They are Donald Trump’s secret political weapon. His campaign paid more than $5 million in September alone to Cambridge Analytica, which claims it can convince voters to back him by tailoring Trump’s political ads to their personalities.

Their approach combines micro-targeting, already in use in political campaigning, with psychological profiling. The company gathers up to 5,000 pieces of data about a potential voter to create a psychological profile, then adapts political ads to his or her personality and beliefs.

Using a survey usually placed on social media, the company invites users to take a personality test. The results of that test help the company group people under personality types measuring openness, conscientiousness, extraversion, agreeableness and neuroticism.

Data-crunchers then combine individuals’ personalities with their voting history, where they shop, what they buy — even what they watch on television.