Browsing YouTube on Wednesday night, I just happened to click on a trailer for the new horror flick Hereditary. The movie looks promising, so I ended up watching a couple of trailers for it.

On Thursday I found one of the trailers autoplaying in a sponsored ad in my Twitter feed, and I very rarely see ads for random movies in my feed. Naturally I wondered if I was being retargeted based on my views of the trailers on YouTube. It’s very possible, and also very difficult to find out for sure. Why don’t I get to know this?

That’s the problem with “free” services like Facebook, Twitter, and YouTube. They aren’t free. They show you all the benefits they provide but they hide the costs—the use of your personal data.

That Hereditary ad gave me an idea. When I go to my Amazon VOD page, there’s a row of movie suggestions the company’s algorithms think will match my tastes. Just above the row of movies there’s a line of text that tells me the personal data that was used to target me. It might read: “Because you enjoyed Spaceballs . . .”

While Amazon targets me with movie content, Facebook and Google target me with ad content. These advertising giants should be required to label ads on their networks with the personal data used to target the ads, and the origin of that data.

If an ad was targeted based on basic demographic information collected about the user by Facebook, the ad should say so. If an ad was targeted based on some snippet of information inside the user’s Gmail, the ad should say so.

If the government wants play a role in protecting consumers from surveillance-based companies like Facebook and Google, this is a simple and clearly defined way of doing it.