Does the SEC really need to define 'best interest'?
Best interest. Act in the client's best interest. Put the client's best interest ahead of your own. Regulation Best Interest.
The "interest" part of that phrase is pretty easy to understand. Words such as well-being, benefit and advantage are tied to it. If something is in one's interest, doing it will better their situation (or at least not worsen it).
It's the "best" part of that phrase that's so hard to nail down.
Perhaps that's why the SEC decided not to define "best intere...