Led by biologist Ana Catarina Vieira de Castro, researchers from Universidade do Porto in Portugal analyzed the behavior of 42 dogs from schools that used reward-based training as well 50 dogs from schools that focused on aversion-training. Over time, dogs that had trainers who yelled and jerked their leashes were determined to be more stressed, with higher levels of cortisol — a steroid hormone that regulates a wide range of processes throughout the body — found in their saliva.
The research team also used video recordings to test the dogs for"stress behaviors." They discovered that the dogs in the averse-based methods group"displayed more stress-related behaviors, spent more time in tense and low behavioral states and more time panting during the training sessions, showed higher elevations in cortisol levels after training and were more 'pessimistic' in the cognitive bias task than dogs from group reward.