What are the two things that are most important to know about a stranger? Or a group of strangers?
Social psychologists know. But so did the early authors of D&D.
The stereotype content model, elaborated by Susan Fiske and other social psychologists, describes how we organize beliefs about other people and social groups - traits and stereotypes. Over the past 20 years, dozens of studies have supported the idea that two key traits, warmth and competence, are major players in our attitudes and behaviors toward other groups.
Warmth is how cooperative the group appears to us. Competence is how strong - how able to do meaningful things - they look. So, jolly halflings might be seen as high in warmth but low in competence. Dour dwarves are the other way around, not very warm but very good at what they do. Kobolds, maybe, are low in both.
The reaction roll is the warmth check. That's fairly easy to see. Do they see you as cooperative and will they be likely to cooperate in turn?
But what about the competence check? Well, competence in the dungeon is largely a matter of fighting. If I see you as better at fighting than me, I might run if things seem hostile. If I see myself as better and I don't like you, I might attack. In other words - competence is morale.
Each early edition of D&D has its own way to handle morale checks, but in general morale is tested mainly after taking some amount of casualties in combat. This reflects the origins of the game in tabletop wargaming, where it was assumed that units started the battle with enough courage to approach each other.
And never mind courage. Even rationally, by Sun Tzu's time-tested maxim, an inferior force should not approach a superior. So, in the probing and testing before battle, if any, is joined - that is when the first morale check, off-table, should happen. (Coincidence that this fog of war is exactly the reason for the position of wargame referee, which evolves into Dungeon Master?)
This all suggests that the first encounter between two forces should include not just a reaction roll from the non-players, but a morale check, to see which side they esteem as the more powerful. This will determine, for example, whether their response upon a negative reaction (from either side) is to attack or retreat.
And on the other hand, because both rolls are on 2d6, I'm tempted to make the reaction roll a "reaction check" - with an unfriendly attitude if rolled over the creature's rating, and both of these checks having greater effects if made or failed by a certain margin.
Next post: The combined reaction/morale chart.
6 hours ago