That said, are high income earners more available for conversation? Seems like a good question for economic researchers within child development.
Anecdotal (I would most likely qualify as high-income for the purposes of that study): Most of the time when I am with my now-18-month-old daughter, I give her a lot of authority. If she can say where she wants to go or what she wants to do or what she wants me to do, then in general that's what happens. I have to imagine that creates a great incentive to learn to express your thoughts.
Knowing this helps parents with limited time to know that it's better to focus on back-and-forth than to just read more books all the time.