I’m not sure what the study says, but I can tell you that union electricians make more money than non-union electricians. This goes for all skilled construction trades: union plumbers, fitters (both pipe and sprinkler), and tinners all make more than their non-union counterparts.

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.