This study was done on dogs but I think the results are even more valid with parrots because dogs are not only a domesticated species that has been bred for thousands of years to be human- oriented AND they belong to a hierarchical society where learning rules and obeying a 'leader' or 'alpha' is part of their genetic make-up. And the study found that aversive techniques create not only much more stress during the training but also affect the animal's outlook, making the dogs that were training harshly 'pessimistic' toward humans intentions, in general.
https://www.mnn.com/family/pets/stories ... a-40378997