Unisex names

[Salmoneus]

I had no idea this was an English thing, too. I associate it with WASPs, particularly those with Southern roots.

It's not done anymore, at least among ordinary people, but it was the norm in the middle and upper classes a century ago. For instance, just look at RAF air force officers in WWII (i.e. people from distinguished backgrounds but normally not actually aristocrats). They all had surnames as middle names. It wasn't very organised, though - some had one, some had two, some had a maternal surname, some had their grandmother's surname. I think it wasn't so much about registering your linneage as about claiming a relation to some respectable name.
So, for example, there's Air Chief Marshal Sir Hugh Caswall Tremenheere Dowding, Lord Dowding, First Baron Dowding of Bentley Priory in the County of Middlesex, Knight Grand Cross of the Order of the Bath, Knight Grand Cross of the Royal Victorian Order, and Companion of the Order of St Michael and St George (in practice actually known as 'Stuffy'), the British leader in the Battle of Britain. His father was Arthur Dowding and his grandfather was Charles Tremenheere - don't know where the Caswall comes from. His son was then Derek Hugh Tremenheere Dowding, and HIS son is Michael Hugh Tremenheere Dowding.
Other examples from the RAF are William Gore Sutherland Mitchell, John Frederick Andrews Higgins, Patrick Henry Lyon Playfair, Wilfrid Rhodes Freeman, and Cyril Louis Norton Newall. Wikipedia only mentions parents for the latter, who was the son of William Potter Newall and Edith Gwendoline Caroline Norton. On the other hand, Bomber Harris was Arthur Travers Harris, son of George Steel Travers Harris - so the Travers isn't maternal there, and he's lost the 'Steel'.

[The exception seems to be 'Sholto' Douglas - 'Sholto' was just a common middle name for Douglasses]

Going back a bit, and showing some more confusion, there's Arthur Conan Doyle, son of Charles Altamont Doyle. In this case, 'Conan' was the surname of his godfather, and it's unclear what status it had in Conan Doyle's name - he was baptised with 'Conan' as a middle name, he was knighted as 'Doyle', and he's in libraries under 'Doyle'. But when he married, his wife became Jean Conan Doyle...

Incidentally, some of these people did use their middle names as first name - the 'american' thing, though, is to keep the first name as an initial. So Sholto Douglas was always Sholto Douglas, whereas if he'd been american he'd probably have been W. Sholto Douglas.

Another very Southern thing is to go by the initials of those names, e.g. J.D., R.J., C.J.. This used to be a predominately if not solely masculine practice, but recently I've come across more female examples. (Some from the publishing industry, where unisex names are often considered more marketable, e.g. J. K. Rowling, K. A. Applegate.)

I think there are two different things here. One of them is using initials in certain contexts - authors have been doing this a long time. Likewise those airmen are generally listed as WR Freeman, PHL Playfair, and so on. similarly, cricketers are always listed like this in official scorecards, and sometimes in other contexts by extension. (most famously, WG Grace - even the official lists list him as "Grace, WG (W.G. Grace)".)

The other is actually going by initials as though they were a name, in daily life. That's something that I think is very rare here but I get the impression is a thing in the UK. [Eg, JK Rowling isn't actually known as 'jaykay' by her friends]. There's a character called CJ in a 70s sitcom (the fall and rise of reginal perrin), but he's intentionally stupid and vacuous, so the americanism may be intentional.
G.E. Moore was commonly known as G.E. even in person. Even he didn't go through with it entirely, though - his wife called him Bill.

[masako]

A few of these are hard to tell. Not sure about Deondre.

[CatDoom]

I would guess that Deondre is a boy, if only because it looks like it would rhyme with "Andre." I'm reminded of the fact that I knew a guy in high school named Jamal who, peculiarly enough, appeared to be of primarily European descent.

Incidentally, my middle name, McNamara, is my mother's maiden name, and my father's middle name, Porter, is his mothers maiden name. In my case it probably comes down to the fact that my grandmother was big into genealogy; by two brothers' middle names are Whitcomb and Harrison, which were surnames held by other ancestors of ours a few generations back, whom she described in a couple books she wrote on our family history.

Perhaps more unusually, my mother uses her own maiden name as her middle name, and pretty much stopped using the middle name her parents gave her altogether. Not sure how common that is, though these days hyphenated names combining the surnames of a husband and wife seem to have gained some popularity in my neck of the woods.

[Copperknickers]

'Shannon' is exclusively a female name in the UK, I've never heard of a male one. 'Cameron' is exclusively a male name except among people who are specifically named after Cameron Diaz.

There are a few gender neutral names that are not surnames in the UK: Alex (I know several people where 'Alex' is on their birth certificate rather than Alexander/dra), Robin (usually Robyn for girls admittedly), Greer, Gerry, Dale, Daryl, Jamie, Lee, Noel (usually pronounced 'Noell' for girls).

[richard1631978]

Air Chief Marshal Sir Hugh Caswall Tremenheere Dowding fellow commander during the Battle of Britain were Trafford Leigh-Mallory & Sir Keith Park, both names after places, though Keith is far more popular.

[din]

Perhaps more unusually, my mother uses her own maiden name as her middle name, and pretty much stopped using the middle name her parents gave her altogether. Not sure how common that is,...

My mother-in-law did the same thing when she got married. Where are you from? She's from NC.

[Jūnzǐ]

For the record, Hawaiian names are entirely unisex, at least according to Wikipedia.

Also, it appears that most Urhobo names are unisex (and proverbial in nature). Even more so with Tibetan names.

[StrangerCoug]

Not strictly part of the "unisex" conversation, but it does fit the OP's stated topic of naming conventions in world languages: I'd like to know how names become associated with a gender in the first place in order to get ideas for conlangs that have few or no unisex names. (For Salmoneus's "transgendering of names", I'm looking in that case for how they became male before they became female.)

[Neon Fox]

Perhaps more unusually, my mother uses her own maiden name as her middle name, and pretty much stopped using the middle name her parents gave her altogether. Not sure how common that is, though these days hyphenated names combining the surnames of a husband and wife seem to have gained some popularity in my neck of the woods.

My mother does that too; it was the standard for married women until quite recently--in fact, I have at least one friend my own age who used to be Megan Grant and is now Megan G. Nichols. (Not her actual names.)

My father's middle name is the Anglicized version of his mother's maiden name. I'm not sure if they were an Ellis Island change or if they continued using the Italian name in America and just gave Dad the Anglicized name.

[alynnidalar]

Interestingly enough, my grandmother and her brother both have their mother's maiden name as a middle name (born in the early 1900s). I'm not sure if that was some kind of common thing at the time or just their parents being a little out there (they totally were).

Not immigrants, either. That branch of the family has been in the US since at least the 1700s.

[Pressed Bunson]

Perhaps more unusually, my mother uses her own maiden name as her middle name, and pretty much stopped using the middle name her parents gave her altogether. Not sure how common that is, though these days hyphenated names combining the surnames of a husband and wife seem to have gained some popularity in my neck of the woods.

My mother does that too; it was the standard for married women until quite recently--in fact, I have at least one friend my own age who used to be Megan Grant and is now Megan G. Nichols. (Not her actual names.)

My father's middle name is the Anglicized version of his mother's maiden name. I'm not sure if they were an Ellis Island change or if they continued using the Italian name in America and just gave Dad the Anglicized name.

The mother of one of my best friends used her maiden name as the friend's middle name. So, D. E. became D. W. and gave birth to M.E.W.