(REUTERS) - The pay gap between men and women spreads across all income levels.

A new study by online salary and benefits information company PayScale of 1.4 million workers shows how stark that gap is.

Amongst its findings: the biggest difference in wages is between married men with children and married women with children.

Dads' median salary: nearly US$68,000 (S$95735.80) while Moms': under $47,000 (S$66170.30)

Aubrey Bach, senior editor at PayScale, said: "We control for factors like education level, experience, location, etcetera and we compare men and women working in the same jobs and that's how we calculate what we call a controlled gender pay gap. So it's an apples-to-apples comparison."

PayScale also found that women's salaries peak much earlier than men's between the age of 35 and 40 versus 50 to 55 for men.

Also, the higher the job level the greater the salary gap.

Behind the differences: research shows women aren't as good at negotiating pay and they face a maternal wall - that is they may have to leave their careers mid term to take care of kids or that's what companies assume.

One bit of good news: while technology is dominated by men it also has the smallest pay gap of any industry.