It is "common sense" that women who return to work after having a baby fall behind men in earning power, said a think tank. The Institute of Economic Affairs (IEA), a free market think tank, was reacting to a report by the Institute for Fiscal Studies (IFS) that found that the gap between hourly earnings of the sexes becomes steadily wider after women become mothers.
The survey, which was funded by the Joseph Rowntree Foundation, found that women in the UK are paid about 18% less per hour than men on average.
However, this gap increases consistently for 12 years after a woman has had her first child to about 33%.
It suggested the shortfall in pay was linked to some women working fewer hours after having children – meaning they miss out on pay rises and promotions.
But IEA director general Mark Littlewood said: "It is common sense that if you reduce the number of hours worked your potential future earnings would also drop."
He added: "When people make the decision to go part time, either for familial reasons or to gain a better work-life balance, this can impact further career opportunities but it is a choice made by the individual – men and women alike."
The IEA also called on the government to scrap plans to force firms to publish gender pay leagues.
The think tank said: "The government should scrap the planned publication of crude league tables as there is little evidence that the gender pay gap results from discrimination. Their imposition raises business costs and creates perverse incentives to not hire women for certain roles."
However, the IFS study also found the pay gap had been closed by better conditions for lower-paid women rather than those with A-levels or degrees who have seen little change over the last 20 years.
Robert Joyce, associate director at IFS and an author of the report, said: "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.
"Understanding that lack of progression is going to be crucial to making progress in reducing the gender wage gap."
The TUC general secretary Frances O'Grady said: "It is scandalous that millions of women still suffer a motherhood pay penalty. Many are forced to leave better-paid jobs due to the pressure of caring responsibilities and the lack of flexible working."
A government spokeswoman said it would press ahead with its plan for businesses to publish gender pay tables.
The spokeswoman said: "The gender pay gap is the lowest on record but we know we need to make more progress and faster. That's why we are pushing ahead with plans to force businesses to publish their gender pay and gender bonus gap – shining a light on the barriers preventing women from reaching the top."