This trait is first available as a child, and it conflicts with the mean and slob traits.
These Sims tend to disapprove of other Sims' improper behavior such as Mean or Mischievous socials, but find themselves happier in Formal Outfits and have a much easier time with Friendly and Romantic socials.
Dressing in formal outfits will grant proper Sims a happy moodlet.
Proper Sims will bow when they greet other Sims, and the "Friendly Introduction" option is replaced with "Respectful Introduction".
More "respectful" romantic interactions also replace the usual ones:
Attempt to Swoon
Propose Scandalous Question
Proper Sims will refuse to use a hot spring until they have bathed or showered first.