How long should sentences be in scientific manuscripts?

As a teacher and editor, I often find myself having to correct mistaken views about what constitutes good academic English writing. Many students and authors believe academic English should be complicated; sentences should be long, with difficult vocabulary and complex grammar. When I tell them the opposite is true, they don’t believe me. Surely I don’t expect them to write like children!

No, I don’t. In scientific writing, you can’t avoid technical vocabulary, but the author has control over the structure and length of each sentence. And sentence length is an important factor in determining the readability of a text. In a nutshell, the longer the sentences, the less readable the text. It’s easy to see this in the following contrived example.

The purpose of this study was to clarify the role of boolic acid in the production of globulum, one of the most important goobonic agents, which is responsible for maintaining proper boolic balance as well as healthy tooth enamel.

Even if words like “boolic acid” or “globulum” had meaning—which they don’t—the sentence would be hard to understand. It’s 39 words long. Dividing it into two sentences makes it more readable:

The purpose of this study was to clarify the role of boolic acid in the production of globulum. Globulum is one of the most important goobonic agents, maintaining proper boolic balance and healthy tooth enamel.

The first sentence has 18 words; the second has 17. I also got rid of “is responsible for” and “as well as,” two wordy phrases—a problem I wrote about in a previous blog. Perhaps now you have a better grasp of what the paper is about.

According to author Mike Markel, 15-20 words per sentence is ideal for most writing (see James Scott). On its website, Oxford University recommends that academic sentences be no longer than 25 words. Once a sentence exceeds the 25-word limit, readers struggle to remember the information that came earlier. When this happens, readers often give up and scan through a text, zooming in only on the information they need. No one wants to struggle through a difficult text.

And this is true now more than ever. The average length of English sentences has been decreasing over the centuries. Consider the first sentence from the American Declaration of Independence, written in 1776:

When in the Course of human Events, it becomes necessary for one People to dissolve the Political Bands which have connected them with another, and to assume among the Powers of the Earth, the separate and equal Station to which the Laws of Nature and of Nature’s God entitle them, a decent Respect to the Opinions of Mankind requires that they should declare the causes which impel them to the Separation.

That’s a whopping 75 words! Most likely, you had a hard time reading this sentence. I did too. When you eliminate the authors’ list of grievances against the king, the average sentence length of the entire Declaration is 54.5 words. I learned this by using a freely available online readability checker, txtcount.com.

Using the same checker, I found that the average sentence length of Watson and Crick’s famous article on the structure of DNA, published in 1953, was 18.2 words. A 2012 article by another Nobel Laureate, Shinya Yamanaka, had an average sentence length of 15.1 words. I found both articles highly readable, even with little understanding of the science involved.

Keep in mind that average sentence length matters most. Some sentences will naturally be a little long, and some may be short. But if sentence after sentence is too long, readers tune out.

I’d advise authors of scientific manuscripts to use a readability checker to see how readable their sentences are, and to make adjustments if necessary. Some of these tools, like the Hemingway Editor, even identify which sentences are most difficult to read. This doesn’t take much time and can help you greatly improve your writing. Most importantly, this will improve your chances of getting published.

By the way, this article has an average sentence length of 12.1 words. Was it readable? If you made it all the way to the end, that’s a good sign!