It's long been known that on average, women earn less than men in the UK. The pay gap is sometimes blamed on women putting their careers on hold to raise a family, while in some cases, women are simply paid less than their male colleagues for doing the same job.

When Octopus HR commissioned a study looking at the relationship between earnings and month of birth we weren't initially looking to add to the debate about the pay gap. However, when we split the data into male and female cohorts, things started to get interesting.

For women, there was a clear relationship between when they were born in the academic year and median earnings. Women born in the autumn months earned most, with a median salary of £25,903, women born in the winter and spring months earned progressively less and those born in the summer months earned the least, with a median salary of £24,211. Those born at the start of the school year earned 7% more than those born towards the end. There was no such relationship for men.

We also looked at whether there was any correlation between when someone was born and the likelihood of them earning £156,000 or more a year, which would place them in the top 1% of earners in the UK. For women, the findings were stark. Women born in the first half of the academic year were 27% more likely than those born in the second half of the school year to fall into this elite category. Again, this relationship didn't exist for men.

Why should women's careers be affected by when they are born in the school year while men's careers aren't? Research by the Institute for Fiscal Studies hints at one possible reason. In their research, the IFS found that compared with children born in September, those born in August are three times more likely to be regarded as below average by their teacher in reading, writing and maths; 20% more likely to study for vocational qualifications and 20% less likely to attend a leading university.

In general then, for women to make it into the C-suite perhaps they are having to prove themselves more than their male colleagues. Perhaps women need to excel in exams and attend a leading university, the sort of academic achievements that, statistically, someone born at the start of the school year is more likely to attain.

Indeed, if you look at the universities attended by male and female board members of the FTSE 100, overall the female board members attend slightly better universities than their male colleagues. We compiled a list of FTSE 100 board members and noted the universities listed on their online profiles in sources such as LinkedIn, Forbes and Bloomberg's Business Week. We then ranked the universities using the The Times Higher Education World University Rankings.

The mean average worldwide ranking for universities attended by female FTSE 100 board members was 70, for male board members it was 74. We also looked at the median rankings of universities attended by female and male board members. For women, the median university ranking was 16, for men it was 23. Both are small differences, but they do suggest that women are having to do better at school than men in order to land top-tier jobs.

If this is the case for FTSE 100 board members then, combined with our data on median earnings, it's not a huge leap of the imagination to think that this is happening for other, less senior roles too. What our research doesn't show is the ultimate cause of these trends. Perhaps women are more likely to avoid starting a family if they're an academic high flyer, or perhaps women are being actively discriminated against unless they display exceptional academic achievements. Whatever the ultimate cause, if there is a bias in the workplace against women born in spring and summer months it's surely a huge waste of talent.

Paul Beaumont is the Managing Director of Octopus HR