Court documents show how Google uses your Gmail messages to profile you.
Gmail profiling is performed by a mysterious device known as the Content OneBox, or COB for short. The COB’s inner workings are shrouded in mystery — the court documents describing it are heavily redacted. But we know that it deploys many distinct data mining methods to place users into these “buckets”. In addition to exploiting demographic or interest-based observables, the COB tries to understand the actual meaning of email messages with advanced “machine learning” algorithms.
It reads as if these algorithm do not sort users into pre-defined categories (i.e. housewives, members of the military, etc.) but make the categories themselves by finding which properties or keywords from messages typically cluster together for the millions of users that Google Mail has. This way, the computers do not need to be trained for each category specifically, but come up with millions of categorizations on their own.
In a second step (the article suggests), Google records which categories of users are more likely to click on which ads and then uses this data to optimize what ads you are shown.
Also keep in mind that Google not only analyzes the emails of many people, but through recording all visits of a user on any site which uses Google’s popular “analytics” product to record statistics or Google Ads, Google also feeds basically all your internet use into its data store.
You can avoid this, however: For email, quit Google Mail and use inexpensive privacy-friendly providers, such as Posteo or Mailbox.org. For your web-browsing, do not use Google Chrome, but instead Firefox, armed with Adblock Edge and Privacy Badger. As a search engine, try StartPage. Taking these simple steps will decrease the amount of data that Google gets from you considerably.