I thought that it was primarily found in people with Down syndrome (but, not that it necessarily means someone has Down’s). I could very easily be wrong, though.

This also allowed them to be more experimental in the studio. The songs they recorded during this period were not intended to be played live, so the arrangements and instrumentation could be far more complex. It wasn’t until “Let It Be” that they wrote songs they intended to play in front of an audience again, which explains the simpler arrangements on that record.