" A third principal states that the more intense the interaction between a student and its models, the more effective the training. Intensity - the extent to which tutors arouse a response in a student - is determined from direct observations of interactants (e.g., by recording emotional responses) or from indirect measures (e.g., blood pressure or hormone levels).
One implication, supported by data reviewed in Pepperberg and Neapolitan, is that, for both humans and birds, intense interaction requires one or more tutors. Of course, increasing the intensity of the interaction may not always increase learning: overly nurturant models may inhibit learning by preventing a student from experimenting on his or her own and overly aggressive models may arouse fear or counter-aggression strong enough to block processing of any input. "

― Irene M. Pepperberg , The Alex Studies: Cognitive and Communicative Abilities of Grey Parrots