According to this report in Science Daily:
[A] study of more than 17,000 people in 28 countries found that married men report doing less housework than men who are live-in boyfriends.
According to [Shannon] Davis, the key finding of the study is that it suggests the institution of marriage changes the division of labor. Couples with an egalitarian view on gender--seeing men and women as equal--are more likely to divide the household chores equally. However, in married relationships, even if an egalitarian viewpoint is present, men still report doing less housework than their wives.
The summary fails to mention whether differences in employment status, type of employment, income differences, parenthood status and hours worked outside the home were considered. Any or all of these might covary with marital status and affect the division of household labor, particularly when people across a wide range of cultures are considered. Perhaps the authors considered these factors, but, if so, it isn't mentioned in the SD link.
My practice partner worked about 50 hour per week in the office prior to her marriage. That dwindled rapidly after she married. Now, she puts in under 10 hours per month. Her husband works more than ever outside the home. He also does less household work than ever. She does less household work, as well. They have help. And, that is the way she likes it.