I have written a memoir about my experiences as a military brat living overseas (Vietnam). I've been sending out query letters and receiving mostly rejections (got one request for a MS from BIG NY AGENCY -- they passed), but I haven't been including any demographic info about my primary target audience: the approximately ten million former US military brats living around the US and world today.
These brats (sometimes called Third Culture Kids) form a large, but mostly invisible sub-culture that has gotten little attention in the literary world. It seems like every week there's a new book about heroin addicts and child abuse victims. I know that I would love to read about the experiences of other military brats, but the shelves at B+N never have such books.
Question: Should I include this demographic info in my query? It seems to me, that if an agents is looking for something to sell, it might be good to make them aware of the huge, mostly untapped market for this type of memoir. On the other hand, I don't want to oversell the MS.
If even one half of one percent (or so) of the former military brats still living wanted to read this particular book, the sales would be in the 40-60,000 range. Does that make sense to you?
A lot of people like to put demographic information in their queries even when they don't belong there. In one case, someone had written about driving around in his RV and said that all 3.4 million (or whatever the number is) RV owners would obviously want to buy his book and that's why it would be a bestseller. That's something not to put in your query. That's something that's so funny that I might mention it two years later in a blog post.
In your case I would say giving a statistic isn't bad. Statistics are good if they're not well-known; I didn't know how many army brats there were before you told me despite knowing a couple. Other cases include rare diseases and other things we might have heard of but not know a lot about. However, it deserves a line and nothing else. It's not a selling point so much as useful information for us.
The reason it's not a selling point is that we know that 40,000 army brats aren't going to buy your book. The truth is some people don't like reading about things they already know; it's a turn-off. I don't care for reading about Crohn's Disease. Someone's saga of doctor visits and bowel resections and screaming, "$39 a pill for Zofran?!?" isn't news to me. People for the most part read books because the things contained in them are new and interesting. In the case of non-fiction it's usually because the topics are things they want to know more about. True, some army brats would probably buy your book, but not a whole lot. We wouldn't lean on that demographic for sales. We would lean on a demographic of people who want to know more about army brats, and off the top of my head I have no idea what that demographic would be.
The story sells your book, not the demographic. Exceptions are made for doctors writing books for patients and the like.
Also, Zofran is worth every penny.