I could swear that I have posted an interview about this research before, but I cannot find it.
The report referenced in the interview is Searching for the Supposed Benefits of Higher Inequality: Impacts of Rising Top Shares on the Standard of Living of Low and Middle-Income Families by Jeffrey Thompson and Elias Leight of the Political Economy Research Institute (PERI) of the University of Massachusetts, Amherst.
Actually, coauthor Jeffrey Thompson states that the research shows that there is a correlation between the rich taking a larger share of the economy and the rest getting less income, not just a lower share of the economy. This is a much stronger result than comparing the share of the economy going to the rich and the share going to the rest. This last result would be merely stating a tautology. (Tautology is the saying of the same thing twice in different words. Tautology also means a series of self-reinforcing statements that cannot be disproved because the statements depend on the assumption that they are already correct).
The tautology leaves open the possibility that the rich getting a larger share could still leave the rest with more income if the economy had grown sufficiently to compensate for loss of share. The research specifically says that this did not happen.
The headline itself, does not pinpoint the actual result either. As the economy grows and the shares stayed about the same, as it has done in the past, the rich would get richer and so would the rest of us.
So to make sure we are not confusing correlation with causality, you could say that things that have occurred in recent history to make the rich get a larger share of the economy make the poor poorer. Which just means that you have to look at the various factors that caused the rich to have a greater share. You can’t just blindly make the rich have a smaller share as it might not help the poor.
The researcher mentions causes to be looked into. One that he did not mention was the increase in GDP from the rich making money by investing in financial derivatives. The GDP increase is on paper only. The increase comes from the bubble caused by such investment. The huge temporary gains divert resources from more truly productive areas of the economy. The diverted resources include top intellectual talent that could be doing more for the world than figuring out how to profit from high-frequency stock trading. This top talent could be inventing products that would benefit many more people than just the top 1% of the wealthy.