&gt; No economic rule I know of guarantees that to be the case.<p>Certainly there is no economic rule, however, I think that most people generally believe that for most of human history it has been the case that at least in the medium-to-long-term, top line growth does accrue pretty cleanly to the middle 50%.<p>There are certainly some people that are starting to wonder if this is no longer the case but personally I&#x27;m always pretty skeptical of &quot;this time it&#x27;s different!&quot; arguments so am unconvinced by the evidence to date. Who knows what the future will hold though!