How many are too many? Optimal expansion of the professional sports league

How does a professional sports league decide on the number of teams to let compete? Even if allowing for each team to be a local monopoly, the increase in the number of teams in a league will reduce the profits of existing teams through reducing the playing talent per team. The league will choose to expand the number of teams if this talent pool increases (or if income increases or costs decrease). The last 50 years of expansion of the MLB, NBA, NHL, and NFL are inspected in light of these predictions. The findings are that the m odel is consistent with the pattern of expansion over the last 50 years for these leagues.