Why Have Sentence Lengths Decreased?

There is a relatively new, practical reason to write short sentences: they are less likely to be mangled by automated translation software. Sentences often become long via multiple clauses. Automated translators can mangle such sentences by (for example) mistakenly applying words to the incorrect clause. If you split such sentences, you make such translations more reliable. Most of our writing now potentially has global reach. So you can be understood by more people if you meet translation software half-way.

What is a sentence anyway... is there something special about a period, as opposed to other punctuation marks? Many are available: the colon is a possibility; also its half-brother; and the comma,of course...also the ellipsis -- even the mighty m-dash!

At first, I thought this post would be about prison sentences.

I got curious and checked if DeepResearch would have anything to add. It agreed with your post and largely outlined the same categories (plus a few that you didn't cover because you were focused on an earlier time than the screen era): "Cognitive Load & Comprehension, Mass Literacy & Broad Audiences, Journalism & Telegraphic Brevity, Attention Span & Media Competition, Digital Communication & Screen Reading, Educational & Stylistic Norms". 

The last one I thought was interesting and not obvious from your post:

• Widespread literacy also had an effect on social norms. It wasn't just that sentences got shorter to accommodate the average reader, but also that it became more socially expected that writers accommodate the reader rather than the reader being expected to live up to the elite demands. This was partially connected to the rise of compulsory schooling. Once you're demanding that everyone learn to read, you kind of have to accommodate the limits of their abilities rather than just telling them "get good or gtfo".
  • DR: More people could read, but to reach this broader audience, authors were compelled to write in a plainer style than the ornate constructions of previous centuries. We can view this as a shift in the social contract of writing: instead of readers straining to meet the text, the text was adjusted to meet the readers. Shorter sentences were a key part of that adjustment. [...] By the early 20th century, the norm had shifted – long-winded sentences were increasingly seen as bad style or poor communication, out of step with a society that valued accessibility.
  • (This claim seems like it matches common sense, though DR didn't give me a cite for this specific bit so I'm unsure what it's based on.)
• DR also claimed that there was a "Plain Language movement" in the 1960s and 1970s, that among other things pushed for simpler sentences. Its only cite was to a blog article on readability.com, though Wikipedia also talks about it. You mentioned e.g. the Flesh-Kincaid formula in a descriptive sense, but it's also prescriptive: once these kinds of formulas get popularized as respected measures of readability, it stands to reason that their existence would also drive sentence lengths down.
  • E.g. Wikipedia mentions that Pennsylvania was the first U.S. state to require that automobile insurance policies be written at no higher than a ninth-grade level (14–15 years of age) of reading difficulty, as measured by the F–K formula. This is now a common requirement in many other states and for other legal documents such as insurance policies.

There were a few other claims that seemed interesting at first but then turned to be hallucinated. Caveat deep researchor.

Related: https://www.lesswrong.com/posts/Pweg9xpKknkNwN8Fx/have-attention-spans-been-declining [LW · GW] 

Another related thing is that the grammar of languages appears to be getting simpler with time. Compare the grammar of Latin to that of modern French or Spanish. Or maybe not quite simpler but more structured/regular/principled, as something like the latter has been reproduced experimentally https://royalsocietypublishing.org/doi/10.1098/rspb.2019.1262 (to the extent that this paper's findings generalize to natural language evolution).

Interestingly, breaking up long sentences into shorter ones by replacing a transitional word with a period does not quite capture the same nuance as the original. Here's a translation of Boccaccio, and a version where I add a period in the middle.

Wherefore, as it falls to me to lead the way in this your enterprise of storytelling, I intend to begin with one of His wondrous works, that, by hearing thereof, our hopes in Him, in whom is no change, may be established, and His name be by us forever lauded.

Wherefore, as it falls to me to lead the way in this your enterprise of storytelling, I intend to begin with one of His wondrous works. By hearing thereof, our hopes in Him, in whom is no change, may be established, and His name be by us forever lauded.

By replacing ", that," with a period, my revision completely changes our relationship with the narrator. In the original translation, the narrator is both announcing his goal and describing what he plans to do to achieve it.

In the revised version, he's describing his plan of action and a potential effect of that plan. We might assume that he's choosing that plan in order to bring about that effect, but it's no longer explicit in the text. Each sentence stands on its own. It's up to the reader to perceive the narrator's intention.

I wonder if inserting periods systematically tends to disrupt explicit links between intention and action. If so, perhaps the shortening of sentences reflects the anomie of the modern era, the gradual decay of an explicit moral framework in the stories we tell.

In China, there was a parallel, but more abrupt change from Classical Chinese writing (very terse and literary), to vernacular writing (similar to speaking language and easier to understand). I attribute this to Classical Chinese being better for signaling intelligence [LW(p) · GW(p)], vernacular Chinese being better for practical communications, higher usefulness/demand for practical communications, and new alternative avenues for intelligence signaling (e.g., math, science). These shifts also seem to be an additional explanation for decreasing sentence lengths in English.

Many short sentences can add up to a very long text. The cost of paper, ink, typesetting and distribution would incentivize using fewer letters, but not shorter sentences.

Humans didn't always speak in 50-word sentences. If you want to figure out how we came to be trending away from that, you should try to figure out how, when, and why that became normal in the first place. 

Literacy seems to make sense to me but I might be missing something in the post. Writing is language and language is communication so at least two sides.

As more people learned to read, they also learned to write, and written communications increases. However, even with modest literacy one can read a long sentence. Or can do that when it is written by a good/skilled writer. But being able to read does not really lead to writing skills in most cases I suspect.

As more people started communicating via writing (think things like schools and education expansion) the skill level of the average writer likely declined. That probably lead to training next generation writes to write in a more simple sentence structure.

This may be because editing has become easier and faster to iterate.

It's comparatively easy to identify sentences that are too long. Is it easy to identify sentences that are too short? You can always add an additional sentence, but finding examples where sentences themselves should be longer is much harder. With more editing cycles, this leads to shorter and shorter sentences.

I suggest additional explanation.

The bigger the audience is, the more people there are who won't know a specific idea/concept/word (xkcd's comic #1053 "Ten Thousand" captures this quite succinctly), so you'll simply have to shorten.

I took logarithm of sentence length and linearly fitted it against logarithm of world population (that shouldn't really be precise since authors presumably mostly cared about their society, but that would be more time-expensive to check).

shorter sentences are better because they communicate more clearly. i used to speak in much longer and more abstract sentences, which made it harder to understand me. i think using shorter and clearer sentences has been obviously net positive for me. it even makes my thinking clearer, because you need to really deeply understand something to explain it simply.