More than two-thirds of the nation's 73.7 million children lived with married parents in 2007, a new census report says. And just over one-in-four children lived with a single parent.

However, unlike past census reports, the 2007 Current Population Report released by the Census Bureau on Monday highlights a relatively small segment of children living with two parents who aren't married.

Some 2.2 million children — roughly 3 percent of the nation's children — lived with two unmarried parents, the report found.

The report marks the first time the Census Bureau has done a detailed survey of the relationship between children and parents, whether biological, step or adoptive. In the past, a child living with two unmarried parents was counted as living with just one parent.

The report doesn't show any state-level data. However, with roughly 31 percent of its residents under 18, Utah is the nation's youngest state, according to July 1, 2007, census population estimates. Utah ranks first in the number of children, said Pam Perlich, senior research economist at the University of Utah.

While Utah is becoming more like the rest of the nation, its demographics remain unique, making it difficult to compare to the rest of the nation, Perlich said.

Karen Crompton, executive director of Voices for Utah Children, points to another statistic that makes Utah unique — 82 percent of the state's families are headed by married couples.

Crompton said the Current Population Survey report does raise interesting questions.

It shows many of the same trends for children in cohabitating households as in single-parent households: They tend to be lower income, people of color, less educated. The rate of homeownership is lower.

However, the children in those households are also younger. Some 42 percent of children were under age 3, compared to 17 percent of children with married parents, the CPS said.

"Maybe it indicates these are younger parents," Crompton said. "It would be interesting to see the statistics over time — how many ultimately do marry."

And for low income couples there is sometimes a disincentive for marrying, she said. She points to an earned income tax credit of up to $4,000 for someone below the poverty level as an example. Marrying someone of the same income level could double a household's income, meaning a loss of at least some of the tax credit.

"You could lose nearly all that income, although your living situation hasn't really changed," she said. "If we think having two married parents is a way to go, we need to look at incentives to encourage that as opposed to building disincentives."