There is a natural difference also in the relative physical strengths (on average) of the sexes, both in the lower body and more pronouncedly in the upper-body, though this does not mean that any given man is stronger than any given woman.
These different capabilities make the genders vary in the types of jobs they are suited for.
Various explanations for the remaining 25% to 40% have been suggested, including women's lower willingness and ability to negotiate salaries and sexual discrimination.
However, multiple studies from OECD, AAUW, and the US Department of Labor have found that pay rates between males and females varied by 5–6.6% or, females earning 94 cents to every dollar earned by their male counterparts, when wages were adjusted to different individual choices made by male and female workers in college major, occupation, working hours, and maternal/paternal leave.
The gap is due to a variety of factors, including differences in education choices, differences in preferred job and industry, differences in the types of positions held by men and women, differences in the type of jobs men typically go into as opposed to women (especially highly paid high risk jobs), differences in amount of work experiences, difference in length of the work week, and breaks in employment.
These factors resolve 60% to 75% of the pay gap, depending on the source.