Full disclosure:  This information about the wage gap between the sexes is from The Center for American Progress, which describes itself as “independent nonpartisan educational institute dedicated to improving the lives of Americans through progressive ideas and action.”  The organization is comprised of researchers and policy experts (generally former politicians and former government administrators) and is regarded as leaning more towards the left/liberal side of the political spectrum.

I posted this though because they include some links back to helpful sources and have summarized a number of issues concisely.