A pen name is the pseudonym, or alias, adopted by a writer who wants to keep her or his identity concealed. A pen name may be a deliberate attempt to sound like a real person, as both Jean Plaidy and Victoria Holt (pen names adopted by Eleanor Alice Hibbert (née Burford), (1906-1993)) were in order to maximise the return for her great productivity. Other pen names are purely disguises, like 'Sapper' (H.C.McNeile, 1888-1937), or 'Flying Officer X' (H. E. Bates, 1905-1974), who both wrote while serving officers in the armed forces (Bates as a PR expert, hired to write morale-boosting short stories).
The use of a pen name may be for different reasons. The Bronte sisters all adopted pen names because, as women writing in Victorian times, they feared that they might not be published; if published, might not be read seriously; and that they might be despised as women seeking fame in a public arena. (Anne Bronte published Agnes Grey and The Tenant of Wildfell Hall as Acton Bell; Charlotte, author of Jane Eyre, as Currer Bell; and Emily, author of Wuthering Heights, as Ellis Bell.) Other writers may use different names to avoid their social circles realising their writing. 'Lewis Carroll', for example, is the pen name of Charles Lutwidge Dodgson, who taught Mathematics at Christ Church in Oxford University: presumably he did not care to be known as the author of children's fantasy. Samuel Langhorne Clemens preferred to write as 'Mark Twain', perhaps to retain some privacy, and perhaps because the two monosyllables are more memorable than his own. (They are the cry of sailors on the Mississippi river boats who took soundings by leadline. When they reported "By the mark, twain!", they meant that the line showed two fathoms (12 feet) of water under the hull, which represented safety.) Joseph Conrad certainly felt that he was better able to publish English novels under that name than under his Polish birth name, Józef Teodor Konrad Korzeniowski - although he had officially changed his name when adopting British citizenship, and it must have helped him when he was employed on British ships.
Screenwriters blacklisted in Hollywood during Senator Joe McCarthy's anti-communist persecution adopted pen names in order to continue working: pen names are often a response to censorship. In the eighteenth century, a still unidentified writer (perhaps a group of writers) published many letters under the pen-name of 'Junius', masking highly political and sometimes slanderous attacks on the then government and the King, George III. Other pen names have been adopted by pairs, or even teams, of writers as a standard by-line for their output. This happens both with political writers seeking to make a point (and often to avoid censorship or persecution) and writers of popular fiction. Not all the 'Saint' thrillers published as being by Leslie Charteris (whose birth name was Leslie Charles Bowyer-Yin) were written by him, and 'Ellery Queen', famous for detective stories, is actually two writers.