I don’t have any real opinion about the direction of causality. If anything the idea of unions contributing to more wage equality makes sense to me. But, linking to a single study as “proof” of anything is a big no no. It is very easy to cherry pick studies so that you get the “true” outcome. The only way to make a halfway convincing argument is to look at every piece of evidence on the question and aggregate them. This is why we believe global warming is man made and severe despite a few studies that suggest otherwise. One other caveat I would add is that if you read the linked study, they’re only reporting correlations. There doesn’t appear to be a causal identification strategy there. Which isn’t to say it isn’t interesting and worthwhile. Only that one should take the results with an appropriate level of caution.
