Education and job experience used to explain much of the gender wage gap but many more women now graduate from university than men and fewer well-educated women drop out of the workforce for long periods.
At least 65-80 per cent of the gap is now not explained by any specific factor, as research published last week by Deputy Prime Minister Bennett showed.
Bennett attributed much of this unexplained portion of the gender wage gap to unconscious bias against women in hiring and promotion. A much better explanation is in the work of the world's top female labour economist, Claudia Goldin.
Goldin explored how the strict time demands of professional jobs drove much of what remains of the gender wage gap.
Goldin pointed to legal and finance sector jobs that disproportionately reward workers who were willing to work long hours, rigid hours and show up whenever the client wants them.
These strict time demands drove the 20-plus per cent plus wage gap for top professionals.
These rat-race jobs severely penalise even the shortest career interruption.
After starting on the same pay, large gender wage gaps emerge in high-powered professional occupations at least five years into a career.
Successful professionals are powering up to become partners or high-flyers by working at least 60 hours a week or more, with no time off for families.
This means some professional jobs can neither be part-time, shared or otherwise family friendly. These jobs do not allow a file to be handed over to a co-worker even for a short time except at great cost and delay.
Customers and co-workers expect to deal with the same person.
You want your lawyer, the same lawyer fully briefed, to show up in court on time, every time and be available to you whenever you are in trouble.
The legal system cannot work in any other way because of the possibility of urgent applications to court and the like.
The criminal justice system would be profoundly unjust if lawyers were not available 24/7 to their clients in custody. Some professional jobs cannot be made family friendly.
In jobs where it does not matter much who the customer is dealing with or who is doing a specific task within a team, women can excel.
Goldin found pharmacy to be the most family-friendly occupation. Pharmacists are completely interchangeable and in America most are employed by Walmart and other big companies.
Patients do not care who fills their prescription.
There are no special rewards for a pharmacist working long hours such as partnership track. Few need to risk self-employment to succeed. There is next to no gender pay gap for pharmacists in the States.
Employers are not to blame for the design of rat race jobs. High-powered professionals must work long hours, rigid hours, and be available for meetings because of the demands of their clients.
Women anticipate this rat race because, as an example, female surgeons tend to specialise in areas where they can schedule operations in advance rather than having to rush in to perform emergency surgery. But these 9-to-5 professional jobs pay less.
Adding to the brew is in an economy increasingly based on brain rather than brawn, more women than men will be in some professions.
This is because the OECD's PISA tests show women everywhere have vastly superior verbal and reading skills, an edge that is worth the equivalent of 6 to 12 months additional schooling.
Superior verbal and reading skills will lead women to favour interactive occupations such as law and medicine rather than science, technology, engineering and mathematics (STEM).
Why should women be asked to enter a STEM profession to even up the gender gap if that profession does not use their superior reading and verbal skills as much as law or medicine?
They would be giving up a natural edge over their less articulate male colleagues.
The last chapter of the gender wage gap is not driven by unconscious bias. It is mistaken for public policy to think so.