Drinking milk (or not)
Interesting tidbit via Wikipedia:

"Since the majority of northern Europeans and some Mediterranean Europeans have the mutation rendering them lactose-tolerant, lactose intolerance is widely regarded as a medical condition in Europe and North America."

"Since the first nations to industrialise and develop modern scientific medicine were dominated by people of Western and Northern European descent, adult dairy consumption was long taken for granted. Westerners for some time did not recognize that the majority of the human ethnogenetic groups could not consume dairy during adulthood."

Also:

"Approximately 70% of the global population cannot tolerate lactose in adulthood. Thus, some argue that the terminology should be reversed, lactose intolerance should be seen as the norm, and the minority Western European group should be labelled as having lactase persistence."

I'm lactose intolerant, and I always just assumed that I was kind of weird for not being able to drink milk past childhood. (I had also surmised that this was an Asian thing, since a lot of the Asian people I knew were in the same boat.) But I had never considered just how prevalent my "condition" was.

(Read the full Wikipedia article on lactose intolerance).