When dog-parents spend extra time scratching their dogs' bellies, take their dogs out for long walks and games of fetch, or even when they feel constant frustration over their dogs' naughty chewing habits, they are gradually shaping their dogs' personalities. Dogs, like people, have moods and personality traits that shape how they react in certain situations. New findings from Michigan State University went where few researchers have gone before to reveal that, also like humans, dogs' personalities likely change over time.

"When humans go through big changes in life, their personality traits can change. We found that this also happens with dogs - and to a surprisingly large degree," said William Chopik, professor of psychology and lead author. "We expected the dogs' personalities to be fairly stable because they don't have wild lifestyle changes humans do, but they actually change a lot. We uncovered similarities to their owners, the optimal time for training and even a time in their lives that they can get more aggressive toward other animals."

Additionally, Chopik found that dogs' personalities can predict many important life outcomes. For example, canines' personalities will influence how close they feel to their owners, biting behavior and even chronic illness.

The research, published in Journal of Research in Personality, is one of the first - and is the largest - studies of its kind to examine changes in dogs' personalities. Chopik surveyed owners of more than 1,600 dogs, including 50 different breeds. Dogs ranged from just a few weeks old to 15 years, and were split closely between male and female. The extensive survey had owners evaluate their dog's personalities and answered questions about the dog's behavioral history. The owners also answered a survey about their own personalities.

"We found correlations in three main areas: age and personality, in human-to-dog personality similarities and in the influence a dog's personality has on the quality of its relationship with its owner," Chopik said. "Older dogs are much harder to train; we found that the 'sweet spot' for teaching a dog obedience is around the age of six, when it outgrows its excitable puppy stage but before its too set in its ways."

One trait that rarely changes in age with dogs, Chopik said, was fear and anxiety.

Honing in on the saying, "dogs resemble their owners," Chopik's research showed dogs and owners share specific personality traits. Extroverted humans rated their dogs as more excitable and active, while owners high in negative emotions rated their dogs as more fearful, active and less responsive to training. Owners who rated themselves as agreeable rated their dogs as less fearful and less aggressive to people and animals.

The owners who felt happiest about their relationships with their dogs reported active and excitable dogs, as well as dogs who were most responsive to training. Aggression and anxiety didn't matter as much in having a happy relationship, Chopik said.

"There are a lot of things we can do with dogs - like obedience classes and training - that we can't do with people," he said. "Exposure to obedience classes was associated with more positive personality traits across the dog's lifespan. This gives us exciting opportunities to examine why personality changes in all sorts of animals."

Chopik's findings prove how much power humans have over influencing a dog's personality. He explained that many of the reasons a dog's personality changes are a result of the "nature versus nurture" theory associated with humans' personalities.

Next, Chopik's will research will examine how the environment owners provide their dogs might change the dogs' behavior.

"Say you adopt a dog from a shelter. Some traits are likely tied to biology and resistant to change, but you then put it in a new environment where it's loved, walked and entertained often. The dog then might become a little more relaxed and sociable," Chopik said. "Now that we know dogs' personalities can change, next we want to make strong connection to understand why dogs act - and change - the way they do."

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