A recent examination of dog bite cases involving children found that two-thirds of the dogs had previously never bitten a kid. In addition, one in five of the 103 dogs studied had never bitten anyone before the documented incident. A history of aggressive behavior (other than biting), however, was common among the dogs.

Led by researcher Dr. Ilana Reisner of the University of Pennsylvania’s School of Veterinary Medicine, the study also showed that both medical and behavior problems contributed to biting.

In kids younger than 6 years of age, the bite was most likely related to the dog guarding food or other “resources.” Older children were more likely to be bitten by a dog guarding territory, such as a yard or house. Similarly, if a dog didn’t know a child, he was more likely to bite him or her while guarding territory, while a familiar kid was more likely to be bitten near the food dish.

Anxiety also played a role in the dogs’ aggression, researchers said. Three-quarters of the dogs showed anxious behavior when left by an owner or exposed to a loud noise, such as a thunderstorm or fireworks.

Because half of the dogs had a medical condition, largely skin or bone related, pain and irritability could have also played a role in the bites, researchers said.

In addition, more than half the dogs that bit had attended obedience classes and almost 100 percent were neutered.

“Although the success of obedience training for individual dogs was not measured, the results of this study suggest that obedience training, like neutering, will not ensure prevention of future bites to children,” the researchers wrote in their paper, which was published in the journal Injury Prevention.