The use of 'they' as a replacement for 'he or she' pronouns has been gaining momentum not only because it lets one slip around those steadfast predefined gender binaries of 'he or she', but also because the American Dialect Society voted it the word of the year in 2015.
A relatively recent article by Ernie Smith details centuries of debate around the use of this term that linguistic anthropologists and anthropologists alike would enjoy. Smith also highlights Dennis Baron's publicly available essay entitled: The Words that Failed: A chronology of early nonbinary pronouns which might make a good read for those wishing to use those failed and absent words to understand linguistic changes over time.
Smith goes on to describe what they believe to be the most interesting comment on the debate from Ruth Walker at the Christian Science Monitor who likens the use of the singular they to a shortcut through grammar law that while irksome for some, will most likely become more acceptable over time.
In any case, this historical investigation of the manner in which linguists and laypeople think around (literally in this case) the topic of gender binary might be a useful brainstorming session for students or a stepping stone to being rethinking one's own vocabulary.
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