Rhinotillexis and mucophagy may help expose the immune system to weakened versions of pathogens. Depending on how well you clean your hands before and after, however, it may introduce new things into the nose, or introduce things in your nose to the people you shake hands with. There is also a minor risk of injury, mostly nosebleed.

Just wanted to say that this sort of interaction is important. I'm glad people question what they're taught and I'm glad that you have responded with a reasonable answer that explains the why behind this social norm. I think the reasoning behind social norms are really important to understand. Not only because they might help reinforce a positive behavior, but we might also come up with a better solution when we discuss it.

Its possible that nose picking and similar behaviors by kids are important for helping build the immune system.

Yes, I think if you washed your hands immediately afterward every single time, that it would be totally fine (from a first principles standpoint).

I'd estimate the majority of what I teach my son is cultural norms.

"No, there's no good reason you have to do it this way, but you do."

I don't actually say that to him (he's 3), but I say it to myself a lot. And the "Don't pick my nose, either" isn't a joke, that's verbatim.

I think it is accurate to say things that boil down to "other people will expect you to {xyz thing}", which is not "no good reason".

Picking your nose is introducing pathogens from the part of your body that encounters the most germs, straight into the part of your body that has no barriers or stomach acid to repel them. I think if early humans had lived in large enough societies to have a lot of transmissible disease, we would have evolved an instinct against touching your eyes or nose by now.

I guess the main reason is that picking nose leaves the finger covered with mucus, and the risk is that the mucus could be spread around - potentially passing on germs.

I suppose if people used hand sanitizer after every nose picking, then maybe it be less of a problem?

If (say) you're starving to death in Donner Pass, then go ahead. Similar for drinking your urine - if at sea in lifeboat and dying for lack of non-salty water. Otherwise, violations of "eww, ick" social standards tend to be punished harshly.

It seemed really weird for a few years, when conspicuously talking to yourself went from being a "this person has a serious mental illness" indicator, to being a "this person has a cell phone" indicator.

I really wish we had stayed in the "this person is an inconsiderate **" intermediate phase.

Obligatory reference to Breaking Bad[0].

[0]: https://m.youtube.com/watch?v=IImhYDUYmzM

I have a distant memory of frequent enough nosebleeds during the dry seasons that the school nurse was worried about me.