Speaking at a press event in Paris, Ms Mayer said: "Personalised search tracks and shows you in your search history the clicks and trends of your searches.
"Based on what we see as your searching pattern it ultimately can enhance your result.
"There's a simple way to turn it on and off. We will only use the data that a user gives us to target personalised search."
Google anonymises that information after 18 months and so the search engine has to re-learn the patterns of user behaviour.
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