Why are double negatives so confusing in scientific writing?

Recently, I was editing a manuscript and found a sentence that reminded me of the importance of avoiding “double negatives” in scientific writing.

The patient had no abnormal blood glucose or insulin levels.

What are double negatives?

Double negative phrases are often used in informal spoken English, for example: "I didn’t do no cooking", "he never ate no food" and "they don’t know nothing".

In mathematics, two negatives always make one positive. Similarly, two “negative” words in a sentence are considered to cancel each other out to make one “positive”.

Therefore, "I didn’t do no cooking", "he never ate no food" and "they don’t know nothing" really mean "I did some cooking", "he ate something" and "they know something".

Many English speakers consider double negatives grammatically incorrect; every child has been told “don’t use a double negative!” at least once by their older relatives!

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Why should double negatives be avoided in scientific writing?

The sentence above uses two “negative” words to refer to blood levels: no and abnormal. While abnormal isn’t exactly a negative word like no, not or didn’t, the word abnormal seems negative in this context.

When reading a thesis or journal paper, double negatives make you pause to “translate” the negative actions to positives. When I first read “The patient presented no abnormal blood glucose or insulin levels”, I had to stop to decide whether the patient actually had abnormal blood results or normal blood results.

Most importantly, double negatives can make your reader misunderstand what you really mean. This is especially relevant in scientific writing, where the goal is to be clear and unambiguous.

How can you avoid double negatives?

If you find you’ve written a sentence containing a double negative, then you should try to rearrange it. Let’s look again at the example from the paper I edited:

The patient had no abnormal blood glucose or insulin levels.

In this case, the writer meant the patient’s levels were normal. It’s so much easier to read the patient’s levels were normal than their levels weren’t abnormal.

Removing the two negative words (no, abnormal) gives a much simpler, clearer sentence:

The patient had normal blood glucose and insulin levels

Another example

Patients without heart disease were not included.

This is another potentially confusing example. As there are two negative words (without and not), you probably had to pause to decide if patients with heart disease were included or not. This sentence can be simplified by removing the negative words.

Patients with heart disease were included.

Or, we can make sure there is only one negative word:

Patients without heart disease were excluded.

Let’s not do no summary, or … let’s do a summary!

Double negatives are phrases with two negative words (such as no, none, without, absent) or one negative word and another potentially negative word like abnormal.

Double negatives make the reader pause while they work out the true meaning of the sentence and can sometimes be confusing.

Double negatives should be rewritten by removing the negatives, then checking the sentence has the correct meaning.

Other examples of double negatives

Here are three more examples of double negatives that could be encountered in a scientific manuscript. Can work out the true meaning and rewrite each sentence to make it clear?

Xyz protein expression was not absent
No cases without complications were reported
Gene Xyz mRNA was not undetectable

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Andrea Devlin PhD

Chief editor and owner of Science Editing Experts

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