All studies in the social sciences are problematic. The more interesting their results, the more ambiguous they are, and the harder they are to replicate.

What's more likely, that poor people would cheat out of greed or out of desperation? Rich folks can't use desperation as an excuse. Greed for them is a luxury.

The notion that bad behaviour is caused simply by human nature is an obfuscation. Greed is celebrated in the upper class, whereas what predominates in poor classes is more like resentment or desperation.

The point about the impossibility of earning a billion dollars ethically is a commonplace:

https://www.ft.com/content/3729c71e-2419-11e9-8ce6-5db4543da632

https://pittnews.com/article/175763/opinions/opinion-there-are-no-ethical-billionaires/

https://medium.com/to-the-pxint/there-is-no-such-thing-as-an-ethical-billionaire-4dd2a6296e2e

https://www.currentaffairs.org/2017/06/its-basically-just-immoral-to-be-rich