A few recent events reminded me of the danger of false presumptions.
Leakage
I received an egregious water bill due to leakage of my house. I tried to isolate the leak by shutting off all sources (e.g. faucets, toilets, sprinklers) one by one, but the meter kept on running.
I thought it must be due to some leaking pipe hidden somewhere in the house. And likely some obscure location; I have put my ears against the walls and could not detect any dripping sounds.
The first group of plumbers came, and could not find anything as well.
The second group of plumbers came, and immediately figured out the sources. Yes, that is the problem: there are multiple concurrent sources of leak instead of just one. That is why I (and the first group of plumbers) could not identify the issue, because we all assumed there is only one source.
Wedding
A friend visited me a few days ago. During the dinner, he said he wanted to share a gossip with me. He got married. I said congratulations. Then he said he married a guy not a gal. I said congratulations again. But I also admitted that I was absolutely surprised because for all these years I thought he is a straight guy and I could not detect a single trace of homosexuality in him. (I thought I would know, given that I have been living in the SF Bay Area for like 15 years.)
So, given my interest in studying humans, the rest of the dinner naturally turns into detailed QA about psychological and physiological characteristics of gays. I have a lot of false presumptions corrected on spot.
Jockey
I tried to find some gossip about me in return to my friend above, but I could not find any, except for the (already public domain) fact that I am within the paternal line of Genghis Khan. My friend’s immediate reaction was: you do not look like a Mongol at all because your physical size is too small.
I guess I am made at least 90 percent of Han Chinese (only the Y-chromosome is passed unchanged from my paternal side of ancestors), so this is no surprise.
But upon further thinking, it is not clear to me why the Mongols, at least my ancestors around the time of the massive expansion, necessarily have large physical sizes. It is probably a false presumption caused by their reputation and images of Mongol wrestlers.
I am not a historian or anthropologist, but the Mongols main military strength lies in the cavalry, not infantry (which mainly consisted of captured slaves anyway). So, it makes more sense to have a small rather than large body build for a jockey who needs to ride from Manchuria all the way to Europe plus round trip, right?