"Using a single earbud is bad for your hearing. Due to psycoaccoustic effects, you will end up turning the volume of a single earbud way up, to the point that it will, over (not much) time, damage your hearing."
Is this for real? He came to me with the question, and I really don't know. While I don't pretend to be an acoustics expert, I have never heard anything like this. Seems like hooey to me, but I'm usually wrong so I thought I would put it out there to those smarter than me here at DIYMA.