the 16th amendment creates the apportioned requirement at the federal level.<p>Also the expansion of the commerce clause &quot;the switch in time that saved 9&quot;  also contributed when used in concert with the 16th amendment  see<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Wickard_v._Filburn" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Wickard_v._Filburn</a>