A summary from YodasEvilTwin on Slashdot:
"The internet is dominated by sexist men, which discourages women from getting involved in related fields."
I add a bunch more caveats, references and empirical data, but that is a good summary of how I interpret the evidence.
There is currently a responsibility-dodging contest between industry and academia over who is to blame for the declining enrollment of women in Computer Science and declining employment of women in software development. I hear people in industry bemoan the "empty pipeline", while academics maintain that women aren't entering their programs because of perceptions of the industry. I have compiled some data that may help resolve the question by highlighting a third factor common to both: access to an Internet-based culture of computing.
Two recent South American studies found that students who had not had Internet access were more likely to major in Computer Science than those with previous experience  They also found that the effect was more pronounced among young women than young men.
I put forward a related inflection point in the classic graph of CS enrollment in the US over time :
Access to personal computers increased both men and women’s enrollment proportionally, whereas rising access to the Internet raised men’s enrollment significantly more than women’s enrollment and women’s enrollment began to decline before men’s peaked. Access to non-social technology led to equal rises in men and women enrollment, whereas access to the Internet increased the gap in enrollment. It becomes even more apparent if we consider that at the same time women’s enrollment was rising, so the percentage of all degrees awarded to women awarded in computer science rose less and fell further than the total number of women studying computer science:
It appears to me that the introduction of the Internet compounded the pre-existing gap in American enrollment.
While often the gender problem in Computer Science is lumped together with the gender problem in Engineering, the enrollment pattern for Engineering in the United States looks very different. Thus I am highly sceptical of research that conflates the two fields, and I will be focusing exclusively on Computer Science.
(Note: though this chart begins at 1970, explicit discrimination was not outlawed until 1972 and almost certainly contributed to very low initial percentages.)
Here I examine the predictive power of Internet access to explain international variations in women's enrollment in computer science.
For these I will compare women’s representation in Computer Science to women’s representation among general college enrollment, in order to capture how women’s enrollment in Computer Science differs from other fields. This avoids confounding factors that may be shared by all college-educated professions such as family responsibilities or sexual harassment in order to focus on those contributing to the specific declines in CS.
Thanks to some data from Eurostat , I was able to produce cross-national comparisons between 30 nations. I use a normalized number: the percentage of all college graduates who are women minus the percentage of CS graduates who are women, divided by the percentage of all college graduates who are women. Thus, 0 is perfect equality and 1 is perfect inequality. Here is the data for the 30 countries available:
The data most widely and relevantly available regarding Internet use was the percentage of the population who had never accessed the Internet in 2006. This is the time frame where the women above would have been choosing their field. Again, the data for those 30 countries:
I then graphed access to the Internet versus the variation from expected participation. I found a statistically-significant negative correlation:
Among these nations there are five Scandinavian countries that have had 100% internet access among young people for over a decade and very high internet access among the general population since 2004. This is fundamentally different than the conditions in all other nations, where near-universal internet access, if it has been achieved at all, occurred less than a year before these data points.
If we exclude those nations that have hit that ceiling, we get an extremely statistically significant correlation:
Among the non-Scandinavian group, 39% of the total variation goes away if we control for Internet access. We would expect the primary difference of women’s enrollment in the field to be national culturally-specific characteristics ; that there is a significant shared mediating factor is notable.
In addition, 23 countries had sufficient longitudinal data to do a temporal analysis. Of those 23 countries, 5 had a positive correlation between rising Internet access and the parity of women’s enrollment in CS relative to other fields, all around the Mediterranean. 3 nations had no significant correlation. 1 had a weak negative correlation and 14, or 61%, had a strong negative correlation:
I conclude that in the last 10 years among many Northern European nations, rising Internet access is correlated with falling interest in computer science relative to other professions among women. The group of Mediterranean nations that show a positive correlation should be a fruitful area for future research, but seem outliers from the Northern cohort.
Given that the same dynamic has be observed at an individual level, changes in enrollment over time at a national level and in cross-national analysis, a causal relationship is plausible, though not proven. Notably, I do not have any longitudinal data prior to the introduction of the Internet for these countries to compare the post-Internet trends to and I have found no studies from other nations comparing individual-level Internet use with interest in Computer Science.
However I will propose two possible causal mechanisms that are supported by current evidence and I believe warrant further research.
I would caution against considering this support for any hypothesis relying on generalizations about the characteristics of women as a group, as the literature does not support gender as a distinguishing factor in career interest, skill or motivation . Instead, the evidence suggests we should be looking to cultural factors amplified by the Internet that affect those identified by others as women, since the one thing women have in common is being perceived as female by members of the community.
