The internet meme has gone mainstream. Once exclusively shared by nerds on message boards, email lists, and other parts of the old-school internet, memes are now spread by the tech-unsavvy billions of Facebook and Twitter.

Memes are best when they are funny. But humor is defined by the people laughing, and some people on the internet are often tickled by hateful racism, sexism, and genocide, reveals a new working paper (pdf) that studies Twitter, Reddit, the /pol/ message board on 4chan, and Gab.

For a year starting July 2016, a group of seven researchers, with funding from the European Union, analyzed the four websites to identify which memes catch on in different kinds of online communities. They collected hundreds of millions of images, then classified the images as belonging to different meme families, like Expanding Brain or Evil Kermit.

The four sites they looked at are very different. Twitter is the most mainstream of them, and as such its memes are mostly innocuous, like the insights of the Roll Safe guy and the subtle anger of Arthur’s Fist.

On the other side of the spectrum is 4chan, an anonymous message board, and its /pol/ community. /pol/, or “politically incorrect,” is best-known for being light on moderating content and heavy on racism, sexism, and alt-right politics. Gab is similar to Twitter in how it works, but like /pol/, Gab is mostly for trolls and alt-righters who think their hate speech will be “censored” on other platforms. The memes on these platforms are mostly alt-right iconography, like Pepe the Frog, MAGA, and the anti-Semitic Happy Merchant. On /pol/, over 10% of all memes are Pepe-related, the study shows.

Reddit lies somewhere in between 4chan and Twitter. It shares some of the alt-right tendencies of /pol/, with many Pepe memes high on the list. That’s particularly true of The_Donald, the Reddit page for Trump superfans. But Reddit is also home to less political jokes. The most common meme on Reddit, for the period studied, was Manning Face, a photo of Peyton Manning looking disgruntled in a balaclava.

That these communities are different will not be news to anyone who has used them. But they can at times be similar. Sad Pepe the Frog appears in the top-20 list for all four, for example.

That’s because these distinct places often interact. /pol/ accounted for 4% of the memes on Reddit, and 3% of those on Twitter, the researchers found. But that only gets at part of the picture. The reality is that many memes begin in the dark corners of /pol/ and find their to the rest of the internet.

“It seems to be that a lot of memes move from /pol/ to The_Donald,” says Jeremy Blackburn, a computer scientist at the University of Alabama at Birmingham and co-author of the study, referencing the Trump Reddit page. “That’s a clear path of progression.” Because the racism of The_Donald is toned down relative to /pol/, memes can spread more easily from there to Twitter or other places.

“We are amazed by the power of these communities with regard to creating and disseminating memes on the greater web,” adds Savvas Zannettou, a PhD student at the Cyprus University of Technology and the paper’s lead author.

Another way to look at this is by looking at groups of memes that involve particular people. On this account, the study finds, Donald Trump is the unquestioned central figure of memes today. He is in the top spot for all four communities, the subject of more internet jokes than anyone else.

Still, the communities are not all the same. In second-place on Twitter is Barack Obama. On /pol/, the second spot belongs to Hitler.

The authors’ takeaway is that, while memes are essentially meant to be funny, they need to be reckoned with as sources of cultural influence. “The whole thing with memes is very serious; they are used to weaponize information and to manipulate opinion,” says Zannettou. “It’s not something that should be taken lightly.”