Search history including Google search history, I’ve found.

I also completely believe that YouTube correlates via IP for at least not-logged-in views (or at least tries to associate to accounts even if they've never logged in); I get bleedover to my iPad from my completely disassociated PC but not my Mac that’s logged into a different account.

(Also maybe I put too much effort into tailoring my own YouTube recommendations, but 99% of the time when they start going awry, I have a pretty good idea what triggered it. Random coincidences don't happen...)

Like others have commented, I too am very certain Youtube uses IP address. My family and I are behind the same cable modem, and oftentimes my family's Youtube "related videos" will start popping up in my Youtube home page, and vice versa, even though I have no interest in or have watched those videos.

Interestingly, I seem to recollect this happening only in the past few months or so, maybe a feature they turned on recently?

> I kinda doubt the last two though. That would provide very little benefit and be a huge privacy risk.

To the contrary, I highly doubt that they don't use it, privacy risks be damned. Using that data is their primary business model after all.

I’ve had enough “coincidences” of the same type that I 100% believe it’s occurring. It’s the simplest answer actually.

Occam's razor I agree.

Easy enough to test, just start talking with another person for about 2-5 minutes straight repeating the words purple dog collar over and over in earshot of the device you suspect.

You'll be amazed when within 30 minutes to an hour all the ads on all devices will suddenly show purple dog collars or pet related items.

Really? I info-dump on my partner all the time about all sorts of specific products. She’s never mentioned one randomly pop up in an ad. I can honestly say I’ve performed your test thousands of times.

As a counter-experiment, try thinking of some product and intentionally DON'T search for it or say or type it anywhere. Make sure it's also not something you heard about recently from a friend or in the media. If you then see ads appear for that product you can be sure it's just baader-meinhof.

Talking with another person won't test whether they're correlating via IP or location.

I have found out that google uses atleast 1 of the last 2 "doubt" ones. My sister was watching my little pony on her laptop without being logged in and her reccomendations would "leak" into my reccomendations on different computer with logged in account. It was very annoying and even flagging the videos as "not interested" or "don't show" wouldn't help much. I think I "fixed" it by disabling yt history or something. I am not sure. This was 2...3 years ago

They do correlation with twitch though. Went there once to support a charity stream, and I got flying sims and among us in recommend for weeks (or maybe only a week?)

I think if the IP is detected as used by one person the recommendation engines relies on it more heavily. For example in an office the same IP is used by many so its weight in the recommendation engine is relatively low. Additionally more and more ISPs are using longer lasting DHCP leases. I know my Comcast IP hasn't changed for over a year. This can better tie a person to the same IP.

There are 6 people using my ip regularly. We all get a mishmash of eachother's YouTube recommendations.

I'm frankly surprised to find some people in this thread don't already know that ip correlation is going on as a matter of course.

>Random coincidence, which you notice in the one case where it happens but not in the 99 cases where it does not.

In addition, there could have been 100 instances in the past but they didn't stick.

I would insert for 2nd place:

- The system knows you two are friends (there are multiple ways for that), and you get recommendations based on what interests your friends