If a dog kept happily hitting the target through these ambiguous tones, the researchers claim, it's probably because it was hopeful that one of them would lead to a reward. On the other hand, the researchers report, the "pessimistic" dogs grew distressed when ambiguous tones didn't result in milk, and avoided repeating the task.

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This study is really meant as a proof-of-concept for a sort of doggy personality test-- one that could help determine the best service dogs for particular tasks. The researchers found that pessimistic dogs, for instance, were doing better in their training to be guide animals for the disabled. They were careful and anxious about taking risks. But a persistent, optimistic dog might do a better job in search-and-rescue missions.

Marc Bekoff, an author and professor emeritus at University of Colorado who wasn't involved in the study, was hesitant to call the dogs who gave up "pessimists."

"The paradigm of the study is great -- most dog studies use 10 dogs or so, and this has 40 dogs of all different breeds and ages. And it's possible that these dogs were pessimists -- but maybe they just gave up," Bekoff said.

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In other words, maybe the dogs who stopped looking for milk that would never come were just realists. To track down a pathologically pessimistic pup, Bekoff suggested, one might see if a failure during the milk and water task led them to be less interested in unrelated reward-based experiments.