When two negative words are used together, readers sometimes struggle to see the resultant positive meaning.
MEANING AND TYPES OF DOUBLE NEGATIVE
A double negative is a pair of negative words that combine together to make a positive meaning. The two words may be both the same (e.g. not … not … ) or different. Here are some examples:
(a) (SAME NEGATIVE USED TWICE) It is not normal not to want to be loved.
(b) (TWO DIFFERENT NEGATIVES) It is not impossible to become a millionaire.
The first example means it is normal to want to be loved. The writer might have preferred the double negative because the focus is not wanting to be loved rather than wanting to. The second example means it is possible to become a millionaire. The reason for using the double negative here might be to show disagreement with somebody who thinks that becoming a millionaire really is impossible (for more on disagreement in texts, see 152. Agreeing & Disagreeing in Formal Contextrs and 51. Making Concessions with “May”).
It is possible for two negatives in English to reinforce each other rather than cancel each other out (see 69. How Computers Get Grammar Wrong 2). However, that is a feature of informal or non-standard varieties, and is not normal in academic and professional writing.
THE DIFFICULTY OF DOUBLE NEGATIVES IN READING
The main reason why double negatives are a problem in reading seems to be the effort they require to work out their logic. The use of two different negative words may be particularly difficult, especially if one or both is “hidden” (see 13. Hidden Negatives). See how good you are at dealing with double negatives in the examples below. Try at least to identify the two negative words each time before you read the explanation.
Double Negative 1
The most important point is that it is inconceivable that people could ever have developed their more complex tools without their ability to manipulate symbols and communicate with one another in symbolic languages.
This sentence gave students difficulty in a reading test. It means that making complex tools is only possible if symbols and language are used. The two negative words are inconceivable (= “not able to be imagined” or “not possible”) and without (= “not having”). Putting them together, “not possible … not having …” means “only possible … having …”. So the sentence could be rewritten “it is only possible that people could have developed their more complex tools by having the ability to use symbols and language.”
Double Negative 2
An important concern in decoding images should be that of undermining the ways in which dominant forms of visual representation reduce complex issues … to a few “recognisable” aspects which appear to constitute an acceptable totality.
This sentence was reported as difficult by a student from Japan (see my posted article What can learners tell us about their reading difficulties?). The two negative words are undermining and reduce. The first means “preventing” or “weakening”, the second “making less”. If you prevent something from being made less, you ensure it remains large. The writer seems to be saying that people who decode images should remember that complex issues have a larger number of aspects than the misleading few shown by the most popular images.
Double Negative 3
For example, if success and failure are to be judged solely by the quality of legislative reform …, the conclusion has to be that recognition of animals as anything other than commodities that exist mainly for human benefit has been limited.
This sentence was reported as difficult by a student from the Indian subcontinent. The use of a double negative is actually one of two comprehension problems here. The other is a lack of clarity about the link between legislative reform and recognition. The reader has to work hard to see that recognition means “recognition in legislative reform”.
The two negative expressions here are other than and limited. The first means “different from” (commodities) and hence is the same as “not” (commodities). It is helping to say something about animals. The word limited means “not much”, and is about recognition – there is not much of it. Thus, the double negative could be restated as “there is not much recognition that animals are not commodities”. In other words, animals are mostly recognised as commodities. What does that mean? It is a criticism. Legislative reform (i.e. changes in the law) sees animals only as commodities (things to trade in and exploit), when the writer thinks they should be respected as living things with rights of their own.