This is a poorly done study. Doesn’t even control for profession. Take union electricians vs. fast food workers. Union electricians will make much more, like the article it saying. So what? What’s the conclusion here as it relates to unions?
Prevailing wage requirements are mandated in government funded construction contracts specifically because non-union labor is paid less and the government is willing to pay living wages for their projects by forcing non-union contractors to pay the union wage + fringes.
FWIW I manage union electricians at a NECA member shop that hires union labor from IBEW locals and only subcontracts labor to union subcontractors.