Readability and Complexity: Diachronic Evolution of Literary Language Across 9000 Novels (2023)

Abstract: Using a large corpus of English language novels from 1880 to 2000, we compare several textual features associated with literary quality, seeking to examine developments in literary language and narrative complexity through time. We show that while we find a correlation between the features, readability metrics are the only ones that exhibit a steady evolution, indicating that novels become easier to read through the 20th century but not simpler. We discuss the possibility of cultural selection as a factor and compare our findings with a subset of canonical works.

## introduction several textual features have been associated with "good style" or narrative in the stylometric and quantitative literature studies. a recent surge of quantitative studies has used large corpora to investigate whether intra-textual features correlate with perceived literary quality. average sentence length @xcite , type-token ratio @xcite , the distribution of parts of speech @xcite and level of redundancy @xcite have been shown to explain literary success partially. also measures of readability are often connected to literary success: it is a widespread conception of both readers and publishers that bestsellers are easier to read @xcite , and readability has recently gained traction in creative writing and publishing, such as in text-editing tools like the hemingway foot_0 or marlowe editors. 2 these applications evaluate texts with simple readability measures and tend to encourage the production of texts that are easier to read, assuming that more readable texts are better. with the evolution of quantitative methodologies, more sophisticated models of texts have also been explored as possible markers of literary quality: the shape and dynamics of novels' sentiment arcs as a way to approximate their narrative development or the complex way parts of speech alternate throughout a text, influencing readers' experience of the story above or below conscious perception @xcite @xcite . literary evolution may show a progressive convergence towards preferred forms of and styles in narrative, perceived as effective and maintained/further evolved through community feedback @xcite . already in the 19th century sherman (1893) observed an evolution of the language of fiction and suggested a positive selection for simple language in literary language. this idea recurs in a theory where the rise of a mass readership is thought to have prompted the language of western fiction to become simpler through the 19th and 20th centuries, as it caters to the progressively lower overall literacy and less spare time of the readership @xcite @xcite . in this study, we first extract multiple textual and stylometric measures connected to perceived literary quality or success from a large collection of english novels, ranging from the most surfacelevel readability indices to models considering the dynamics of their sentiment arcs. we examine whether any systematic, diachronic trend of these measures can be observed in the period covered by our corpus , as has been noted in theories of slow but continuous change in literary language or narrative style into the 20th century @xcite @xcite . secondly, considering readability as a measure linked to literary success or quality, we test the correlation between readability and other stylistic measures, as well as two measures with a higher level of abstraction that have previously been used to estimate narrative complexity in relation to reader appreciation: fractality and entropy. these measures are based on the sentiment arcs of novels, which are the sentiment scores (often extracted through dictionaries or machine learning) over the course of a whole novel. certain shapes or sentiment arc dynamics have been connected to reader appreciation, considering both simple and more complex narratives @xcite , and @xcite have shown that sentiment features, such as measures of sentiment arc progression, have an effect even compared to the predominantly stylistic features usually employed for this type of task @xcite . these more complex measures that take into account the sentiment-arc of novels are interesting insofar as they are not direct measures of style, and insofar as they have proven effective in approximating literary quality for different types of quality-standards that may reflect tastes of different reader communities: distinguishing higher-rating works on large user-platforms such as goodreads @xcite and telling works of nobel laureates from those of contemporary authors @xcite . finally, we estimate the same measures and diachronic trend for a subsection of the corpus defined through a combination of different "quality resources" chosen to reflect canonicity: authors most often appearing in english literature syllabi, major literary anthologies and titles defined as "classics" on the large user-platform goodreads. we show that while there is a correlation between surface-level and arc-based metrics, their change through time is significantly different, with readability metrics being time-dependent, and more sophisticated measures time-independent. ## results as we show in figure 4 and the clear trend of all readability measures indicates an overall simplification of the literary prose, beyond the characteristics of the authors' individual