Specifically I'd like to know when you would say "at the behavioral level" and when "on the behavioral level." It feels like there is a difference, but I can't put my finger on it.
Google's Ngram shows that "at the [something] level" is more idiomatic than "on the [something] level":
This is a lot like being "on a bus" as opposed to "in a car". If you are able to move around, then "on" is preferred, although there's a grey area for middle-sized conveyances, e.g.
- He arrived on an A380.
- He arrived on a Learjet. He arrived in a Learjet.
- He arrived in a Cessna.
"At" fits into a scheme of movement, e.g. you get on an elevator and go up. You arrive at the fifth floor. When you step off the elevator, you are on the fifth floor.
For a general term like level it depends on context. If you want to emphasize that your description has moved from one level to another, use "at". If you are intending to talk at length about a level, use "on".
Well, first there's the idom "on the level," which means fair, not cheating, not corrupt.
"Can I trust him?" "Definitely. He's on the level."
"At the level" can mean "operating at this level of skill" - for example: At the level of softball my girls play, you're just happy they run from first to second without getting distracted by a passing butterfly.