A new study suggests that dog training methods based on negative punishmentscan cause long-term harm to the animal.Much research in the past has studied training methods in general,including for dogs working with police or with search and rescue operations.But the latest study centered on dogs kept as companion animals for humans.Such dogs are often considered part of the familyand need training on how to behave around people.Researchers from Portugual's Unviersity of Porto led the study.The researchers carried out experimentsinvolving two kinds of dog training methods – aversive and reward-based.Aversive methods depend on the use of some kind of negative actionin answer to unwanted behaviors.Examples of this include shouting, pushing or pulling the dog to force it to do somethingor using special collars that put pressure on the neck.Reward-based methods involve giving the dog food, praise or attentionwhen the animal completes wanted behaviors.The study included 92 dogs that were attending training schools in Portugal.Fifty of the dogs received aversive training,while 42 were trained using reward-based methods.The experiments were designed to measure both short-termand long-term effects of the two training methods.The dogs were video recorded during training sessionsso researchers could observe their reactions to the training.Researchers also collected mouth fluid from the dogs before and after the trainingto test for levels of a stress-causing hormone called cortisol.Researchers reported that dogs from the aversive training groupwere observed to have more stress-related behaviors than those in the reward-based group.They also showed increased levels of cortisol.The study suggests these results clearly demonstratethe short-term effects of aversive training methods.Another part of the experiment was designedto measure the long-term effects of the two different methods.This involved the dogs taking part in an exercise about a month after the training sessions.The dogs were put in a room containing food bowls.Researchers observed how quickly and excitedly the dogs went to the bowls.The researchers reported that the dogs receiving aversive trainingwere observed to be more "pessimistic" in behaviors in the roomthan the ones trained with rewards.The latest study supports earlier research on the effectiveness of reward-based training.Many other studies have suggested that food is the best rewardto get dogs to perform the behaviors we want.One of those studies was led by Erica Feuerbacher,a professor at Virginia Tech, in Blacksburg, Virginia.Her study compared a food reward to a reward of petting or praising the animal.Feuerbacher told The Associated Press the dogs were clear about what reward they liked better."They'll work harder and respond faster for food than for social interaction," she said.Feuerbacher noted, for example, that research has found that dogs were likely to stay near a personpraising them for the same amount of time as if they were being ignored.Zazie Todd is the writer of a forthcoming book called "Wag: The Science of Making Your Dog Happy."She told the AP that people clearly should not expect a dog to obey just because they love them."If only it was like that," she said.Todd added: "If your boss stopped paying you,you'd probably stop going to work pretty quickly. You need to motivate your dog too."Some dog trainers teach the use of "life rewards,"which could include play or taking the dog for a walk.Todd says these can be useful, especially to help keep behaviors the dog has already learned.However, for most everyday behaviors most people want to teach,food rewards are just "quicker and easier," she said.