Is it possible that anyone will have an effective income tax rate of 37%? Why or why not?

Not really.   When you are in the 37% bracket , it means all of your income ABOVE  518,000 is taxed at 37% rate......all of your income BELOW 518000 is taxed at lower rates , so your EFFECTIVE rate will be less than 37%.     If you earn A LOT of money , your effective rate will get closer and closer to the 37% mark.