The standard assumption is that women's lives have dramatically improved over the last 50 years. They have considerably more personal freedom; and opportunities for education and employment have been transformed. As a result they have much greater financial independence, which has given them more power to shape their lives. So far, so easy.

But something odd is going on that no one can explain. These huge social changes are not making women happier, and, according to several significant studies, women's happiness relative to men's has declined in the last 25 years. This includes women of all age groups, and it is evident in many countries, particularly in the US and the UK.