While I think you also meant to say that they’ll call superiors by their first name, I just wanted to say that in the Army, officers calling same rank and subordinate rank officers by their first name is common practice. So, down the chain, a captain might call a lieutenant by their first name, then the lieutenant will say “yes sir?”
Some officers have a sort of unofficial one pay grade first name limit. For instance, when I was a lieutenant, my boss (a Major) called me LT Jman or just LT. As soon as I made Captain (where I’m at now), he calls me by my first name. Odd. Anyway, I call most lieutenants by their first name. Of course…a lot of them are friends of mine, since we were LTs together and I got promoted first, so it’s not that odd.
I meant both. I think that it is taking somewhat of a liberty to call someone by their first name, superior or not, and it isn’t done unless there is an established and favorable history of working together. This is true even in the civilian world, to some extent. I’ve known some NCO’s and officers to get fuming mad when superiors used their first name “as if we were bunk buddies or something.” I do think this was often an indirect expression of resent for the all-around slackness of degreed medical officers. People who got their rank through hard work and military professionalism don’t like to be reminded of those who received it for what they see as no effort. That’s why gung-go medical professionals are esteemed among the line units a lot more than the slack ones.
I’ve observed this first-name calling in the Army, though. In some situations I felt it was a way of creating a sort of paternal context to the relationship, and in others it felt like a demonstration that the unit cohesion and professionalism was so tight that titles didn’t need continual verbal reinforcement. In both cases I felt like it was almost intentional or calculated.
Of course a lot of times, like you said, there’s a history of out-of-work socialization that bleeds over into the workplace, or a culture where everyone has been there so long that a significant rank differential can develop between people who were equals, or near equals, for longer than they have been of different ranks.