Want to improve your financial net worth? Get married

But that isn't the whole story. It may explain why incomes increase so much after marriage, but the Lupton and Smith study also showed that singles who married increased their net worths by 400 to 600 percent within five years. Married households had about double the income of the divorced and never-married, but four times or more the net worth.

"Marriage changes behavior," Whitehead says.

It helps people be responsible to each other — and to their children. It helps them focus more on the future.

Waking up

When Freed "woke up" at age 37 and tied the knot, everything changed in his life. He and his wife paid off student loan debts and saved money for a down payment on a house. They bought a home and two rentals in North Bergen, N.J., which provided not just a nice place for them to live, but provided rental income.

"When you get married, you start to think," says Freed. "You have a family. You buy a house. You start being more responsible. … When you have a family it is easier to focus and prioritize things. You think more about the bigger picture."

Part of the bigger picture for the Freeds was having a baby girl.

Whitehead says having children is one of the big reasons people's views on personal finance change within marriage.

"For couples who are married and have children, they become personally responsible for new lives," she says. "It encourages them to think about what lies ahead — such as education — and how to save and prepare for that."

Cohabitation

It might be easy to think that merely living together would bring some of the same economies of scale that a marriage does. The Institute for American Values report, however, found that in cohabitation, partners are not typically financially responsible to each other and instead spend as individuals, and as they please.

Whitehead says people who live together keep their finances separate and don't pool their resources — one of the main financial advantages married couples enjoy for building wealth. The tentative nature of the relationship doesn't have the partners planning and saving for things such as retirement together.

Whitehead also says relatives are not as involved in the lives of people who cohabit as they are if they are married. There is a difference between helping out a son-in-law who lost a job versus helping out a daughter's boyfriend.

Charles Murray says when upper-middle-class couples cohabitate, they may have higher joint incomes and be more apt to pool resources, but seldom have children until they decide to marry. Working class people who live together are another story, he says. "When a (working-class) woman shows up cohabitating, what it means is her boyfriend is living off her income," he says. A study by Kathleen Kiernan and Valerie Estaugh in 1993 showed cohabitating mothers are almost five times more likely to have an unemployed partner than married mothers.

Preaching what they practice

Murray says marriage is doing very well among the upper middle classes, where people are college-educated. He says 84 percent of adults in that class, ages 30 to 49, are married. "That is very high," he says, "and it hasn't changed much since the 1980s."

He asks upper-middle-class college students how many come from a home with both biological parents. He once found that in a class of 18 students, 17 came from such homes.

But there is a problem.

"What they will not do in their jobs and lives is preach what they practice."

Murray said his students don't act like marriage is a value that should be shared with others. "They won't treat it as superior," Murray says.

Instead, they talk about other family structures as being equally valid and effective as two-parent families.