So there are a few potential skewing factors here.
Yea…..there’s more than a few. I called out only one of them. But, I’m waiting for the book to come out. :-)
On unionization: generally levels of unionization will disassociate from income levels once sufficient labor market protections are in place. With most market abuses curtailed, labor market outcomes will only be distorted by rent-seeking (by either unions or business) in non-competitive environments.
I’ve made this argument several times over the years. By the 1970’s, unionism existed only to raise wages; all the working conditions goals that were at all reasonable had been achieved. With the advent of international competition, the corporations could (and did) make the case that further wage gain put business viability at risk; then actual demonstrations of this in real life (steel industry) showed up. At that point, the unions had difficulty justifying their union dues to the rank and file, who were seeing less and less return on their money.