Learning something new can make it harder to recall what you learned before

Retroactive interference happens when new learning disrupts recall of older information, the mirror image of proactive interference, where old memories block new ones. It's strongest when the new and old material are similar and learned close together in time. The concept dates back over a century, to Georg Müller and Alfons Pilzecker's 'rückwirkende Hemmung,' or 'retroactive inhibition.'

— Georg Elias Müller & Alfons Pilzecker, Interference theory — Foundational memory research, early 20th century