Why Memories Compete
When old knowledge crowds out new (and vice versa)
Your memories don't exist in isolation. They compete with each other. Proactive interference happens when old memories make it harder to learn new things (like dialing your old phone number when you mean to dial the new one). Retroactive interference is the opposite: new learning erodes old memories. This is why similar information is so much harder to keep separate than entirely different information.
Move offices, and you'll spend a week walking toward your old desk. Learn a second language, and the first one starts to slip. Park in a different spot every day, and you'll lose your car at least once a year. Interference is also why studying two related subjects back-to-back (Spanish then French) is harder than studying unrelated ones (Spanish then math). When memories share features, your brain has to fight to keep them straight.
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Interference