Not married to your life partner? It could be costing you. See what the WSJ study shows.

The wealth gap between partnered and married couples is larger than one might expect, said Ana Kent, a senior researcher at the St. Louis Fed. “It’s so intriguing,” she said.

Over the past two decades, Americans are moving in together at higher rates, according to data from Pew Research Center. The share of U.S. adults who are currently married steadily declined from close to 60% in the 1990s to under half in 2019, according to Pew. Over the same period, the share of adults age 18 to 44 living with a partner climbed to 59%.

Many young couples now approach marriage as a “capstone” event, said Andrew Cherlin, professor emeritus of sociology and public policy at Johns Hopkins University, who studies marriage.

“If you build an arch, the cornerstone is the first piece you put in and the capstone is the last,” he said. “What this means is people see an economic bar they need to clear before they get married. Couples wait until they have good jobs, a car that won’t break down, maybe even a house. Then, they get married.”

Melissa Mowery, a 30-year-old communications manager in Asheville, N.C., has been with her boyfriend for five years and living together for nearly four. The two don’t share a joint bank account, but they split the cost of rent and other bills. Even so, Ms. Mowery said she can’t make sense of the financial gap between her relationship and that of married couples.

“We’re already saving a lot of money and splitting the cost on most things,” Ms. Mowery said. “I don’t understand how married couples are accumulating wealth in a way we’re not doing.”ROSS MARTIN FOR THE WALL STREET JOURNAL

“We’re already saving a lot of money and splitting the cost on most things,” she said. “I don’t understand how married couples are accumulating wealth in a way we’re not doing.”

While there are legal and tax benefits to marriage, research suggests the financial security and long-term mind-set of those who tie the knot may also be a powerful driver of wealth. More married couples pool their money—such as sharing savings accounts and investing together—to achieve certain goals, Ms. Kent said. Cohabiting couples are less likely to combine finances and investments.

Working with two incomes and combining their investments to maximize compound interest can significantly increase a couple’s financial prospects, said Emily Garbinsky, associate professor of marketing at Cornell University, who has studied couples’ financial behavior. Simply put, married people may be more likely to be on the same page financially, she said.

Ms. Mowery and her boyfriend, Alex Feiszli, first moved in together after dating for a year. Now, five years later, they share rent and other bills but otherwise don’t combine their finances.ROSS MARTIN FOR THE WALL STREET JOURNAL

“Married people may be much more likely to have these conversations around what goals they have for their financial future,” she said. “There seems to be something very special and unique about deciding to share finances.”

Unmarried couples may be less willing to commingle their money, said Prof. Garbinsky.

“Our money, our income, represents a huge part of who we are,” she said. “[Sharing] that can be scary for people, so they tend to be very protective.”

Photos: Teaming Up for the Homeownership Dream

PHOTO: VERONICA VEST

Both married and unmarried couples who do pool finances also experience greater relationship satisfaction and may even stay together for longer, Prof. Garbinsky said.

Housing is one of the biggest factors in establishing a couple’s wealth. Compared with single people and cohabiting couples, married couples hold a larger concentration of housing wealth, according to data from the St. Louis Fed.

“Most of my married friends have bought a house,” Ms. Mowery said, noting high housing costs in her area. “I just don’t know how they did it. Everyone talks about how when you get married, you accumulate wealth but I don’t know what that means.”

In the current hypercompetitive housing market, as smaller, more affordable starter homes vanish and housing affordability declines, single people and cohabiting couples are often at a disadvantage.

Unmarried couples are less likely to own a home than their married counterparts. After living together for a few years previously, Ms. Mowery and her boyfriend now rent a house in Asheville, N.C.PHOTO: ROSS MARTIN FOR THE WALL STREET JOURNAL

“These [housing] prices are so high that you really need pooled resources to be competitive in some of these markets,” said Lowell Ricketts, a data scientist at the St. Louis Fed.

Socioeconomic factors play a role in the difference between married and partnered wealth; the higher your income, the likelier it is that you’ll marry, a 2017 report by the American Enterprise Institute found.

Plotting a path forward as a couple without much money isn’t as easy as getting hitched and suddenly seeing your wealth grow, Prof. Cherlin said.

“Someone looking at the data would say, ‘Well, these married people are much more successful than their cohabiting people. If these people would just get married, they’ll do better,’” he said. “Whether or not there is truth in that, people don’t tend to believe in it anymore. People who aren’t doing well financially don’t see a clear path to financial success.”

Marriage rates are lower among Black and Latino groups, and those same households of similar ages held far less wealth than their white counterparts, whether married or partnered. Family structure also influences the overall net worth of a household. Partnered couples with young children tend to have less wealth than partnered couples without children.

Some cohabitating couples are refashioning their financial goals. Instead of buying a house, Ms. Mowery and her partner recently looked into a house share that would allow them to spend part of the year working from Belize. They have discussed getting married, although haven’t made plans to do so any time soon.

“I care a little bit less than I thought I would about marriage,” she said. “Once you start living together, it almost feels like you made that commitment.”

Write to Julia Carpenter at julia.carpenter@wsj.com