"It's a well-known pattern," said ISR economist Frank Stafford, who directs the study. "There's still a significant reallocation of labor that occurs at marriage—men tend to work more outside the home, while women take on more of the household labor. Certainly there are all kinds of individual differences here, but in general, this is what happens after marriage. And the situation gets worse for women when they have children."

The good news is that in the past thirty years the time dedicated for housework during a week by men and women tends to be equal.

For example, in 1976, women did an average of 26 hours of housework a week, compared with about 17 hours in 2005. Men did about six hours of housework a week in 1976, compared with about 13 hours in 2005.

Both the men and the women who got married did more housework than those who stayed single, the analysis showed. "Marriage is no longer a man's path to less housework," said Stafford, a professor in the College of Literature, Science, and the Arts.

The scientists also examined how children, age and social status influence the time spent for housework.

According to their findings single women in their 20s and 30s did the least housework—about 12 works a week on average, while married women in their 60s and 70s did the most—about 21 hours a week. Men showed a somewhat different pattern. Older men did more housework than younger men, but single men did more in all age groups than married men.

Married women with more than three kids did an average of about 28 hours of housework a week. Married men with more than three kids, by comparison, logged only about 10 hours of housework a week.

The findings are part of a detailed study of housework trends, based on 2005 time-diary data from the federally-funded Panel Study of Income Dynamics, conducted since 1968 at the U-M Institute for Social Research (ISR).