I would be more interested in a 'door height' study. In lots of rural areas you have doors that are clearly built with the expectation that nobody taller than 5

I would be more interested in a 'door height' study. In lots of rural areas you have doors that are clearly built with the expectation that nobody taller than 5'6" would darken the doorway. I believe hats were more prevalent in these pre-motorcar times so there was even less height consideration made. Doors tell their own story, big cities have been redeveloped (and bombed) whereas rural areas or places that have not got much of a look in since the early days of the industrial revolution have not. Given that there is no theory that fully holds water on why we grow tall (I personally blame the Green Revolution), you could probably blur correlation/causation in a 'door and mattress size' study to conclude that 'if we make the doors bigger then people grow to fill them'.