Google offers much more information than a few years ago, Twenge notes, although the material is far from complete; the archive contains just 4 percent of all books published in the U.S. since 1800. But Twenge and her colleagues concluded that gender was not a factor in which books Google included.
"It seems very comprehensive and well done," Pennebaker said. "There are two types of data, imperfect data and no data. If you're going to wait around for perfect data, you are going to wait around forever."
Women wrote nine of the top 10 books on USA Today's best-seller list. According to the market researchers Simba Information, around 60 percent of those purchasing books are women.
The study confirms women's advances in education and in their publishing successes, said Erin Belieu, co-director of the nonprofit organization VIDA: Women in Literary Arts.
"Women have certainly increased their 'literary output' in the last two decades," she wrote in an email. "Women fiction writers specifically have been able to achieve a large economic impact within the publishing industry."
But more books by women does not mean more books are getting reviewed or more women getting to write for literary publications. For the past two years, VIDA has released studies showing that such magazines as The New Yorker devoted far more space to male writers than to women.
"Women as writers are much more likely to be ghettoized into marketing that wants to define who women are as writers and what it is women supposedly want to read," wrote Belieu, an associate professor in the English department of Florida State University. "This is true in literature as well as in contemporary journalism. Women authors being stuffed into YA and 'Chick Lit' publishing. Women journalists being assigned more 'personal' stories."
The continued prevalence of male writers/male reviewers is "very much the old guard hanging on, as they always do," Belieu adds. "But the progressive mind wins in the long ball game."