Bluematter.

For background, read part 1.

Now, picture two companies competing with each other. They are identical in every respect, other than the gender of their staff. The first one (Girl Co.) employs only women, while the other prefers men (predictably, the founders went for Guy Co.)

When joining their respective companies' graduate schemes, guys and girls have equivalent levels of IQ, education and charisma. Guy Co. and Girl Co. are reputable, law abiding companies, so they pay the same salary for the same job. Being identical in every respect, they have the same costs of production. Competition is stiff.

A few years pass. The guys get married to the girls and they decide to have children. Guy Co. barely notices: its employees go away for a few weeks after each child is born, and perhaps have the odd day off every once in a while thereafter to cater for measles, chickenpox and the like.

At Girl Co., however, things look decisively bleak. One by one, its employees give birth and go away for months on end, while Girl Co. still has an obligation to pay them a large fraction of their salary. Temporary replacements are hired, causing the company's payroll and training costs to go up dramatically.

But that's the least of Girl Co.'s worries. Many of its star employees never come back, choosing instead to stay at home or look for a less onerous job. Those that do return tend to leave earlier than they used to, and showing up for that crucial meeting depends on whether the antibiotics they gave to their baby the night before proved to be as effective as it said on the package.

To cut a long story short, Girl Co. goes bust while Guy Co. ends up dominating the market. From the point of view of the employer, hiring a girl is that much riskier than getting a guy with similar qualifications, training her or promoting her to a business-critical position that will allow her to eventually reach the top even more so.

The real trouble with women is that they have a visible characteristic (their gender) that is negatively correlated with life-time productivity. It's not that there aren't many women that are as career oriented as the most driven of men. On average, however, women tend to not be as good an investment for an employer as men are - and it's impossible to tell who the 'committed' ones will turn out to be. By the time it all becomes clear, most female high-flyers will have lost valuable time in second-rate jobs, without the experience and contacts that will secure them a job running the world and a fat pay package to go with it. Add to that the calculus of household welfare maximisation, and there you have it.

Unfortunately, the saying that a woman has to be twice as good as a man to succeed is all too true. Blame culture, history and the tax system - but keep in mind that these would account to nothing were it not for your less eager sisters.