Starting from now on, Google personalizes search results of all users, even when they are not signed in. The search engine learns which links individual users click most and will rank corresponding sites higher in the future.
If you’re searching for recipes and often click through to the results on a specific website, the search results will show results on that site more prominently, according to Google’s idea. This same idea is already working for a while for users that are logged in and have enabled the Web History feature in their Google Account. From now on, search results are personalized for users that are not signed in, too.
To achieve this, Google makes use of the search history within the last 180 days, by using an anonymous cookie. Google confirms that these data are not linked to your Google Account, but handled separately.
You can see that a search result is personalized when you see a link “View customizations” in the top right edge of the search results page. Clicking the link informs you about how the results have been personalized. You can also disable the personalization there.