Sexism is not the only reason men earn more than women

Exciting research from Cambridge last year suggested that making money was all about testosterone. Or at least, it showed that City dealers who took big risks and earned high profits had higher levels of testosterone at the end of their working day than at the start.

If there's something alpha male about making money, that would be one way of accounting for the gender pay gap in the City. According to a study commissioned by the equalities watchdog, men in finance take home an average bonus five times that of their female counterparts. In terms of basic pay, women earn 39 per cent less than men.

The other, more plausible, way of accounting for the gap, according to Harriet Harman, the minister for women and equality who commissioned the study, is sexism. Too many all-male company boards being too unsympathetic to family-friendly working hours. Ergo, creating a culture inimical to women.

That may be true. Women may not be being discriminated against as women, more as actual or potential mothers, with a tendency to go for part-time work and take school holidays off. Though outright sexism shouldn't be discounted: I fondly recall the phone call I got from a partner of Cazenove, the stockbroker, when I got a job with them. "Our normal candidates, Miss McDonagh," he said, "have backgrounds that are Army, Navy, public-school and male."

But it's a mistake to take an undifferentiated approach to City pay. Some jobs simply attract higher rewards than others and they're not always the ones that attract women. Bonuses are, at best, a reward for intelligent risk-taking (although, post-credit crunch, that sounds like a contradiction in terms.) If an investment banker or a fund manager makes more money over the year, it's reflected in his bonus, in cash or stocks. That's not gender, it's a matter of performance. Same goes for traders.

And plainly, there are jobs – heads of department, directors of financial institutions – that simply don't lend themselves to job-sharing and family‑friendly hours, though it helps that women who earn as much as Nicola Horlick can afford a couple of nannies.

Women may be deciding to pass on the jobs with unsociable hours and high pressure in favour of seeing their children. The trouble is, that way they also miss out on the rewards. The pay gap may, in part, be a matter of women's free choices. That makes it harder to resolve.