Flat Design vs. Traditional Design: Comparative Experimental Study (2015)
As a personal anecdote, I've spent quite a bit of time watching toddlers interact with the world.  In my experience, if it looks like a button, a toddler can tell it's a button and will be interested in pushing it.  (And might even say "button"!)  If it looks like a switch, a toddler will be interested in switching it.  If it looks like a button on a touch screen, a toddler might still think it's a button.  If it's a flat blob that looks like every other flat blob designed by that designer (but looks nothing like a flat blob by other designers!), toddlers don't recognize it as a button.  Shocker, I know.<p>Obviously, the majority of computer users aren't toddlers, but I think there's a lesson here.  I really miss the old Windows 3.1 / 95 / 98 days when controls were more or less standardized and you could reliably tell which were buttons, which were radio buttons, which were okay buttons, etc.