Have you ever felt like your dog was judging you or someone the two of you have just met? It turns out that your assumption is right; your dog is using its senses to determine whether the people it encounters are good or bad. That said, your dog's ability to sense that other person's attitude and personality is somewhat limited.
A study published in the journal Neuroscience and Biobehavioural Reviews found that dogs hold grudges against people who don't treat their favorite people very well. For example, one exercise conducted in the study showed that dogs were less likely to accept treats from a person who refused to help their owner open a container. Although that may make sense, it doesn't entirely explain how a dog decides to dislike a person it had just met.
A dog may react negatively to a person it has met for the first time for a variety of reasons. Canines may use their incredible sense of smell to sniff out the chemical changes in the pheromones people emit when they have less than honorable intentions in mind.
A person's aggressive behavior prompts that change in brain chemistry and dogs are talented enough to pick up on that change right away. This scenario will not apply to psychopaths, however, because they experience few emotions and are unlikely to have changes in brain chemistry that dogs can pick up on.
Dogs also rely on their intuition and trust their own feelings much more than humans do. Therefore, a dog might sense a stranger's bad attitude or thoughts directly through a gut instinct or a sixth sense. Canines are more attuned to the world than people around them tend to be, so if your dog is uneasy around a person, your best bet is to trust it.
Finally, dogs might be so connected to their owner that they can perceive when their owner is uncomfortable or uncertain about something. Their distrust of a stranger may be more a reflection of the owner's anxiety about that new person than anything else. Either way, the dog feels what is happening to its owner and echoes the owner's innermost concerns in its own behavior.
The next time your dog reacts negatively to another person, give it the benefit of the doubt; it may just know the person is bad. Dogs have better instincts and sense than people do, and their abilities and instincts deserve respect. Science has proven dogs to be highly perceptive beings.