Families with children, especially single-parent households and families with three or more kids, are finding it tougher to own a home, a study shows.

While the overall U.S. homeownership rate is at a record 68 percent, the rate for families with children is below its 1978 level of 70.5 percent.

Among low- and moderate-income working families with children, only 56.6 percent were homeowners, the survey showed, down from 62.5 percent in 1978.

The study was conducted by the Center for Housing Policy, the research arm of the National Housing Conference.

Working families are defined as those earning less than 120 percent of the local median income but more than minimum wage.

The overall U.S. ownership rate dipped between 1978 and 1991, then rose and by 2001 had surpassed the 1978 level.

The lag for lower- and middle-income working families can partially be explained by the rising number of single-parent households. In 1978, only 18 percent of all working families were headed by a single parent. By 2001, that share was 39 percent.

Homeownership among couples with children has declined from its pre-1980s level as well. The rate was 66.8 percent in 1978 and fell to 63.2 percent in 1991 before rising to 65.5 percent in 2001.

The rising costs of ownership explain some of the numbers, according to the study. The median price of a new home in 1978 was $55,700, about four times the $14,300 median income of a working family with children.

By 2001, the median cost of new home had jumped to $175,000, five times the median income of $35,000.

Meanwhile, the annual minimum wage went from $5,512 in 1978 to $8,840 in 1991 and to $10,712 in 1999, where it has remained.

From 1978 to 2001, the U.S. median mortgage payment has climbed 282 percent, while median income has grown just 166 percent.

But income growth in real terms over that time was essentially zero, since consumer prices rose 172 percent.

Homeowners' insurance also costs went up 175 percent while property taxes rose 126 percent and average utility payments shot up 108 percent.