Literature is a form of expression whose temporal structure, both in content and style, provides a historical record of the evolution of culture. In this work we take on a quantitative analysis of literary style and conduct the first large-scale temporal stylometric study of literature by using the vast holdings in the Project Gutenberg Digital Library corpus. We find temporal stylistic localization among authors through the analysis of the similarity structure in feature vectors derived from content-free word usage, nonhomogeneous decay rates of stylistic influence, and an accelerating rate of decay of influence among modern authors. Within a given time period we also find evidence for stylistic coherence with a given literary topic, such that writers in different fields adopt different literary styles. This study gives quantitative support to the notion of a literary "style of a time" with a strong trend toward increasingly contemporaneous stylistic influence. cultural evolution | stylometry | culture | complexity | big data W ritten works, or literature, provide one of the great bodies of cultural artifacts. The analysis of literature typically involves the aggregation of information on several levels, ranging from words to sentences and even larger scale properties of temporal narratives such as structure, plot, and the use of irony and metaphor (1-3). Quantitative methods have long been applied to literature, most notably in the analysis of style, which can be traced back to a comment by the mathematician Augustus de Morgan regarding the attribution of the Pauline epistles (4) and the late nineteenth century work of the historian of philosophy Wincenty Lutasłowski, who brought basic statistical ideas of word usage to the problem of dating the dialogues of Plato (5). It was Lutasłowski who coined the word "stylometry" to describe such an approach to investigating questions of literary style. Since then, a wide range of statistical techniques have been developed toward this end (6), generally with the goal of settling questions of author attribution (see, e.g., refs. 6-11). Stylometric studies have also been pursued in the study of visual art (12, 13) and music [both in composition (14-16) and performance (17)], and are part of a growing body of work in the quantitative analysis of cultural artifacts (18). In this paper we report our findings from the first large-scale stylometric analysis of literature. The goal of this work is not author attribution-for the authorship of all the works is well known-but is instead to articulate, in a quantitative fashion, large-scale temporal trends in literary (i.e., writing) style. This type of study has been, until now, impossible to undertake, but the advent of mass digitization has created dramatic new opportunities for scholarly studies in literature as well as in other disciplines (19). Our literature sample is obtained from the Project Gutenberg Digital Library (http://www.gutenberg.org/wiki/ Gutenberg:About). Project Gutenberg consists of more than 30,000 public domain texts, music, audiobooks, etc., is freely available online, and is among the digital archives that have become, over the past 60 yr, crucial components of the preservation of cultural artifacts (18). In scope, our work is related to, but quite different from, recent studies in the dating of literary works (20), the analysis of the coarse-grained structure of literary history (and the evolution of genre) (21), and most notably, a recent analysis of Google Books (22), wherein the temporal trends in content-word usage were articulated. Content words also form the basis of the various topic model analyses that have been applied to large corpuses of science texts (see, e.g., ref. 23). In contrast, the work presented here focuses on the usage of content-free words as the basis of the first large-scale study of the similarity structure of literary style. Content-free words are the "syntactic glue" of a language: They are words that carry little meaning on their own but form the bridge between words that convey meaning. Their joint frequency of usage is known to provide a useful stylistic fingerprint for authorship (8, 11) , and thus suggests a method of comparing author styles. When we consider content-free word frequencies from a large number of authors and works over a long period of time, we can ask questions related to temporal trends in similarity. The primary results of our analysis are that time provides the most coherent means of clustering work and a trend of diminishing stylistic influence as we move forward in time. Such a finding is consistent with a simple evolutionary model for stylistic influence, which assumes that imitation attends preferentially to contemporary authors. In addition, we uncover quantitative support of the previously purely anecdotal notion of a literary "style of a time." Taken together, these two findings suggest the utility and perhaps the creation of a new field of stylometric analysis in culturomics. In our experiments, we studied a subset of the authors in the Project Gutenberg database composed of those who wrote after the year 1550, had at least five works in English in the Project Gutenberg collection, and for whom we had birth and death date information. This left us with 537 authors. For each author, we created a representative feature vector by aggregating the content-free word frequencies for each individual work by that author. In total, we analyzed 7,733 works. In our experiments, we used a list of 307 content-free words that included prepositions, articles, conjunctions, "to be" verbs, and some common nouns and pronouns (see Table S1 for a complete list). We did not attempt to semantically disambiguate between occurrences of homographs in situ (e.g., when using "to" as a preposition or as an indicator of an infinitive verb). Doing so would require a sophisticated grammatical model, and it was not our aim to model this particular aspect of word usage. We believe that ignoring these distinctions is not likely to greatly affect our results, because words that account for the greatest differences in usage frequency among Author contributions:
Another related study is the one by REF that investigates the evolution of the style of 537 authors of the Project Gutenberg collection by looking at the usage of grammatical words.
Quantitative patterns of stylistic influence in the evolution of literature
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
Medicine Art