The studies most often cited to support this finding are studies of London bus drivers vs conductors (ticket takers) on the double decker London busses. They te

The studies most often cited to support this finding are studies of London bus drivers vs conductors (ticket takers) on the double decker London busses. They tend to be from similar socioeconomic backgrounds, start at similar young ages, and so on, controlling for many confounding factors, but the drivers spend all day sitting, while the conductors spent most of the day on their feet. However, when trying to find the actual study to see if they had controlled for any pre-existing conditions, I found this paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1008580/pdf/brjindmed00119-0013.pdf http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1008580/pdf/brji... which seems to indicate that in fact, people of heavier builds were more likely to become drivers than conductors, and conductors with heavier builds were more likely to switch to becoming drivers. So actually, it seems that the original study may be flawed, in the way you describe. While it did study people from a young and healthy age, it hadn't considered pre-existing weight or genetic differences between drivers and conductors, and it looks like that may actually play a substantial role in the difference in findings.