What actually causes deja vu?

The answer

Deja vu, experienced by approximately 60-70% of people, most likely results from a momentary mismatch in memory processing. The dominant theory, supported by cognitive psychologist Anne Cleary at Colorado State University, proposes that deja vu occurs when a current scene shares spatial geometry with a genuinely experienced but consciously forgotten memory, triggering familiarity without recollection. A 2016 study at the University of St Andrews using fMRI found that deja vu activates brain regions associated with conflict monitoring - the prefrontal cortex was checking the hippocampus's false familiarity signal. This suggests deja vu is actually your brain's fact-checking system working correctly, detecting and flagging a memory error in real time rather than a glitch.