Two people in the same job, performing to the same standard, should be paid similarly. This is not a controversial statement.

Yet from nearly 6,000 New Zealand academics shows that even when women achieve the same research performance scores as men, women¡¯s odds of being ranked, and paid, as associate or full professor are half the odds of men with similar scores and age. These findings are important globally because we are the first to control for research performance, standardised across all fields, for every academic in a whole country.

We did it using New Zealand¡¯s Performance Based Research Fund (PBRF) mechanism, which scores everyone for research performance every six years: a sort of individualised version of the UK¡¯s research excellence framework. We found three gaps between academic men and women. First, not controlling for PBRF score, there is an average lifetime pay gap of NZ$400,000 (?200,000): about 80 per cent of the cost of an average house in our home town of Christchurch.

Second, there is a performance gap, in which women score an average of 50 points (out of a possible 700) lower on PBRF. And, most concerningly, there is an average lifetime ¡°performance pay gap¡± of NZ$200,000 (?100,000) between men and women with the same PBRF scores: 40 per cent of the cost of an average house. ?

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The literature on gender pay gaps is broad and deep, but our unique dataset has allowed us to dismiss some popular and persistent explanations of the gap.

The explanation is not babies. Few would argue that having babies improves women¡¯s research productivity, but even when?a woman attains a world-class A-grade in the PBRF, her odds of being an associate or full professor are still half those of an A-grade man. Babies might explain part of the 50-point performance gap, but not this ¡°performance pay gap¡± among people with the same performance rating.

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The explanation is not age, either. Age was a strong predictor of pay, but it explained neither the pay gap nor the performance pay gap. Men and women of the same age had significantly different ranks and salaries.

Some have looked for an explanation in women¡¯s preference for lower-paid fields, such as nursing and education, compared with men¡¯s tendency to cluster in engineering and medicine. But we find that even female-dominated fields like education have a performance pay gap.

It¡¯s unlikely to be just because women don¡¯t ask for promotions. Recent evidence suggests that times have changed; a 2018 study found that?.

And our findings are unlikely to be unique to New Zealand. We are simply the only country able to measure a performance pay gap to this level of detail. New Zealand has had a female prime minister for 14 of the past 25 years. Few batted an eyelid when Jacinda Ardern, our current prime minister, took maternity leave, or when her new baby accompanied her to New York to ?address the United Nations General Assembly. Yet still we have a performance pay gap.

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That gap is unlikely, either, to be caused by women doing less teaching and administration. Indeed, others have that employers and students expect women to do more of the tasks for which they are less likely to get : , pastoral care and .

This brings us to our ¡°double-whammy effect¡± theory, according to which universities over-demand and under-reward women¡¯s teaching and administration. This higher burden might cause women to score lower on research performance while still failing to meet those expectations. But while this might partly explain both the performance and pay gaps, it doesn¡¯t explain the performance pay gap.

Universities should look at both hiring and promotions. Our study suggests that men tend to get hired at higher ranks. Starting at a lower salary compounds through a career. Promotion disparities are less clear, but the odds ratios described above suggest that promotions are not making up for women starting at a lower rung.

Universities should also look at the performance gap, which is the elephant in the room.

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A simple explanation, favoured by former Harvard University president Lawrence Summers, would be that men are at high-impact research. Another would be that .?A more subtle explanation is that academia might be a self-reinforcing system so that women can succeed only by playing by others¡¯ rules, rather than having the freedom to find their own route to excellence.

suggests that when people don¡¯t perceive their workplace to be a level playing field, they lose motivation to excel. When women feel they¡¯re working in an equitable meritocracy, they are just as ambitious as men, whether or not they have children. So perhaps fixing the performance pay gap will also narrow the performance gap.

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Ann Brower and Alex James are associate professors in the colleges of science and engineering respectively at the University of Canterbury, Christchurch, New Zealand.