hangs head My daughter’s middle name is Lynn. I think the popularity of those three has a lot to do with the fact that they just seem to flow so well with so many names.
My name is Firstname A. Lastname, and *everyone *assumes the A. is for Ann. It’s not, though.
But since I hate hate hate it, I won’t share it. 
My middle name is actually Lynn. I hate my first name, which is an Anglicized version of a common Italian female name, and which nobody can pronounce.
My wife’s middle name is Marie (she was born in the US but raised in Ireland). My younger sister’s middle name is Marie. My older sister doesn’t have a middle name, but her first name is Anne-Marie.
I could give several more examples, but one sums it up: When I asked my friend Tricia what her middle name is she said, “I have the same middle name everyone my age does.” I knew right away that her middle name is Marie.
My niece was born in January and Grace is her middle name!
My mom insists that most any Catholic girl will have the middle name of Ann or Marie/Mary.
Her mom was Mary, sister was Marie. Four girls in the family and they all have either Ann or Mary as their middle name.
Me, I’m not a Catholic. I got myself a different middle name 
I do not have one of those for my middle name, nor does my twin sister. If I know my mom she may not have even known about the popularity of that practice, which is fine in this case. My middle name is Louise because both my grandmothers’ middle names were Louise. I use my maiden name as my middle name now, however.
My stepmother’s two daughters have Marie and Lynn . Despite the cliche of these names they do flow with their respective first names. My husband’s niece has Anne and it doesn’t flow with her first name at all, nor does it flow with the other first name they were going to give her. I think my sister-in-law thinks Anne is the only middle name.
My wife’s Catholic. My daughter’s middle name is Marie.
ETA: My daughter is Catholic too.
No need to make anything up - homogeneity.
Born in Canada, in the early 80s. I was stuck with Roxanne for my middle name. Bleh. Everyone who finds that out immediately starts singing RAAHHHXANNE - always, always the shriekiest line of that damn song - and telling me I don’t need to walk the streets for money.
My mom and her sisters all have saint names for their middle names. They all have multiple middle names, come to think of it. Marie, Lucie, Therèse, Catherine, Claire… not sure at this point which name belongs with which aunt but they’ve each got at least two and I think they all have a Marie.
And the rest all have Jane as far as I can tell (a generational thing, for women born in the 60s/70s it would seem).
My parents generously/bravely/rashly (delete as appropriate) allowed my sister (aged 7) and my brother (5) to choose both my first and middle names. My brother’s best friend at the time was called Philip, so my middle name is Philippa. I guess I should be grateful he was too young to have yet discovered Lord of the Rings or the like.
To be fair to him, he has since named his daughter Philippa, so he obviously knew his own mind from a very young age!
(for those still reading, I escaped unscathed by my sister’s choice too. I have her middle name as my first name, and it’s quite respectable).
Huh. My middle name is also Renee. Funny.
Julia Ann here. I don’t really think those two names flow together very well, but that just might be my mild dislike of going by Julia. I’m a Julie, dammit!
One sister has a middle “Marie.” The other has a middle “Jean.”
My middle name is Leigh - a ton of people of both genders in my age group (I was born in 1985) have Lee or Leigh as a middle name. I too have noticed that Grace is an extremely popular middle name for the 12-and-unders.
Anne, with an E. I have queens’ names: Elizabeth Anne
Middle-named after my mother AND it’s an “Ann” variant. Not sure what that proves.
I answered in behalf of my non-Doper wife, whose middle name is Ann. Her sister’s is Marie. My 2 brothers’ wives are Anne and Marie. One of my nieces is Anne.
Wow, what a boring family!
My family tended toward last names as middle names. American. But my uncle’s wife had a daughter “Bobby Lou,” who was born in the 40s. I think at that time, “Lou” was a very common middle name for girls (especially for southerners, which my aunt was), but it has since largely fallen out of fashion.
I agree that the prevalence of "Mary"s and "Ann"s as middle names is probably a Catholic influence. But Lynn is, like Lou, just a name that flows well with other names.
Mine’s Anne. Born in Canada in the 1970’s. All my female friends growing up seemed to have Lynn, Leigh, Anne, or Jane as middle names.
Mine’s Dawn but almost every friend I had growing up was either a variation of Ann or Lynn, and my best friend’s middle name was Marie. Renee was popular too.
It was Nicole but I changed it to my maiden name after I got married. Nicole was very popular as both first and middle name in Brooklyn in the 1980’s. Grace and Rose seem to be popular as middle names today.