By Christine D. Calder,
DVM, DACVB
Calder Veterinary Behavior Services, www.caldervbs.com
Why Does My Dog Behave This Way?
You may ask why your dog seems nervous around strangers or why your friendly puppy suddenly growls at another dog in the park. These questions are common. The latest research shows that how a dog acts as an adult is strongly linked to their earliest experiences and their breed.
Two large studies followed thousands of dogs to find out what really shapes their behavior. What scientists learned is simple. Puppies who receive positive reinforcement, safe places to explore, and lots of positive experiences early on are much more likely to grow up confident and friendly. On the other hand, puppies that face stressful experiences, punishment, or scary situations, especially in the first six months, are more likely to have problems with fear, frustration, or default to aggressive behaviors when they are older.
What Makes the First Six Months Special?
Think of the first half-year of your puppy’s life as building the foundation for everything that comes next. During this period, your puppy learns who to trust, what is safe, and how to handle new things. Your choices and your dog’s experiences in this time make a big difference.
If your puppy experiences rough punishment, such as being hit, yelled at, rolled onto their back, or having their muzzle grabbed; or if they are subjected to aversive training tools like prong collars, choke chains, or electronic (e or shock) collars, or are left alone for long periods without comfort or hurt or attacked by another animal, these moments can cause lasting negative effects. Similarly, serious accidents or frightening interactions with people, such as being grabbed, chased, or threatened, can deeply impact a puppy’s emotional development and damage the dog-owner relationship.
What About Breed and Other Factors?
You may wonder if your dog’s breed also matters. The answer is yes. Some breeds are naturally more sensitive to stress while others can handle challenges better. For example, Labrador Retrievers tend to stay friendly and resilient even after tough starts. In contrast, Siberian Huskies or American Eskimo Dogs are much more likely to become fearful or default to aggressive behaviors after early negative experiences.
Smaller dogs, those under 20 pounds, are often more excitable, fearful, or harder to train than larger breeds. If your dog is a mixed breed, especially from a shelter, he may carry extra stress from his past. Studies have also found that dogs that have been neutered are sometimes more likely to show fear or aggression than those who are not.
What Can You Do?
There are steps you can take to support your dog:
1. If you bring home a new puppy, ask about his first few months. If you adopt, try to find out about early socialization or past stressful events and experiences.
2. Use reward-based training methods rather than punishment. Make early experiences positive and fun. Give your puppy time to learn about the world at his own pace.
3. Remember that breed, size, and background all shape behavior. Be flexible, patient, and adapt your approach to your dog’s individual needs.
Setting Up Your Puppy for Success
Your choices in your puppy’s first six months make a big difference. Focus on reward-based training, appropriate socialization techniques, and understanding your dog’s background and breed and you can help your puppy grow into a resilient and social companion. The science is clear: what you do early truly does matter for your dog’s life as he ages.
References:
Espinosa, J, Zapata, I, Alvarez, CE, Serpell, JA, Kukekova, AV, & Hecht, EE (2025) Influence of early life adversity and breed on aggression and fear in dogs. Scientific Reports, 15(1), 1-12.
Li Y, Sexton CL, Fitzpatrick A, Ruple A (2025) An analysis of behavioral characteristics and enrollment year variability in 47,444 dogs entering the Dog Aging Project from 2020 to 2023. PLoS One 20(9): e0330257. https://doi. org/10.1371/journal.pone.0330257