One explanation for the persistent differences is that men and women, except the most highly educated ones, often work in separate labour markets. Women are concentrated in teaching, health care, clerical work, social care and sales; they are underrepresented in manual and production jobs, maths, physics, science and engineering and in managerial jobs, particularly at the senior end. They are also much more concentrated than men in just a few job categories. Half the employed women in rich countries work in just 12 of the 110 main occupations listed by the International Labour Office (ILO). The jobs in which men work are spread far more widely, from construction workers to top managers.