My theory is that most humans have been colonized with alien mind-control slugs that hold the earbuds for them, and the ones who can't wear earbuds are the only surviving free ones.

"Ears", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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My theory is that most humans have been colonized with alien mind-control slugs that hold the earbuds for them, and the ones who can't wear earbuds are the only surviving free ones.