I disagree based on personal experience. I am currently a line manager who is a developer himself and manages 5 other developers and my salary is not the highest and I have absolutely no issue with that. One of the devs has been with the company for 8+ years, has knowledge of literally every corner of the software and has a great work attitude which adds a lot of value by helping other devs to become as productive as possible. This person earns a lot more than me and I have no issue with that, because as a line manager I am more afraid to lose this person than anything else.

EDIT: My main responsibility is to keep the team happy and productive. If this developer would leave the team our productivity would drop and we wouldn't achieve our goals we set for ourselves this year. It would have a direct impact on my bonus as well and it is in my highest interest to keep great people at every possible cost. If I think I don't earn enough then I will have that chat with my line manager, but I will never try to destroy my team based on some personal ego crap.

Agreed. But I think that's why encouraging a servant leadership model in your company is beneficial.

Which is better for the company? (A) Leader acts in such a way to make him or herself 150% effective, or (B) Leader acts in such a way as to make all of his or her direct reports 115% effective?

If a leader can't make their team more effective, then why have them in leadership?

(Edit: Essentially, what dustinmoris said!)

It's true that some supervisors operate in this manner (i.e. oppressing employees based on their own compensation), but those are egregiously sub-optimal supervisors indeed.

Even if they are accepting of you earning more, their own boss and/or Human Resources may well have a different opinion. Woe unto those whose supervisors are also underpaid.

Can you provide a source for your "90%" assertion, please?