Megan McArdle has an interesting post on how to get women interested in science fiction. Some of her suggestions seem plausible. For example, she notes that the closely related fantasy genre is more popular with women than SF (although even fantasy probably has fewer female fans than male ones). However, overall I'm somewhat skeptical. Casual empiricism suggests that most people with a strong interest in science fiction or fantasy literature developed that interest very as children or teenagers. I think it's very difficult to persuade an adult to take an interest in these genres if they never had previously.
Somewhat surprisingly, Megan doesn't mention the most common explanation for the relative paucity of female SF fans: that the genre is mysogynistic and/or lacks strong, well-rounded female characters. Although this conventional view probably had some accuracy forty or fifty years ago, I doubt that it accounts for the gender gap in SF today. Over the last several decades, many left-wing and libertarian writers have entered the SF and fantasy fields, portraying women very differently than in the early days of the genre. And even those early days weren't quite as completely sexist as some think. Say what you will about Robert Heinlein's Starship Troopers, which has plenty of flaws; but it did portray women serving in combat units on an equal basis with men back in the 1950s. Today, there are even quite a few prominent explicitly feminist SF and fantasy writers, such as Marion Zimmer Bradley, Octavia Butler, and Ursula LeGuin.
Even more fundamentally, the sci fi genre - like other genres - responds to market incentives. If there were a large unment demand for feminist SF or other types of science fiction that may be of special interest to women, publishers and writers would have a strong incentive to meet it. The portrayal of women in science fiction has been debated for at least forty years, and publishers are certainly aware of the issue, and would act on it if they smelled profit. The underlying reality, I think, is that SF has less appeal to women than to men independently of the ideology of the writers and the way they portray female characters.
That said, recent data suggest that the gender imbalance in SF fandom may be overstated. This 2001 National Science Foundation surveyshows that 31% of men say they read science fiction books or magazines - a number statistically indistinguishable from the 28% of women who claim to do so.
The NSF's results are so contrary to conventional wisdom that I wonder if there's something wrong with the methodology. The most obvious potential flaw is that many of the women say they read SF only do so on rare occasions and aren't real fans of the genre, whereas the men read more often. However, the NSF did a follow-up question in which 17% of female SF readers say they do so "regularly" compared to 16% of the male ones. The NSF data do still suggest that SF has greater appeal to men than to women. Other studies reveal that women generally read far more than men do, especially in most fiction genres. So if men and women read SF at roughly equal rates, that suggests that the genre is of greater interest to men once you control for their generally lower propensity to read. The same is true of fantasy. Even the wildly popular Harry Potter series, which successfully reached beyond traditional genre readers, apparently has more male readers than female ones.
Even so, I remain skeptical about the NSF data. I wonder if the study simply suffers from random error (i.e. - even a methodologically sound poll will sometimes get an unrepresentative sample just by random chance). Are there more recent surveys that confirm the NSF result or reach different conclusions? Alternatively, if you are a survey research expert, can you point out methodological errors in the survey that I have missed?
UPDATE: Various commenters point out that women in Heinlein's Starship Troopers can serve in some types of combat units (e.g. - piloting starships), but not others (e.g. - not in the infantry). Fair enough, I had forgotten this detail. However, the idea that women should serve in any combat units was well in advance of mainstream opinion in 1959, when the book was published. For that matter, the US military does not allow women to serve in the infantry even today. The main point still stands: Heinlein was not entirely free of sexism (far from it, in fact). But he was more egalitarian than the conventional wisdom of his time.