This is not always stated explicitly, but will turn up in board minutes if you have access to them. Many professional boards have added a DEI committee or incorporated language into their mission statement in recent years as well (Oregon bar statement: https://www.osbar.org/diversity/programs.html).
I'm never clear whether the hypothesis is "more DEI = more efficacy && competency," or whether the hypothesis is "benefits of (DEI) > benefits of (efficacy && competency)." The former hypothesis at least seems more testable, but I'm not sure whether anyone is trying very hard (meta-analysis: https://academic.oup.com/tbm/advance-article/doi/10.1093/tbm...).