This book, which unfortunately I am unable to provide any link for as it remains an unpublished and thus publicly unavailable work, is the undergraduate dissertation of Andrew Stovell - my father. It's a dispassionate and rigorous study of the phenomenon of glossolalia, more commonly known as "speaking in tongues", from both a biblically-considered and linguistic pair of angles. I've just read the whole thing in one sitting as I stumbled upon a chapter in another book about this and my mum mentioned "oh, dad wrote his thesis on that!" so I gave it an appropriate detour.
It's a really interesting read. The first part lays out a groundwork for what glossolalia is, how it is portrayed and discussed in the Bible to start with, then briefly looking at how it has manifested throughout history, leading up to contemporary practices in charismatic churches. The second part is a literature review of other linguists' studies into the phenomenon, which deftly summarises the scant but deep insights of other academic perspectives, most of which this thesis's author largely supports in his argumentation. The third and final part comprises an in-depth phonetical and phonological analysis of ten recorded (and meticulously transcribed using the International Phonetic Alphabet in the lengthy appendix) examples of actual purported glossolalia, with some reflections as to the nature of these and what that implies as to whether they can be understood as language at all.
Ultimately my dad concludes that the biblical accounts of "speaking in tongues" are quite clearly talking about "xenoglossia" (speaking a different real language) rather than "glossolalia" (speaking incomprehensible *possibly angelic* languages) and all scholarly analysis of modern such speakers drives to the point where we cannot confirm these instances to be of any real human language; indeed he goes further to point out that examples of glossolalia are very often characterised by frequent repetition of simple phonemes with no clear syntactical or grammatical logics at play, and thus even if we are to understand these utterances as divinely-inspired language of some form, it is odd to note that those divine languages must be overwhelmingly composed of the sounds babbling toddlers are wont to make. Which is to say, the actual spiritual gift of speaking in tongues is biblically historical and useful, and the modern phenomenon of it is conversely essentially gobbledegook.
If your curiosity is piqued by this post that you actually want to read this, and if you do have a linguistic interest in the veracity of speaking in tongues I would recommend this as a resource certainly, but as I mentioned it's not published anywhere so if you find yourself desperate drop me a comment and I'll ask my dad whether we can scan it into a more easily distributable .pdf or something.
