre-coronavirus world — there are so few potential workers on the sidelines that companies had to offer decent salary and benefits to attract and retain them.

Eric Lothenbach, who has lived in Minnesota all his life, called it “a bit of a shock” to hear that retirement plans for his fellow maintenance professionals weren’t the norm elsewhere.

Lothenbach, who now oversees maintenance technicians in 30 (and counting) buildings in the Twin Cities area, said he had been pleasantly surprised to find his retirement accounts were on track.

“At 45, I’ve still got a ways to go,” he said. Yet, the closer he gets to retirement, the happier he is to see his savings starting to add up. Eventually, he says, he and his wife, Delora, dream of moving to a cabin on one of the state’s ubiquitous lakes. Delora also works in the industry.

The company he works for, Sherman Associates, develops and manages residential, retail and commercial properties in Minnesota, Iowa and throughout the region. It has 600 employees, 80 of them in maintenance or facilities — an area which, like construction and retail, tends to get retirement benefits more often in Minnesota.

The area around the Twin Cities, the most populous metro area on the Mississippi River, is home to all of Minnesota’s fastest-growing counties. Lothenbach, who recruits, interviews and trains new workers when he’s not wielding tools himself, suggests the booming real estate industry there has pushed companies to offer more generous benefit packages.

“When you’re dealing with tight labor markets and low unemployment, you’re looking at what I can do as an employer to stand out,” said Ron Nutting, chief operating officer at Sherman Associates, adding later that “anything we can do to make our retirement plan stronger can be a competitive advantage.”

Collective bargaining doesn’t seem to play as big a role as you might think. States with more workers covered by private-sector unions tend to have more workers with retirement plans. But the relationship isn’t strong, and both Iowa and Minnesota fall below average in our measure of how many private-sector workers are covered by unions at work.

A related idea, employee ownership, plays a supporting role. When asked about benefits, Dave Moe, director of marketing for Minnesota-based Braun Intertec, was quick to mention the employee-owned company’s employee stock ownership plan (ESOP), which grants employees ever-growing stakes that become fully vested after about six years with the company.

There are no perfect measures of ESOPs by state, but with the help of Nancy Weifek, research director at the nonprofit National Center for Employee Ownership in Oakland, Calif., we estimate that for its size, Minnesota easily tops any other state for ESOP plans at privately-owned companies. Iowa, while above average, doesn’t rank quite as high.

But like others, Moe was quick to mention the tight labor market. He said that in recent years the “war for talent” likely pushed employers to keep offering retirement plans in Minnesota, even as they cut back elsewhere.

Haley Dudrow of the Weitz Company, a Des Moines-based construction behemoth, suggested another intriguing theory: Maybe it’s the insurance. Des Moines is a national hub for insurance companies, and locals’ connection to the industry might foster a long-term, practical mind-set.

We can’t speak to a causal or cultural relationship, but Labor Department data shows both states ranked in the top five for concentration of insurance-industry employment in 2018, based on how much each industry contributes to the local economy in the form of wages.

In the end, though, insurance employers probably contributed not by changing the culture, but by doing what all growing companies do: hiring workers and doing their part to tighten the state’s labor markets.

Decades of intense competition for workers helped make retirement plans commonplace in both states. It illustrates the benefits workers can receive late in an economic expansion, when everyone who wants a job has one, and when competition for employees means many could expect a 401(k) or ESOP from any potential employer. It’s a case for running the economy hot, and allowing unemployment to fall and worker leverage to increase.

Lothenbach, the maintenance manager in the Twin Cities, said he now preaches the retirement-saving gospel to his three sons, all in their twenties. When he was asked why Minnesotans were more aggressive in saving for retirement, he admitted he’d never really stopped to think about it.

“They probably want to retire so they can move somewhere warm,” he joked.