At most, the unexplained pay gap is between 4.8% and 7.1%
The 77% figure is often criticized because it is a raw aggregate based on several assumptions. For example, women tend to work fewer hours, in lower paying professions, with less work experience. The 77% figure for median full-time woman, does not take any of those differences into account.
A study commissioned by the United States Department of Labor, Gender Wage Gap Final Report concluded that:
"There are observable differences in the attributes of men and women that account for most of the wage gap. Statistical analysis that includes those variables has produced results that collectively account for between 65.1 and 76.4 percent of a raw gender wage gap of 20.4 percent, and thereby leave an adjusted gender wage gap that is between 4.8 and 7.1 percent."
The vast majority of the difference can be explained by the occupations that women tend to work:
[Bayard, Hellerstein, Neumark, & Troske, 2003; Groshen, 1991; Sanborn, 1964] Groshen (1991) explains that, within her most detailed categories, which essentially consist of detailed occupations within individual establishments, the pay of men and women who work in the same category is almost equal. Based on those categories, which generally are either predominantly male or predominantly female, the proportion of workers within an occupational category who are female can account for between 50 and 60 percent of the raw gender wage gap. With less detailed categories, the percentage is smaller, but still ample.
A detailed statistical analysis of interindustry differences in the size of the gender wage gap has been conducted by Fields and Wolff (1995). Their analysis has examined three increasingly detailed sets of industry categories, and has found that, as the industry categories become more detailed and more numerous, the range of sizes estimated for the gender wage gap in different industries increases. [this can] account directly for as much as 22 percent of the gender wage gap. Further, the observed difference in the distributions of male and female workers among the industries can account for an additional 19 percent of the gap
Work Experience is also a major factor:
[Blau & Kahn, 2006; Boraas & Rodgers, 2003; Gabriel, 2005; Light & Ureta, 1995; U.S. Government Accounting Office (GAO), 2003 (since renamed U.S. Government Accountability Office)] In particular, Blau and Kahn (2006) report that results from their statistical analysis indicate that women's gains in work experience during the 1980s account for about one third of the total narrowing of the gender wage gap over that time.
Women tend to take time off for childbearing and motherhood:
Examining the reductions in earnings that have been observed after career interruptions that have lasted at least one year, Light and Ureta (1995) [...] account for as much as 12 percent of the raw gender wage gap.
[Anderson, Binder, & Krause, 2003; Budig & England, 2001; Dey & Hill, 2007; Johnson, 2008] Budig and England report that, in their baseline analysis, having children is associated with a 7.3 percent reduction in the wages of mothers. After the effects of the mothers' absence from the labor force and their consequent diminished accumulation of pertinent experience are taken into account statistically, however, the reduction in wages is decreased to 4.7 percent. Then, after accounting statistically for job characteristics that might be especially appealing to mothers, such as part-time status or flexible work schedules, the reduction is decreased further, to 3.7 percent.