Employment rates for women in the year before and after childbirth also fell significantly: by 16 percent for university graduates and 33 percent for those with basic state education. Male employment rates were barely affected by child birth, according to the study.
"The gap between the hourly pay of higher-educated men and women has not closed at all in the last 20 years. The reduction in the overall gender wage gap has been the result of more women becoming highly educated and a decline in the wage gap among the lowest-educated," said Robert Joyce, associate director at IFS and one of the report's authors, in a press release.
"Women in jobs involving fewer hours of work have particularly low hourly wages, and this is because of poor pay progression, not because they take an immediate pay cut when switching away from full-time work."
Mark Littlewood, director general at the Institute of Economic Affairs, criticized the IFS report, arguing that it showed the wage gap had nothing to do with gender discrimination.
"As the study itself notes, women who take time off work and return doing fewer hours are not getting paid less per hour," he said in a press statement.
"To compare the salary of a part-time worker to their potential salary had they remained full-time skews the numbers. It is common sense that if you reduce the number of hours worked your potential future earnings would also drop.
"And it's hard to cry 'employer discrimination' when, according to the Office for National Statistics, women in part time work last year earned on average 6.5 percent more than their male counterparts."
Another report out this week from the Chartered Management Institute found male managers were 40 percent more likely to be promoted to senior roles than their female counterparts. The report argued that the discrepancy in promotion rates was one of the main causes of the wage gap issue.
"Promoting men ahead of women is keeping us all back. Diversity delivers better financial results, better culture and better decision making," Ann Francke, chief executive of CMI, said in a press release. "Transparency and targets are what we need to deal with stubborn problems like the gender pay gap."
The gender pay gap issue isn't isolated to the U.K. ABC News reported on Monday that figures from the Australian Council of Trade Unions showed the gender pay gap in Australia had grown steadily over the last ten years, with men being paid on average 20 percent more than women.
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