If you were to analyze the ownership of fixed income savings instruments, I think you would find that the rich own disproportionately more. In other words, bonds make up a higher percentage of wealthy people’s portfolios than they do for middle class (or poor) people. Include indirect ownership through pension funds etc.

I admit I have no source for this, but I recall seeing it several times over the last 20 years from a variety of decent sources, so I think there is real data to support it. If I look, I will post any sources either way.