Today marks Equal Pay Day, when based on the difference in pay between men and women, the latter are effectively working from free from now until the end of the year. It's an emotional topic, but what does science have to say about the whole issue?

A common argument for why the gender pay gap exists is that women tend to work in lower-paid occupations. But new in-depth research has found even when contributions to their employers are the same, women are still paid only 84 cents for every $1 men receive.

Why? Well, it's sexism, they say.