With their blocky heads and square jaws, pit bulls have a fearsome reputation in popular culture. They have often been portrayed as overly aggressive toward humans, or natural-born fighters and guard dogs.

The dogs known by the catchall phrase as pit bulls fall into four breeds: the American pit bull terrier, the American Staffordshire terrier, the Staffordshire bull terrier and the American bully. But pit bulls are also known as any mixed-breed dog with a blocky head and muscular build.

Online, they are either vilified or defended. There are videos of pit bull fights, gruesome bite photographs and archives of attacks. Animal experts say that the dogs’ reputation is unfounded and that blame for aggressive behavior should fall on their handlers.

“There is this idea that somehow they are unstable, relentlessly aggressive and treacherous,” said Bronwen Dickey, whose book “Pit Bull: The Battle Over an American Icon” also questioned the basis for the fearful reputation of the animals, such as that they have extraordinarily strong biting jaws and are prone to using them.

“There is always this narrative that they turn on you, that they are unpredictable,” she said.

The dogs are generally about 65 to 80 pounds. Centuries ago, they were bred to be used in slaughterhouses to prevent bulls from escaping, said Dr. Bonnie Beaver, a professor at Texas A&M University’s veterinary medicine college. As terriers, they tend to be territorial and individualistic.

“They are not more aggressive than any other dog,” she said in an interview. “But the terriers tend to have, I would call them, a shorter temper,” she said, meaning they switch “from their thinking brain to their emotional brain faster.”

“If the human owner doesn’t have the ability to prevent that switch into the emotional brain, then that dog is off on its own,” she said. “Most of the times when these bans come into effect, it is one or two very tragic events that involved big dogs.”