Traditionally, fish has been thought of as a 'brain food', though this is in fact wrong. A couple of chemists from the 1800s thought that intelligence was increased by eating phosphorous, which fish has a lot of. That turned out to be false, but the myth stuck, because people never look into this sort of stuff.
I guess people need to eat more fish.
Sources:
Mailbits.com
Ever Wonder Why? By Douglas B. Smith