A global technical culture was noted by scholars as early as 1988  and has since migrated online . In many ways it has been one of the strengths of the field, but it may also be amplifying and spreading the alienating and exclusionary aspects of the current occupational culture.
The first hypothesis I propose is that Internet culture supports a belief in a meritocratic environment , which has been linked, ironically, to an increase in biased behavior  as it provides moral cover for prejudiced beliefs. Encountering overt, covert or benevolent sexism undermines both women’s performance and interest . Even if such beliefs were prevalent in professional spaces before the Internet, as masculine gender performance is common, aggressive and publicly visible in online forums  women no longer have to be the target of such behavior themselves before college in order to associate it with the industry and choose an alternative career.
The second hypothesis is that the Internet encourages a sense of belonging  to the masculinized culture of software development , which alienates many women  by causing them to feel excluded from a camaraderie-focused profession . Again, while this culture may have existed before the Internet, women with Internet access are likely to encounter such attitudes earlier and more frequently. To the best of my knowledge, whether the Internet has changed the culture of computing itself, either in America or internationally, is an outstanding question.
The one thing this correlation does imply is that programs to expose girls and young women to technology wouldn't be effective atboost the number of women entering software development. In fact, they might be counterproductive if they lead to earlier encounters with the existing occupational culture. Basically, free samples only boost sales of products people find enjoyable and we are therefore unlikely to solve these problems by writing off the experiences of the current generation and placing all our chips on the next.
It is much more controversial to say “we must change our culture” than “little girls should get to play with robots”, especially since many current programmers identify with that culture strongly . Yet, if women aren’t becoming programmers because the occupational culture is unappealing, cultural change is the only solution that would lead to integration. The fears among those of us who do identify with the culture may also be unfounded: the aspects that are alienating are not necessarily central to the culture as a whole.
There is a second option: those programmers who do not wish to participate in the current exclusionary culture may spin off and create their own spaces with which the alienating members of the prior culture no longer identify. Unfortunately, the recent push I have seen towards such an alternative culture is even more sexist than what came before . On the other hand, new fields such as Biological Computation, Analytical Mathematics and Library and Information Sciences involve software development in different cultural contexts, producing vastly different gender balances . By respecting, celebrating and promoting such fields we may be able to create a more inclusive discipline without significantly altering current norms of Internet behavior.
Of course, such respect for other fields would, itself, be a significant cultural change .
Where ever possible I have included a URL to the full text of the article or data set.
L.F. Seymour, M. Hart, P. Haralambous, T. Natha, and C. Weng, "Inclination of scholars to major in information systems or computer science", South African Computer Journal, 2005, pp.97-106.
Jacobs, C. and Sewry, D.A. Learner Inclinations to Study Computer Science or Information Systems at Tertiary Level. South African Computer Journal, 2009.
This data is from the Integrated Post-Secondary Education Data System, available from: http://nces.ed.gov/
All the data used to create these charts is available from: http://epp.eurostat.ec.europa.eu
Trauth, E.M., Quesenberry, J.L., and Huang, H. “A Multicultural Analysis of Factors Influencing Career Choice for Women in the Information Technology Workforce,” Journal of Global Information Management (16:4), 2008a, pp. 1-23.
Gerpott, T. J., Domsch, M. and Keller, R. T. (1988), CAREER ORIENTATIONS IN DIFFERENT COUNTRIES AND COMPANIES: AN EMPIRICAL INVESTIGATION OF WEST GERMAN, BRITISH AND US INDUSTRIAL R&D PROFESSIONALS. Journal of Management Studies, 25: 439–462. doi: 10.1111/j.1467-6486.1988.tb00709.x
Turner, Fred. "Cyberspace as the new frontier? Mapping the shifting boundaries of the network society." June 6. 1999. Red Rock Eater News Service.
Also, a case study from Brazil:
Takhteyev, Y. Jeeks: Developers at the Periphery of the Software World, the Annual Meeting of the American Sociological Association, New York, NY, August 10-17, 2007.
Quesenberry, J.L. and Trauth, E.M. (2008) “Revisiting career path assumptions: the case of women in the IT workforce” Proceedings of the International Conference on Information Systems (Paris, France, December).
Hannemyr, Gisle. "Technology and pleasure: Considering hacking constructive" First Monday [Online], Volume 4 Number 2 (1 February 1999) http://www.firstmonday.org/htbin/cgiwrap/bin/ojs/index.php/fm/article/view/647/562
Also, for a historic instance in the creation of culture, see:
Turner, F. (2005) ‘Where the Counterculture Met the New Economy. The WELL and
the Origins of Virtual Community’, Technology and Culture 46: 485–512.
