Under-representation of women in science subjects is a global phenomenon no matter what the country's level of development, according to Unesco's third World Education Report.
Girls in many regions are staying longer in education than boys, however, particulary in higher education. This is the first time any agency has detected this as an international trend, although the United Kingdom, France and Canada had spotted it in their own systems.
The report, published in conjunction with Oxford University Press, gives a gender breakdown in its international education statistics. The university "science barrier" is analysed by students' gender and study areas in more than 100 countries in 1992.
There is a big variation in those countries' total percentage of women students. But in every country the percentage of women in natural sciences, engineering and agriculture is lower than female share of total enrolment. The opposite trend is apparent in the humanities with better gender balance in law and social sciences.
The findings on women and science subjects are so consistent that they appear to constitute an "iron law" of education, says the report.
Gender segregation increases as the percentage of women in higher education grows.
Unesco claims that high female participation in courses oriented towards commercial and service occupations between 1980 and 1992 has not necessarily disadvantaged women, because of the rapid growth of these sectors.
The report reviews theories for women's course choices, including economists' emphasis on the logic of comparative advantage and rational choice and educational psychologists' search for female and male "abilities".
It urges caution on both. Economic interpretations ignore discrimination and gender-biased psychological explanations which can confuse ability with performance.
Despite low enrolment rates for females, girls who go to school often do better than boys. This suggests that education systems are "fairer" to girls than are the economic, social and cultural conditions that limit their access in the first place.
Higher rates of female enrolment are to be found in most industrial countries, many Latin American, Caribbean and Gulf countries, as well as in Sri Lanka and the Philippines.