ELLEN SPERTUS, a graduate student at M.I.T., wondered why the computer camp she had attended as a girl had a boy-girl ratio of six to one. And why were only 20 percent of computer science undergraduates at M.I.T. female? She published a 124-page paper, “Why Are There So Few Female Computer Scientists?”, that catalogued different cultural biases that discouraged girls and women from pursuing a career in the field. The year was 1991.

Computer science has changed considerably since then. Now, there are even fewer women entering the field. Why this is so remains a matter of dispute.

What’s particularly puzzling is that the explanations for under-representation of women that were assembled back in 1991 applied to all technical fields. Yet women have achieved broad parity with men in almost every other technical pursuit. When all science and engineering fields are considered, the percentage of bachelor’s degree recipients who are women has improved to 51 percent in 2004-5 from 39 percent in 1984-85, according to National Science Foundation surveys.

When one looks at computer science in particular, however, the proportion of women has been falling. In 2001-2, only 28 percent of all undergraduate degrees in computer science went to women. By 2004-5, the number had declined to only 22 percent. Data collected by the Computing Research Association showed even fewer women at research universities like M.I.T.: women accounted for only 12 percent of undergraduate degrees in computer science and engineering in the United States and Canada granted in 2006-7 by Ph.D.-granting institutions, down from 19 percent in 2001-2. Many computer science departments report that women now make up less than 10 percent of the newest undergraduates.