ABC News’ Christina Capatides reports:

Most dog owners have been there.  Their dog – usually so sweet and social, the apple of their eye – goes over to sniff another dog on their walk and it turns ugly. A sniff.  Then, a low growl.  Before long, it’s a full-fledged fight with the owner left embarrassed and apologizing, “I’m so sorry. He never does this.”

Well, it turns out, he does and so do a lot of his friends.

 A new study out of Mendel University in the Czech Republic, which is set to appear in the journal Applied Animal Behaviour Science, attempts to explain the reasons by examining various factors that can affect a dog’s level of aggression toward other dogs on walks. 

Specifically, the study spearheaded by Petr Rez?c, an associate professor in the Department of Animal Morphology, Physiology and Genetics, found that a dog’s sex and age can affect his or her inclination to threaten another dog.  Perhaps more surprisingly, by observing 1,870 interacting dogs in 30 different public places in the city of Brno from May to September of 2009,  the study also found that the sex of the person walking the dog and whether the dog is on a leash affect that dog’s propensity to threaten or attack another dog.

In terms of age, the study found that dogs are much more likely to threaten or bite a dog of a different age than a dog in the same stage of life. For example, adult dogs threatened puppies 24.64 percent of the time, while adult dogs only threatened other adult dogs 13.16 percent of the time. Similarly, senior dogs only threatened each other 16.67 percent of the time, while they threatened adult dogs 32.88 percent and puppies 58.82 percent.

Dr. Stefanie Schwartz, a board-certified veterinary behaviorist and the founder of PetBehavior.org, says this makes total sense. “Adult members of canine societies are not going to be particularly inclined to play with obnoxious puppies that are not their own,” says Schwartz of the Virginia-Maryland Regional College of Veterinary Medicine.

“Play with puppies by adults is not always encouraged. Plus, they’re often just not in the mood.  It’s no different than the way adult humans may not be in the mood to play with their kids, when they get home from work.

 While the study found that dogs behave better in interactions with members of their own age group, it found that dogs behave worse to members of their own gender. Male dogs, for example, were nearly three times as likely to threaten another male as a female. Female dogs threatened other females 23.08 percent of the time and males only 8.59 percent.

“There are dominance issues at play,” Schwartz says. ”Dogs need to establish who they are in relationship to each other.  They’re just like people. Take male humans when they shake hands, for example. Little boys in this culture are raised to have a firm handshake and look other men directly in the eye. That’s dominance posturing. If they fail to do so, they appear weak.”

Rez?c’s study discovered that the gender of the person walking the dog affected its level of aggression, as well. ”The occurrence of threat and biting between dogs in public places was higher when both owners were men than when they were women,” the study reads.

“For the average owner, males probably rely more on strength in controlling their dogs, whereas women have to rely more on skill and anticipation of what a dog or dog owner is going to do,” Schwartz says. “So, female dog owners may develop their own more acute sense of surrounding. That may be part of why dogs with male handlers behaved more aggressively in this study. Dogs with male handlers may not get the same kinds of cues that they would if they were walking with a woman.

The last factor the study examined, as it pertained to dogs’ aggression on walks, was they were on a leash. 

“The occurrence of threat was two times higher between dogs on a leash than off a leash,” concluded Rez?c and his team. “In some cases the dog on a leash may feel more vulnerable because it is unable to run away and may therefore show a threat when another dog gets too close.”

For the most part, it seems that this finding echoes a truth that most dog owners have experienced for years.

Shannalee Pauli, the owner of a 2-year-old female Jack Russell Terrier in British Columbia, says her dog, Charlie, “won’t let other dogs in her space, when she is on a leash or in a fence; but off a lead and in an open field, she is fine.”

Jennifer Davis, a teacher from Georgia, who has been fostering dogs for years, agrees.

 ”I think it depends on the dog; how secure or insecure it is, how well trained it is. However, I do know that my own dog is better behaved off-leash than on, and that our dogs at adoption events tend to be less aggressive toward other dogs when they are out of their crates than when they are in them. So, perhaps confinement can create a feeling of being trapped or not in control for the dog, which can create insecurity or fear, which can lead to an aggressive display.”

Melissa Bain, an assistant professor of veterinary behavior at the University of California, Davis, explains, “Sometimes we will see this when dogs are on a leash. Perhaps they feel more confined, unable to get away from another dog, and therefore, display aggression. Perhaps it is how the other dog approaches them, staring straight forward when it is more appropriate to circle around and approach more from the side. 

“Perhaps the owners become worried about how their dog is going to react to another dog and, therefore, shorten up on the leash, which can signal to the dog that the owner is worried and that the dog should now be worried.  Perhaps it is actually frustration leading to aggression.  The dog is frustrated and pulling on the leash, and if the owner gives a leash correction to punish that behavior, it can lead to the dog displaying aggression.”

While most experts agree with the study’s primary findings, some take issue with its methodology and omissions.

 ”There is one major characteristic of the dogs included in this study that we have to consider, if we want to use this article as a ‘starting point’ for a discussion about American dogs during walks,” says Carlo Siracusa, a resident at the University of Pennsylvania’s School of Veterinary Medicine.  “The dogs in the study, as well as a good number of European dogs in general, are not spayed or neutered, while most American dogs are. This hormonal component may have a significant influence on the interaction between two dogs.”

And Schwartz notes that the study neglected to address how the kind of leash used affected the dog’s response. 

“Leashes, at least in this country, generally come in 2, 4, 6, 10 and 20 foot variations,” she says.  “Then, there are retractable leads and they are just a nightmare waiting to happen because they provide no control of the dog.

“The leash itself is a tool that transmits information to the dog about what it can and cannot do; what their owner is permitting them to do. The phrase keeping someone on a short leash means something. If a dog feels that someone has control over them, they are not as empowered to make independent decisions and get aggressive.”