Mx was created on the model of the other personal titles Mr, Mrs and Ms for a person who doesn’t identify themselves as either male or female or doesn’t want their gender to be known.
An article in the Guardian on 17 November — prompted by the news that the Royal Bank of Scotland (RBS) was considering introducing it — noted the quiet rise in the use of Mx as a gender-neutral title, particularly in the UK. Mx is accepted as a valid title by a number of organisations, mostly in the public sector. The Post Office was first in 2009; it has since been joined by several governmental bodies, including the National Health Service, HM Revenue and Customs and the Driver and Vehicle Licensing Agency. But other than those, acceptance is patchy and uncommon; the proposal by RBS marks a potential shift into the private sector, though RBS is 80% owned by the government.
Research by Nat Titman shows that Mx was created in online discussion groups in the early 1980s as a way to avoid identifying oneself as male or female or avoid specifying one’s marital status. It’s hard to say how often it was employed in real life in the following two decades but its sporadic appearances online argue for its being very rare. Around 2000 Mx began to be discussed by transgender and androgynous people, who have since led efforts to gain recognition for it.
This is the earliest example I can find in a British newspaper:
Official forms in Brighton and Hove will include the title “Mx” to cater for the city’s transgender community after a review of services. Brighton and Hove Council’s trans-equality scrutiny panel recommended removing the need for people to identify themselves as male or female at GP surgeries and introducing gender-neutral lavatories and changing rooms.
The Times, 4 May 2013.
It’s said as mix or mux, sometimes mixter.