Wouldn't the fourth theory only make sense if companies routinely hired executives internally? In the old model of "assembly-line worker eventually works his way up to Ford CEO" I could see high compensation at the top being used as a kind of carrot to keep people motivated about climbing the internal career ladder, instead of jumping elsewhere. But afaict most companies nowadays don't hire execs from internal candidates. The COO in question here, for example, was not a Yahoo employee prior to being brought in as COO (he was hired away from Google, not promoted from within Yahoo).