Why are double negatives so confusing in scientific writing?

scientific writing double negativesSomeone stole my identity on Facebook.  They copied my profile picture, opened a fake account in my name, and then sent friend requests to my Facebook friends.  Such profile cloning attempts are becoming more common.  Criminals try to con people into sending money or to share malware and viruses.  If you’re worried profile cloning, then Hoax Slayer has a useful guide on protecting your privacy on Facebook.

Thankfully, my friends spotted something wasn’t right, reported the fake profile, and it was quickly blocked by Facebook.  One of the most interesting aspects of the whole event was a message I received from Facebook support.



While I was panicking as I though my profile had been hacked and someone had obtained my password, I found the underlined sentence of the message really confusing.  This message is really unclear!!  Don’t do anything?  Do something?  Delete the message?  Don’t delete the message?

A few days later, I was editing a manuscript and found the following sentence:  The patient had no abnormal blood glucose or insulin levels.  Together, that sentence and the Facebook message reminded me of the importance of avoiding “double negatives” in scientific writing.


What are double negatives?

The sentence in the journal manuscript uses two “negative” words to refer to blood levels: no and abnormal.  This is called a “double negative”.  Yes, I know abnormal isn’t exactly a negative word like no, not, didn’t, but abnormal functions as a negative word in this context.

Double negative phrases are also 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 referring to the same point 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 literally mean I did cook, he ate something and they know something.  However, most people would understand that the person really means I didn’t cook, he ate nothing and they know nothing.


Why should double negatives be avoided in scientific writing?

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

Secondly, 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 or normal blood results.

Thirdly, double negatives may also make the reader misunderstand what the writer really means.  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 say (and read) that 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


Patients without heart disease were not included

This is another potentially confusing example. As there are two negative words, without and not, I bet you 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


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

Double negatives are phrases with two negative words such as no, none, without, absent.

Double negatives make the reader pause while they work out the true meaning of the sentence and can be very 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 scientific writing.  See if you can work out the true meaning and rewrite each sentence to make it clear.

Protein expression was not absent

No cases without complications were reported

Gene Xyz mRNA was not undetectable


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