So on average men work 40 hours, while women work 35. What this study doesn't show is whether the women wanted to work the full 40 hours and were not allowed to, or if they chose not to for external reasons (child bearing, social activities, etc.) There could still be discrimination within the hours allocated for working.
What this study doesn't show is whether the women wanted to work the full 40 hours and were not allowed to,...
It helps to read a report before claiming it doesn't show something.
According to the report I just cited, 32% of part time men wanted to work full time but were unable to, but only 20% of part time women women wanted to work full time.
The page formatting is awful which makes it difficult to read, and the columns I mentioned (total worked) showed 35.6 hours worked for women, and 40.4 for men.
Where in the data does it show whether men or women wanted to work full time? All I see are vague labels "for economic reasons, for non economic reasons, usually work full/part time". (With the poor formatting, I can't be sure which labels are nested, but it looks like they are.) How could any assumption about the subject's intentions or desires be inferred from those headers? Is there another part of the report you're referring to?
ftp://ftp.bls.gov/pub/special.requests/lf/aat22.txt
So really, the gap is only 8% (not 20%), assume a strictly linear relationship between hours worked and money earned.