Astrology is experiencing a resurgence, particularly among Gen Z: Co-star, an app which offers detailed and personalized forecasts, has been downloaded by 25% of women aged 18 to 25 in the US. The app requests information including the location and precise minute of the user’s birth, runs it through an algorithm and produces “hyper-personalized” advice about various aspects of life, as well as an extensive personality analysis. Despite Co-star touting its use of NASA data, it is widely accepted that there is no scientific basis for astrology. So what is the appeal?

It may well be a classic example of the Barnum Effect, a phenomenon whereby people identify strongly with even the vaguest descriptions of their personalities. In 1949, the psychologist Bertram R. Forer administered a ...