Ashkenazi Jews traditionally practiced a form of semi-arranged marriage (or "matchmaking") called Shidduch. Rather than people just dating each other willy-nilly, matches would be proposed to parents by members of the community or a professional shadchan (matchmaker). If the parents approve, the couple is set up on dates by the shadchan for a bit before the engagement is finalized. Unlike a full arranged marriage, in theory the kids have the ability to back out if they don't like each other. But everything about their relationship has been arranged for them by their families so there can be a lot of pressure to go along with the match that your parents and your community approve of, especially if it would be politically beneficial to connect two families through marriage.
This practice has fallen out of popularity in most American Jewish communities. Older Jews still see it as a mitzvah to try and set their single kids up with people, but you'd be hard pressed to find a Reform Jew employing a shadchan.
That said, it's still practiced by many families in more insular Jewish communities in Brooklyn or heavily Jewish suburbs. Growing up, I had a childhood friend who, at age 16, was already arranged to be with a particular boy her parents had picked out for her even though he lived in Brooklyn and we lived in Greater Boston. They wouldn't be formally engaged until 18 but it was planned. When she developed a crush on a goyish boy she knew and started dating him, thus calling off the shidduch, her parents semi-disowned her.
At the time, I thought it was an incredibly barbaric practice. I knew it was a part of our culture but I also knew that not even most Jews did this anymore and it felt so antiquated. Truly, I didn't understand why we ever even did shidduch in the first place? It seemed horrible even in the old world to do. The reason I read given at the time was "to match the right people together the first time to prevent divorce." Sure, it's a nice idea, but punishing a child for not going through with it seemed to be its own form of conflict worth preventing!!
Only as I got older did I come to appreciate the reason why we started doing Shidduch in the first place. To prevent incest. In isolated insular communities like shtetls, genetic diversity is very low, which leads to a lot of health problems that are prevalent in Ashkenazim.
The shadchan can easily keep track of who everyone is related to and not just match the people who are compatible but also prevent matching people who are too closely related. It's hard to know you're second cousins with someone, but the shadchan knows.
There's this new cultural staple of Ashkenazi Jewish communities specifically in America that we call "Jewish geography" or the "Jewish geography game." This is where whenever you meet a new Jewish person you try to find out who you know in common and if you know their family etc etc. A lot of younger more progressive Jews have been trying to downplay it because it makes it obvious when someone is a convert and tend to dig up sensitive issues like if you're on bad terms with family. But most Jews in America still do it, especially the baby boomer generation and Gen X.
The most extreme version of this I experienced was a urologist trying to pinpoint if he knew my parents while actively inspecting and touching my genitals. He determined that he did in fact know multiple of the kids I went to kindergarten with and some of my neighbors. But he didn't know my parents because we didn't go to shul since in the 90s interfaith marriages were still not permitted by the conservative movement.
This game has always been amusing to me but I only just now realized it's actually an extension of Shidduch.
Because with the American Jewish community spreading out over the continent, yet nearly all originating in NYC only a few generations ago, it's quite likely that sometime you'll meet in even another city could be related to you.
Playing the Jewish geography game with everyone you meet will cause you to learn not just if you know their parents (the usual goal) but who their extended family is and thus, if your are related to them. I don't think most people even realize that this cultural practice serves a function of helping us avoid incest but it absolutely does. It totally does.
Anyway that's my late night ramble on that.
EDIT: I think the point I was trying to make at 2am is that Jewish Geography took the place of Shidduch as a cultural practice for preventing incest. Instead of arranging marriages we just habitually start every interaction by trying to figure out who someone's parents are so we can know if we're related. Although younger Jews aren't doing this so much either, but also as generations go on it's not really as important to try to prevent incest because we have more genetic diversity now. Also with intermarriage and conversion becoming normalized, we have even more genetic diversity.