Although the details have changed since George Orwell wrote the following paragraph in 1945,* his summary of the basic mechanism still looks apt:
“Unpopular ideas can be silenced, and inconvenient facts kept dark, without the need for any official ban. Anyone who has lived long in a foreign country will know of instances of sensational items of news – things which on their own merits would get the big headlines – being kept right out of the British press, not because the Government intervened but because of a general tacit agreement that ‘it wouldn’t do’ to mention that particular fact. So far as the daily newspapers go, this is easy to understand. The British press is extremely centralised, and most of it is owned by wealthy men who have every motive to be dishonest on certain important topics. But the same kind of veiled censorship also operates in books and periodicals, as well as in plays, films and radio. At any given moment there is an orthodoxy, a body of ideas which it is assumed that all right-thinking people will accept without question. It is not exactly forbidden to say this, that or the other, but it is ‘not done’ to say it, just as in mid-Victorian times it was ‘not done’ to mention trousers in the presence of a lady. Anyone who challenges the prevailing orthodoxy finds himself silenced with surprising effectiveness. A genuinely unfashionable opinion is almost never given a fair hearing, either in the popular press or in the highbrow periodicals.”
Tony Norfield, 26 August 2015
* The quotation is taken from a Preface to George Orwell's Animal Farm. It did not get published in 1945, or in (many) later editions, and was first published in 1972 in The Times Literary Supplement.