styles. interestingly, the trend can be observed for the corpus at large as well as for the "high prestige" subsection of titles we outline in the lack of lasting diachronic changes in the other two stylistic measures considered, type-token ratio and textual compressibility, seems to confirm this picture: if novels become easier in terms of basic readability metrics, they do not lose complexity at many other levels, not becoming overall more repetitive nor lexically poorer. in other words, it might be that there has been a large, overall tendency to favor texts that manage to simplify the most surface level aspect of style, without compromising their linguistic diversity nor their narrative arcs' complexity. ## discussion the different trends we have shown between surface-level readability measures, other metrics of style, and arc complexity through time seem to point towards a large-scale evolution of literary language towards prose that favors increased readability without compromising the novels' linguistic or narrative versatility. as we have seen in regarding the trends towards readability alone, it is reasonable to exclude that they are the effect of an overall change of the english language. similar tendencies towards simplification have been found in narrative @xcite but is not as obvious in other domains @xcite . moreover, scientific and journalistic prose has even shown an opposed trend, with texts becoming more difficult to read @xcite . if this trend is not an effect of language change, its presence in literature can give way to intriguing hypotheses. the emergence of what scholars have called mass readership @xcite and a widening of the alphabetized population might have pushed the success of easier books, 12 while the increasingly pressing market logic applied to the editorial world might have helped shaping literary style into simpler and simpler forms, easier to consume in a shorter time @xcite . it is also possible that in the last century, difficulty of reading has shifted from a virtue to a vice in the view of the english writing world, with novelists and publishers alike slowly favoring more direct or transparent prose. a central question to ask is whether we are seeing an actual transformation of literary prose, or whether we are witnessing an effect of cultural selection. in theory, there might have been no evolution at all through the 120 years in question, but less appreciated exemplars might have been progressively lost or overlooked. if texts are undergoing a constant process of selection, it is possible that the "books worth keeping" maintain more complex stylistic features, while the larger number of more easily readable novels is progressively forgotten, leading to the illusion of a historical change through survivor bias. a look at the absolute number and percentage of "canonic" books in our corpus, as defined by various indicators of prestige (see section 3.1), seems to point to this competing view: while the number of texts in the corpus increases with each decade, the percentage of canonic titles decreases drastically through time. however, when looking at the canonic subset alone, we see changes similar to those that we observe in the whole corpus: what we have defined 12 the us national reader survey in 1993 found that 48 percent of the adult population have difficulties reading above 5th-grade level texts @xcite as canonical literature has also become more readable from 1880 to 2000 ( ## conclusion and future works in our analysis of a curated corpus of 9000 englishlanguage novels published between 1880 and 2000, we employed specific readability metrics -i.a., the flesch-kincaid readability score -as well as complexity indices like the hurst exponent of sentiment arcs and classic stylometrics, i.a., type-token ratio and compressibility. our data indicates some clear trends: most readability scores have increased through time, displaying spearman correlations of up to 0.34 with the publication year, signifying a gradual simplification of the overall narrative language. in contrast, richness and complexity metrics remained relatively unchanged over the same period. these divergent trends might suggest that authors are increasingly focusing on making their works more accessible while maintaining a consistent level of narrative complexity and lexical diversity, which we might interpret as a literary strategy to engage a broader audience without sacrificing depth or complexity. it's worth noting that our study does not account for genre-specific trends and is based on available works, thus introducing potential selection bias. future research could expand upon these findings by exploring how these trends vary across different genres and cultures. naturally, exploring this further would require properly discussing and deploying a system of judgments for literary quality, an undertaking beyond the scope of this work. in the future, we would like to both conduct qualitative analyses to assess these results on individual work level and repeat the experiment on different and possibly larger literary data sets. we also plan to collect more textual features, such as model perplexity, as well as develop more sophisticated models for the sentiment analysis that underlies measures of arc dynamics (hurst, apen), such as using lms, and examine the change through time of these features with more sophisticated mathematical models.