Modality is the aspect of language that allows us to hypothesise. Certain words like 'could' and 'must', allow us to express the possibility or desirability of something happening. For example, the headline "Foreign intervention in Syria may cause chaos in the Mideast" suggests the possibility of a certain situation occurring through the use of 'may', while the 'must' in "David Cameron must make the Conservative party look like the nation" suggests that a certain action on the part of David Cameron is desirable or necessary.
Words such as 'may' and 'must' are modal auxiliary verbs. A modal auxiliary is added to a verb ('cause' and 'make' in the examples above) to express the speaker or writer's level of commitment to the proposition of the sentence, for example how true they believe the statement is (foreign intervention may cause chaos), or how desirable or necessary they think it is (Cameron must make changes to the Conservatives' appearance). By looking at the different modal auxiliaries — which include 'will', 'would', 'shall', 'should', 'may', 'might', 'can', 'could', 'must', 'ought' and 'need' — we can see the range of different meanings that modality makes possible, and also how modality tends to be split into that which expresses the speaker/writer's level of certainty, and that which expresses how desirable or necessary a speaker/writer thinks something is.
'The Republicans [could / would / may / might / will] have won the election.'
The basic proposition of the sentence above is 'The Republicans have won the election'. This is a categorical assertion, with no modality, expressing a simple fact. However, as we can see, the insertion of any one of the modal auxiliaries immediately makes the statement into one that makes a hypothesis about the possibility of something having happened, rather than the statement of a straightforward fact. Note that 'could', 'may' and 'might' all sound quite tentative, giving the reader the distinct impression that the opposite — a Republican defeat — is also quite possible. 'Would' seems to imply the existence of a clause beginning with 'if...', for example 'The Republicans would have won the election, if they weren't nuts', which serves to put a precondition on their election chances (an election win might have been a possibility if they weren't nuts). 'Will', meanwhile, sounds much more confident; indeed, the reader may feel convinced that this choice of auxiliary presents a Republican win as a certainty. However, it is important to note an important fact about modality: any modal form turns a proposition into a hypothesis. Because 'will' refers to an event in the future, which we cannot be certain will happen, it is necessarily modal.
'The Democrats [should / ought to] have won the election.'
In the Democrats example, there is much more of a sense of something being desirable: we can imagine that the writer may go on to give their reasons why they think the Democrats ought to win, e.g. 'The Democrats ought to have won as they are less nuts than the Republicans'. Alternatively, these modal auxiliaries could be read as implying that the Democrats are in some way obliged to win the election: for example, the writer might go on to explain why the Democrats ought to be able to achieve election by saying, for example, 'The Democrats ought to have won the election, because the Republicans appeared so unelectable'.
The way modality implants certain possibilities into hearers'/readers' minds can have a warning or scaremongering effect. The British tabloid The Daily Mail is notorious for such stories, particularly in relation to the possible causes and cures of cancer. While the use of the modal 'may' in a headline like 'Coffee may raise child cancer risk' acknowledges the fact that a link between coffee and cancer in children is not actually proven, the proposition nonetheless puts a possibility into a reader's mind, perhaps causing them to suddenly make changes to their child's diet. 'Cure' headlines seem barely less manipulative: the suggestion that 'Aspirin could stave off cancer and help victims to survive' may also cause unnecessary fear, or worry readers into changing their behaviour in a way that may not actually be beneficial. It also, of course, makes a (tentative) promise that could give readers false hope about the chances of victims surviving cancer.
Verbs such as 'think', 'believe' and 'know' can be used by a speaker/writer to express their commitment to a proposition. If a finance minister tells us that 'I believe budget cuts are necessary', then people with a different viewpoint might mock them for acknowledging that this is a matter of belief, rather than something they are convinced is true and provable. On the other hand, even if they said 'I know budget cuts are necessary', the way in which 'know' implies the existence of some sort of 'scale of certainty' automatically makes the statement modal, and its veracity therefore open to debate.
Other verbs, such as 'want' and 'need', express the speaker/writer's desire that something should be the case, e.g. 'I want the prime minister to admit he was wrong', 'The government needs to change its policy'.
Adjectives such as 'possible' and 'certain', and adverbs such as 'probably' and 'definitely' make it clear that what is being stated is a hypothesis, and not an assertion of fact: 'It is possible that there will be an uprising', 'The economy is definitely heading for troubled times'.
These were alluded to in the discussion of the election examples above. They allow a speaker/writer to place a precondition on the possibility of something happening, as in 'The Republicans would win the election, if they weren't so nuts'. Note that conditional structures include modal auxiliaries, helping make clear that what is being expressed is a mere possibility or matter of desirability/obligation: 'If European leaders find a way to work together, then there will be hope for the single currency'.
Modality's potential for ideological impact is linked to the hearer/reader's relationship with the speaker/writer. All of us trust particular figures of authority in our lives, whose opinions we will listen to carefully, or even take as fact. This could be a particular journalist who writes for a newspaper with which we affiliate ourselves politically, a religious text which we turn to for guidance, or a friend who tends to offer good advice on gardening. If such sources inform us what they 'believe' the answer to a particular political problem is, how we 'ought' to live our lives or how 'possible' it is that our hellebores will be the victim of ground elder, then the fact that they are the ones expressing these hypotheses is likely to have a persuasive effect on us.
One famous and much derided example is the way American politicians discussed the possible existence of weapons of mass destruction ahead of the invasion of Iraq in 2003. Senior political figures frequently used modality to portray their 'knowledge' of the situation in Iraq. For example, Donald Rumsfeld said that "We do know that the Iraqi regime has chemical and biological weapons" (quote in USA Today), while George Bush claimed that "We know that the regime has produced thousands of tons of chemical agents, including mustard gas, sarin nerve gas, VX nerve gas" (quoted in The Guardian). The use of such strong modality by powerful figures is important, especially in such serious matters as (potential) war, because as elected officials they can be sure of a certain degree of trust on the part of the people who have elected them.
Pay careful attention to the use of modality in opinion pieces in newspapers and on the radio and television. Writers of these texts are in a powerful position, and are aware that many hearers/readers will listen closely to their views on important issues. In particular, think about the use of categorical assertions to express absolute certainty — what is being portrayed as fact? Where modality is being used — for example 'might' and 'could', references to future events using 'will' — is a sense of something threatening being created?
The section on 'Hypothesizing' in Jeffries' introduction to Critical Stylistic tools introduces the idea of modality:
Jeffries, Lesley. 2010. Critical Stylistics: The Power of English. Hampshire: Palgrave, 114-129.
Richardson., John E. 2007. Analysing Newspapers: An Approach from Critical Discourse Analysis. Hampshire: Palgrave Macmillan, 59-62.
The 'Advertising discourse: methods for analysis' section of the following textbook, aimed at students, helps demonstrate the significance of modality in advertising:
Simpson, Paul and Andrea Mayr. 2010. Language and Power: A Resource Book for Students. Oxon: Routledge, 86-96.