When Jean Aitchison was appointed three years ago as Rupert Murdoch Professor of Language and Communications at Oxford University, her inaugural lecture, extracts of which were published in The Times, elicited numerous hostile letters.

Professor Aitchison’s correspondents, most of them retired, did not take kindly to being told that to worry about split infinitives or the “correct” use of the word “hopefully” was simply to fuss about trivia. They accused her of helping to destroy the English language. Professor Aitchison can expect many more such letters when she delivers this year’s Reith Lectures, beginning on Radio 4 next Tuesday. Her first lecture is a robust attack on those language purists who refuse to accept the idea that language changes.

“People worry about language: they always have,” explains Professor Aitchison. One of the earliest recorded worries is that of a 14th-century monk, who complains of the “strange stammering, chattering, snarling and grating-tooth gnashing” brought about by an influx of foreigners.

“Purists behave as if there was a vintage year when language achieved a measure of excellence which we should all strive to maintain,” she says. “In fact there never was such a year. The language of Chaucer’s or Shakespeare’s time was no better and no worse than that of our own - just different.”

Much of the present concern about language, she maintains, derives from the 18th century: from its admiration for Latin - which appeared to have a fixed, correct form - and from the conviction of its philosophers that English, like Latin, should be logical. Samuel Johnson’s famous dictionary was an attempt to codify a “correct” form of the English language. But when Johnson said he had “laboured to refine our language to grammatical purity and to clear it from colloquial barbarisms, licentious idioms and irregular combinations”, he had in fact, in many cases, simply favoured the usage of the middle- and upper-classes.

The 19th century witnessed a great confusion of etiquette, morals and speech. A Book of Etiquette, for instance, from the 1880s, instructs people not to drink from a saucer or wear diamonds in the morning, as well as reminding them not to say “gents” for “gentlemen”, or “it is him” instead of “it is he”.

Some of this confusion even persists today: Professor Aitchison quotes a remark of Lord Tebbit’s in 1985, to the effect that slipping into “bad English” could be a part of the general decline in standards which helps turn some young people to crime.

A large part of the problem is that many people hang on to trivial rules and precepts that have been handed down, instead of really thinking about language. A new awareness, helped by the spread of linguistics since the 1960s, is beginning to percolate down, but the process is slow. “We need to understand the way language works, rather than try to control it. We need to understand that it is biologically-programmed behaviour: it is in no sense wrong for human language to change - any more than it is wrong for humpback whales to alter their songs every year.”

Jean Aitchison’s own passion for languages began when she was a very small girl, discovering Greek texts on her father’s bookshelf. She took a first class degree in classics at Cambridge in 1960, and became fascinated by the study of Greek dialects: Greek and Latin were never to her “dead” languages. After an MA in linguistics at Harvard, she returned to a lectureship in Greek at Bedford College, London University, but found herself increasingly frustrated by the narrowness of traditional Greek scholarship. In 1965 she moved to the London School of Economics, where she was a Reader in linguistics until 1992. Among her key publications are The Articulate Mammal: An Introduction to Psycholinguistics (1989), Language Change: Progress or Decay? (1991) and Words in the Mind: An Introduction to the Mental Lexicon (1987).

Small and neat in appearance, in her speech and in her writing Jean Aitchison has the gift of making her ideas readily accessible. Her vigorous treatment of our common linguistic worries is both controversial and attractive, and will undoubtedly find many sympathetic ears in the teaching profession.

On political correctness, for instance, a vexed issue in schools, she takes a sturdy long-term view. “I would far rather that people worried about political correctness than dropped h’s or split infinitives. In its origins I believe it is a good movement, in that people are becoming more sensitive to language and to each other. It has been overdone in some ways, but we don’t need to worry too much: thousands of words get coined every day and only a few will actually remain in use - it’s like catching raindrops in a bucket. So the longer, stranger phrases of political correctness - like “visually-challenged” instead of blind - will just disappear in time.”

Professor Aitchison also pours her “we-don’t-have-to-worry” balm over the question of whether a reliance on computers in school will have a damaging effect on language. “Every time there is a new development in communications, people worry about how it will affect language - they did when the telephone came in. But language is biological behaviour, and you can learn new conventions, add extra ways of communicating; these, in many ways, enrich language.”

On the contentious matter of “standard English” versus dialect in the national curriculum (an argument which has been softened, but not entirely laid to rest), she agrees with the authors of the curriculum that there needs to be a written standard of English, in terms of spelling and grammar; this changes much more slowly than spoken English. But there is no reason, she says, to discourage spoken dialects - whose sentence formation, in any case, differs relatively little across England. There should, however, be much greater tolerance of different accents than there is at present. “As long as people can understand one another, I like different accents.”

What is important for children to learn, Professor Aitchison maintains, is that different types of speech are appropriate in different situations. “Giving a lecture in Oxford, for example, I would use a fairly formal style - which wouldn’t be at all appropriate when I’m buying potatoes at home in Hackney. ”

Teaching children what is appropriate is also her advice to parents (and I confess I am no exception here) who berate their children time and again for dropping the ‘t’ at the end of “what”. “Because it is not now polite to say ‘wha(t)’ in some situations, a child could be taught other ways of saying it, such as ‘sorry’. But sloppiness really does not exist in the way we think it does. A child who says ‘wha(t)’ is just a stage further advanced, in terms of how language is developing - a development due to the general and inevitable weakness of articulation of sounds at the end of words. There really is no point in resisting it. A parent who is busily correcting ‘wha(t)’, will themselves be saying ‘ho(t)-water-bottle’.”

Children, Professor Aitchison believes, are particularly attuned to language, in a sensitive period which stretches from infancy to about 13, discussed in her third lecture. “In my view, this is biological behaviour: children are tuned in to certain aspects of language at different stages - beginning with sound patterns, then grammar, then vocabulary.”

An average educated adult knows and can actively use about 50,000 words, and children build these up at an average rate of ten words a day. According to her recent research, children’s language undergoes an extra spurt at around the age of 13, when their vocabulary will often jump to 20,000 words - “a level at which they can start to interact efficiently”.

But for this language development to occur, Professor Aitchison emphasises, children need to read and, above all, they must be talked to. “There must be face-to-face interaction. If you just sit children in front of the television, they will not learn.”

A vital part of understanding language is to understand something of its origins. In the second of her lectures (as well as in a forthcoming book, The Seeds of Speech), Jean Aitchison overturns the assumption passed down by John Locke that the primary purpose of language is to pass on information, suggesting instead that language developed out of the ability, common among primates, to deceive. A chimp, for example, can lead another chimp away from hidden food which it wants to return to alone. “The ability to deceive demands the ability to imagine another point of view. Once you can do this, you can communicate about things that are absent - which, in humans, led to the ability to vocalise.”

The deception that she believes is integral to language is what makes Professor Aitchison so impatient with those who fuss about trivia: worrying about the wrong things can lead to people being manipulated by language - the subject of her sixth and final lecture. “It is very important to worry about language in the right way, and not get sidetracked by trivia. Otherwise you’ll be so busy saying to yourself, this person must be OK because he says ‘different from’ and not ‘different to’, that you will be manipulated without even knowing it.

“The language of newspapers, for instance, is very skilled indeed. Instead of grumbling about ‘journalese’, we should admire the skill, but watch out for the way we may be being manipulated.”

The political language of nuclear “shields” or “assets” should not, she says, blind us to the potential danger of the weapons. And we need to be more aware of how we use metaphor: metaphors about anger invariably use the image of liquid in a container - steamed up, erupting, boiling with anger - but this may lead people to feel that it is justifiable, and natural, to explode and cause damage, just as a container under pressure would do.