I don’t know…I wave or nod at everyone, all the time. Walking in a park. Walking in my neighborhood. One day in a city park, but not our usual area, my wife said I have got to stop. I asked why…flabbergasted. And she said, if she were in a park by herself, she would never make eye contact or wave generically because as a female, she doesn’t want to portray vulnerability to strangers. Look tough and in charge.
Maybe women learn early on that they might be targets, even if it’s very unlikely. So they keep their guard up. And it just carries over to other random things like driving.