Live-in boyfriends are more likely to share the load in household chores over married men, suggests a new study involving more than 17,000 participants.

"Marriage as an institution seems to have a traditionalizing effect on couples — even couples who see men and women as equal," lead author Shannon Davis, a sociology professor at Virginia's George Mason University, said in a release Tuesday.

Researchers interviewed individual members of couples in 28 countries for the study published in the September issue of the Journal of Family Issues.

A total of 17,636 participants were asked questions about their partner's earnings and household responsibilities to gauge how time, availability and resources factor into the division of work.

'It's the way the society has defined what being married means, the institution itself, that affects behavior.'—Shannon Davis, researcher

In all countries, there was a discrepancy in hours of housework among the sexes, with men reporting a mean of 9.41 hours weekly and women 21.13 hours weekly, the study said.

Respondents said that in partnerships where the male earns a greater income, women take on more household responsibilities. In couples where women earn more money, men perform more chores as compared with men whose partner contributes the same amount to the household income.

Davis suggests that the gap between husbands and boyfriends exists because of differing expectations. Unmarried couples that live together have higher equality expectations over married couples, she said.

But expectations tend to change once the couple marries, Davis said.

"It's the way the society has defined what being married means, the institution itself, that affects behavior," she said.