There's no denying it, America's waistline has ballooned significantly. What we don't know, however, is precisely why. A host of different factors may be to blame, including dietary changes, increased sedentary behavior, the prevalence of novel food additives, and genetics.

While men have grown the most rotund (almost 70% are overweight or obese), women have certainly swelled as well. The Kaiser Foundation estimates that 56.6% of American women were overweight or obese as of 2011, a sharp rise from decades past.

Looking to explain this trend, a research team based primarily out of the University of South Carolina has suggested another possible agent of women's obesity: the drastic reduction in time spent on housework. The team published the results of their study yesterday in PLoS ONE.

Using a large data set of self-reported time-use information dating back to 1965, the researchers determined that the time women (aged 19 to 64) spent on housework decreased from 25.7 hours per week in 1965 to only 13.3 hours per week in 2010. By their estimation, that translates to a reduction in caloric expenditure of about 1,857 calories per week, a drastic shift (see table below). Positive energy balance -- when caloric intake exceeds expenditure -- drives weight gain.

Over the same period, the researchers found that the time women spent on leisure time physical activity (exercise) increased, but the change wasn't nearly enough to account for the energy expenditure deficit from diminished housework time. Moreover, they found that women spent an average of 8.2 hours of additional time each week watching television and using computers compared to 1965.

According to the researchers, technology -- not laziness -- played a large role in the decline of time spent on housework. For example, food preparation has become significantly easier since the 1960s, thanks largely to the microwave, and post-meal clean up has become automated thanks to the dishwasher.

Women's role in society has also significantly changed, as full-time, often sedentary employment has replaced and diminished the amount of time women spend on housework.

A reduction in housework alone certainly doesn't fully explain the rise in obesity among women. The overall obesity epidemic in both men and women is infinitely more nuanced. Additionally, the study is limited by the nature of the data, which is self-reported. Furthermore, the authors didn't seem to take into account the caloric expenditure from office work and other sedentary activities that may have displaced housework. Accounting for this would further cut into the overall decrement in caloric expenditure, but definitely wouldn't make up the deficit.*

*Post edited 2/21

Source: Archer E, Shook RP, Thomas DM, Church TS, Katzmarzyk PT, et al. (2013) 45-Year Trends in Women’s Use of Time and Household Management Energy Expenditure. PLoS ONE 8(2): e56620. doi:10.1371/journal.pone.0056620