And the manifestation of this in mainstream offline publication, Wired magazine:
White, Keith. The Killer App: Wired Magazine, Voice of the Corporate Revolution The Baffler 1994 -:6, 23-28
Castilla, Emilio J., and Stephen Benard. “The Paradox of Meritocracy in Organizations.” Administrative Science Quarterly 55 (2010): 543-576.
Logel C, Walton GM, Spencer SJ, Iserman EC, von Hippel W, Bell AE. Interacting with sexist men triggers social identity threat among female engineers. J Pers Soc Psychol. 2009 Jun;96(6):1089-103. Erratum in: J Pers Soc Psychol. 2009 Oct;97(4):578.
Marshall, Jonathan. "Online Life and Gender Vagueness and Impersonation."Encyclopedia of Gender and Information Technology. IGI Global, 2006. 932-938. Web. 4 Jun. 2012. doi:10.4018/978-1-59140-815-4.ch147
Herring, S. C. (1999). The rhetorical dynamics of gender harassment on-line. The Information Society, 15 (3), 151-167.
Herring, S. C. (1995). Men’s language on the Internet. Nordlyd, 23, 1-20. (Proceedings of the 2nd Nordic Language and Gender Conference, November 2-4, 1994.) http://ella.slis.indiana.edu/~herring/men.1995.pdf
Ren, Y., Harper, F. M., Drenner, S., Terveen, L., Kiesler, S., Riedl, J., et al. (Under review).
Increasing Attachment to Online Communities: Designing from Theory. MIS Quarterly.
Trauth, Eileen M.; Joshi, K. D.; Kvasny, Lynette; Chong, Jing; Kulturel, Sadan; and Mahar, Jan, "Millennials and Masculinity: A Shifting Tide of Gender Typing of ICT?" (2010). AMCIS 2010 Proceedings. Paper 73. http://aisel.aisnet.org/amcis2010/73
Current, taking an intersectional approach:
Trauth, E., Cain, C., Joshi, K. D., Kvasny, L. and Booth, K. (2012). "The Future of Gender and IT Research: Embracing Intersectionality", Proceedings of the ACM SIGMIS/CPR Conference, Milwaukee, WI, May 31- June 2.
Historic, and how it came to be this way:
Haigh, Thom. Masculinity in the History of Computing(s). May, 2008. CBI Workshop on History.
Ensmenger, Nathan. The Computer Boys Take Over
a short version of which is available for free here: http://www.ischool.utexas.edu/~nathan/files/ensmenger-gender.pdf
Sapna Cheryan, Andrew N. Meltzoff, Saenam Kim, Classrooms matter: The design of virtual classrooms influences gender disparities in computer science classes, Computers & Education, Volume 57, Issue 2, September 2011, Pages 1825-1835, ISSN 0360-1315, 10.1016/j.compedu.2011.02.004.
Side note: it is interesting how much more diverse actual science fiction & fantasy (48% WorldCon attendence), World of Warcraft subscriptions (40%), video games (33%) and Star Trek fan sites (34-48%) are than computer science itself. This may explain the presence of women in software development who also identify strongly with the culture, despite others viewing it as highly masculine.
Jane G. Stout Nilanjana Dasgupta. When He Doesn’t Mean You: Gender-Exclusive Language as Ostracism Pers Soc Psychol Bull June 2011 37: 757-769, doi:10.1177/0146167211406434
Case, P. and Pineiro, E. (2009) Stop whining, start doing! Identity conﬂict in project managed software environments. ephemera, 9
(2). pp. 93-112. ISSN 1473-2866
For some reason this hasn't made it into an academic journal yet, but Business Weekly has a piece on brogramming:
Wendy Cukier, Denise Shortt, and Irene Devine. 2002. Gender and information technology: implications of definitions. SIGCSE Bull. 34, 4 (December 2002), 142-148. DOI=10.1145/820127.820188
Indira R. Guzman, Jeffrey M. Stanton, Kathryn R. Stam, Vibha Vijayasri, Isabelle Yamodo, Nasriah Zakaria, and Cavinda Caldera. 2004. A qualitative study of the occupational subculture of information systems employees in organizations. In Proceedings of the 2004 SIGMIS conference on Computer personnel research: Careers, culture, and ethics in a networked environment (SIGMIS CPR '04). ACM, New York, NY, USA, 74-80. DOI=10.1145/982372.982388 http://doi.acm.org/10.1145/982372.982388
Full text unfortunately behind a pay wall.