Working fathers earn more than their "childless counterparts", a new study has found.
On average, dads working full-time will enjoy a "wage bonus" of 21% according to think tank IPPR for the Trade Union Congress.
The study, based on 17,000 male and female workers, also found dads with two children earned 9% more than dads with just one child.
In contrast, full-time working mothers suffered a "wage penalty", typically earning 11% less than their childless colleagues.
"[The study] says much about current attitudes that men with children are seen as more committed by employers, while mothers are still often treated as liabilities," said TUC general secretary Frances O'Grady.
Data for the study was used from the 1970 British Cohort Study which follows the lives of more than 17,000 people born in England, Scotland and Wales.
The report, named 'Pay and Parenthood', said the reasons for a fathers' wage bonus "were not clear", but were likely to relate to hours worked and "increased effort".
The study noted full-time working men with children worked on average half an hour longer each week than men without children.
The TUC referenced international studies cited in the report, finding CVs from fathers were scored higher than identical ones from non-fathers.
"While men play a much more active role in raising their children nowadays, many are afraid to request flexible working or time off in case it damages their career prospects," added O'Grady.
"We won't break this cycle unless fathers are given access to independent paid leave to look after their kids, that isn't shared with their partners."
Full-time working dad Andrew Halliday, 29, said he doesn't believe all dads work "longer and harder" when they have a family.
Halliday, who is dad to a three-year-old and seven-month-old, said he even works less than in his previous company to spend time with his two kids.
"I think it's probably more that fathers would tend to be older, therefore have more experience and earn more," Halliday told The Huffington Post UK when commenting on the study.
"I don't think its anything to do with them being parents, but I do believe that once you become a parent, your priorities change and you want to provide for your children.
"Some will be more committed and hard working - which could result in promotions and larger bonus.
"I work my contracted hours and actually work from home at least once a week so I can spend more time with the family, they are only young once and I wanted to spend as much time with them as possible.
"When I now get home, it’s all about them – the work phone is off and I can’t get emails – so there is no distractions from having some great father son’s time."
The analysis, which was carried out by the Institute for Public Policy Research to mark International Women’s Day, found women who choose to have children aged 33 or younger are sacrificing part of their pay packet.
However, women who have their first child at 33 or older have a wage increase of 12% compared with women the same age who haven't had children.
The analysis concluded that the "pay penalty" is caused by younger mothers being "more likely" to have a significant period out of work.