A woman wants to return to work after maternity leave but doesn't want to put in the hours she did previously. So she asks her boss for what seems like the best of both worlds: a part-time role. In a 2012 Pew Research Centre survey of mothers, nearly half thought such a scenario would be ideal, while significantly lower percentages said they would prefer to stay at home or work full time.

But are part-time jobs really so perfect?

Not according to my research. After using time-diary studies to track 1,001 days in the lives of women who earn six figures and also have kids, I found most of those on official part-time schedules put in more than 35 hours a week. One part-time consultant logged 47 hours one week and 53 hours in another, which wasn’t necessarily typical but was nevertheless comparable to the hours logged by full-timers at equally prestigious firms in their time-diary periods.

Even though the part-timers had often taken pay cuts, and risked being seen as less committed to their careers than full-time colleagues, they weren’t necessarily working that much less.

Mounting evidence confirms that "full time" encompasses a host of lifestyle options. As Boston University professor Erin Reid showed in a recent study of a major consulting firm, even though few male employees ask for reduced hours, many are still able to keep their work weeks in the 50-hour range by cultivating local clients, covering for each other and tending to personal issues without calling attention to their absences.

Research by the sociologist John Robinson and his colleagues also shows professionals tend to exaggerate full-time work weeks, sometimes by 25 hours or more.

I saw the same phenomenon in my time-diary studies. There was significant variance in the hours logged by full-timers in similar roles and professions; 10-hour differences (that is, two hours per work day) weren’t unusual.

While part-timers tend to keep focused at work and tie up any loose ends outside office hours, full-timers feel comfortable taking social media breaks, lingering over lunch or running errands while officially on the clock. To be sure, this may not seem fair or right. One woman working an 80 per cent schedule (at corresponding pay) told me she was pretty sure she was working just as much as full-time colleagues. But, she said, “I choose to do what I perceive to be the right thing.”

I have a lot of respect for her integrity, but I think the attitudes expressed by the men in Reid’s study better reflect how forward-thinking organisations expect knowledge workers to behave nowadays.

There are ways to legitimately work full time (not overtime) without spending all of it in the office. Plenty of parents choose what I call a “split shift”: spending morning and early afternoon in the office, leaving at a reasonable hour, then going back to work after the kids are in bed.

More than half of the women did some work on Sundays, while 40 per cent logged a few Saturday hours, too. Their compensation was the ability to spend an afternoon chaperoning a field trip. Reid’s consultants used similar strategies. They earned the same performance ratings as peers working longer hours.

Women – and men – who are tempted to shift to a part-time schedule should first consider whether they could work a full-time schedule more flexibly. That seems to be the real ideal. – Copyright Harvard Business Review 2015 Laura Vanderkam is the author of I Know How She Does It: How SuccessfulWomen Make the Most of Their Time and four other books.