Women are paid less than men. That's a fact. The role of discrimination in that situation is harder to analyse.
Women's median pay in OECD countries in 2010 was 15 per cent lower than men's, according to the PwC Women in Work Index. That's down from 19 per cent in 2000, so the trend is clear. But countries differ greatly - the difference is six per cent in Poland and 39 per cent in South Korea. Almost everywhere, women are also more likely to be registered as unemployed, and less likely to hold full-time jobs.
Indisputable discrimination - you receive less pay simply because you are female - undoubtedly explains some of the difference. In many developing countries, this may be the predominant factor.
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But almost all countries now prohibit gender-based pay scales, and in most of the richest economies, those laws are actually enforced. Still, the pay gap persists. One reason may be that men are simply and unfairly favoured for promotions - the old boys' club reflex.
Other, more complicated factors are also at play. Many women in developed economies seem to choose relatively low-paid professions, for example in caring work. The lower pay, though, may reflect men's social judgment that women's work is worth less. The easiest way to eradicate this source of lower women's pay is to work on those social prejudices. So far there are few signs of a major shift, but it would be relatively easy to adjust pay scales.
Personal judgments and behaviour is harder to change. Women may work in part-time and lower-stress jobs because they devote more time and energy to family responsibilities. That preference reflects traditional views of gender differences, which are evolving slowly. The result is that women's average pay will be lower than men's as long as a significant number of women want, or are forced, to favour unpaid family labour over paid jobs.
The economic gap between women and men has narrowed dramatically since International Women's Day was created in 1911. In some countries it may have narrowed enough that the simple pay gap is no longer an accurate indicator of injustice. In most others, overcoming the remaining discriminations is a work in progress.