It takes a brave character to ask a new partner to take a personality test. But thanks to a study from the Oregon Research Institute, science can accurately predict whether your boyfriend will eat his vegetables or if your girlfriend is into online trolling.

First, you need to corral your crush (or your colleague) to “answer a few fun questions.” After 100 queries—you might need to break out the prosecco midway—most run-of-the-mill online personality type tests will tell you which Big Five persona your partner possesses: Openness, Conscientiousness, Extroversion, Agreeableness or Neuroticism.

The general traits for each are obvious. Conscientious characters are methodical and reliable. Neurotics are unstable and tense. But a new in-depth investigation discovered how each persona...