This actually borders on being a rather comical point. People in prison have an income of zero. So if we're going to arrive at 0.77 by dividing one average by another, I for one would like to know, is the prison population being included?
Where can I fund a journalistic investigation into the matter right away?
0.77 isn't actually the pay gap. It's much, much smaller than that, below 10%. It's one of the biggest victories of feminists that they have been able to shape discourse around a completely wrong number.
I would say the income of a person in prison is a large negative. "According to the Vera Institute of Justice, the average cost of housing an inmate in the U.S. was $31,286 in 2012." [1]
Where can I fund a journalistic investigation into the matter right away?