A small part can be explained by the so-called "motherhood gap" - the amount that women lose long term by taking time out from work to have children. Unsurprisingly this gap increases the longer a woman stays out of the workforce. It is difficult to resolve because in some cases women choose to sacrifice pay gains for what they and their partners regard as a more important family role. That may be changing as more men give up work to be full time fathers but overall the parenting debate is a red herring in the wider context of pay inequity.
Research for the Ministry of Women has calculated that 80 per cent of the pay gap is unexplained by time out for parenting or other factors, such as women choosing lower paid jobs than men. That leaves two main possibilities which are equally difficult to determine - either many women do not push hard enough for pay rises, as Retirement Commissioner Dianne Maxwell suggested this week, or many employers are either consciously or unconsciously biased against them.
Common sense suggests the two factors are working together. Most people do not consider themselves sexist or racist, but when faced with a big decision - such as who to choose for a promotion - they tend to play safe and pick people most like themselves.
That is probably why we have only two female chief executives leading our top 50 companies. It also explains why many women are so reluctant to put themselves forward, as they anticipate rejection, often correctly. The fact that Maori and Pacific women are also paid considerably less than Pakeha women suggests a rethink is long overdue.