Aggression or reactivity toward new people is one of the most common and misunderstood behavior problems. Aggression is often a response to fear but can also be a defensive behavior and not fear-related at all.

CAUSES OF AGGRESSION 

1. Fear due to lack of early socialization

2. Genetics

3. Learned behavior from mom

4. Traumatic experience

The most common cause of aggression and fear around new people is lack of socialization during the first 12 weeks of life. Dogs have a small window of socialization, and if this is missed, there are often aggression/fear issues later on. The golden rule of socialization, from Dr. Ian Dunbar, is that puppies should meet 100 new people by 12 weeks of age. Most puppies meet only a handful.

“BUT MY DOG IS ONLY AGGRESSIVE TOWARD CERTAIN PEOPLE!”

This is very common, and there are lots of reasons your dog may be more aggressive toward certain people than others. People have varying energy levels, physical sizes, volume, body language, and interest in the dog. A calm, small person ignoring your dog will likely trigger less of a response than a large, loud person who immediately tries to pet your dog or children who move quickly and are loud and unpredictable.

Most commonly, dogs have increased aggression toward men and children. 

DEFENSIVENESS

Some breeds are more genetically prone to being defensive toward new people, such as cattle dogs, Akitas, German shepherds, Great Danes, mastiffs, rottweilers, etc. For these breeds it is even more imperative to heavily socialize between 3 and 12 weeks to help override their genetic predisposition. These defensive dogs are not afraid but rather view it as their job to keep strangers away, which they will very happily do.

NOT PROTECTIVE

Many owners mistakenly believe their dog’s stranger-aggression is due to protectiveness - wanting to keep the owner safe. The vast majority of the time this isn’t true, it’s just wishful thinking. It’s easier to believe that your growly dog is just “protecting” you from every single person in the world instead of admitting your dog is aggressive and defensive or insecure. Dogs who are actually trained for protection work do not growl and lunge at every person they encounter. They are generally calm and composed until given the command - they have to be TAUGHT to protect.

PUNISHMENT DOESN'T HELP

It seems counterintuitive, but punishment is the single worst thing you can do if your dog displays aggressive behavior. One time at the vet, while waiting in the lobby, I saw the perfect example of this. A woman had a clearly nervous German shepherd sitting in the corner. This is important because the dog had no escape. She was quite literally backed into a corner. A man walked in, locked eyes with this tense shepherd, and thrust his hand in her face (this is a stupid move, never do this). As he’s doing this, the owner is tensing up on the leash. Unsurprisingly, the dog loudly growled.

The dog’s owner, clearly embarrassed even though it wasn’t her dog’s fault at all, harshly yanked back on the dog’s prong collar and loudly scolded her dog. Here’s how the dog saw this event: she is trapped in a corner on a tight leash at a scary place she does not want to be. A man intrudes on her space by entering the building, thrusting his hand aggressively in her face, and making hard, direct eye contact - THREAT. Her human uselessly stands by, so she takes action by warning this aggressive man off. She lets him know he better back off or she’ll escalate.

And then her owner punishes the dog for growling. So next time she feels threatened, she will remember - Don’t growl. Mom doesn’t like growling. Just bite. And this owner has now taken away her dog’s warning signal, so she will likely be dealing with a bite “out of nowhere” in the future because she’s taught her dog to not growl first.

THE BEST TREATMENT

The best treatment depends on the cause. If your dog is growly out of fear, it’s important to address your dog's anxiety around new people by teaching them that new people are great! For dogs who are not fearful, a different approach - usually involving obedience training, would be more effective. I always recommend working with a trainer when dealing with an aggression issue.