When I was first asked to take part in a discussion about women in physics, sometime in the late 1970’s when I was a graduate student, it was generally thought that the dearth of women in science was due to women not studying these subjects at school and university. This was called a pipeline problem: women were not becoming scientists because there were none in the pipeline. By the 1990’s, people noticed that while the numbers of women studying science were increasing the numbers of women becoming scientists were not increasing proportionately. In particular, the number of academic scientists at any particular stage was less than would have been expected from the number of undergraduates the appropriate time earlier, see for example figure 2.5 of the ETAN Report Science Policies in the European Union: Promoting Excellence through Mainstreaming Gender Equality, European Commission, 2000, http://cordis.europa.eu/improving/women/documents.htm. Another study in Italy looked at researchers at the Italian National research Council who all started in the same year and found that after ten years 26% of men and 12.8% of women had been promoted to research director. Staying Competitive: Patching America’s Leaky Pipeline in the Sciences, Marc Goulden, Karie Frasch and Mary Ann Mason, 2009 (http://www.americanprogress.org/issues/2009/11/pdf/women_and_sciences.pdf) documents the preferential loss of women in academic science between receiving a Ph.D. and achieving tenure. This preferential loss of women scientists is called the leaky pipeline since women started out in the pipeline but were subsequently lost. Confusingly, the phrase is sometimes also used to describe any circumstances in which there are fewer women at higher levels in an organization compared to lower levels regardless of the reasons for the situation.
The American Institute of Physics published a report in 2005, Women in Physics and Astronomy, 2005, http://www.aip.org/statistics/trends/reports/women05.pdf, which concluded that, in Physics and Astronomy in the US, ‘women are represented at about the levels we would expect based on degree production in the past.’ This may reflect the fact that women needed to be exceptionally committed to go into physics in the first place, particularly in the period 1967-1980, when the average percentage of Ph.D. awarded to women by US Physics Departments was 4%. (How accurately would you be able to measure a decline from 4%?)
So, is there data demonstrating the ‘leaky pipeline’ in the sense of demonstrating the preferential loss of women from academic science? Yes, there is. Do data of the form women are 47.9% of lecturers but 18.7% of professors (HESA Press Release 131, http://www.hesa.ac.uk/index.php/content/view/1397/161/, figures are for 2007/08) demonstrate a leaky pipeline? Not by themselves: together with the information that in 1970/71 42% of all full-time undergraduates were women, and, more relevantly for potential professors, 23% of all full-time postgraduates students and by 1980/81 these figures were 41% and 34%, respectively [Source: Social Trends 2009, http://www.statistics.gov.uk/downloads/theme_social/Social_Trends39/ST39_Ch03.pdf], they do, though perhaps not so dramatically as the contrast between 47.9% and 18.7% suggests.