Language Log
Mark Liberman
http://languagelog.ldc.upenn.edu/nll/?p=4126discussion of Male and Female Pronoun Use in U.S. Books Reflects Women's Status, 1900–2008 at
http://www.springerlink.com/content/l323p74567638354/As I observed in an earlier post, apparent historical trends of this kind in word frequencies or in ratios of word frequencies can have several qualitatively different sorts of explanations:
The mix of kinds of books published changes over time (e.g. more romance novels, fewer collections of sermons); different kinds of books use words differently; therefore the relative frequency of words changes.
The mix of kinds of books selected for the Google Books ngram collections changes over time; so the relative frequency of words changes, for similar reasons as in (1).
The distribution of concepts or conceptual frames changes over time, even in the same sorts of books.
The choice of words to express a given concept (in published books) changes over time, even in the same sorts of books.
David Brown ("Gender Pronouns in the News", Grammar Lab 8/12/2012) tries to address part of this uncertainty by calculating a similar ratio (of gendered pronouns) using data from an independent source, namely the Corpus of Historical American English. David looked only at he and she, rather than the full set of masculine and feminine third-person singular pronouns, but this turns out not to make much difference.