An overly anxious pup, for example, is most likely to remain so as an adult. The most assertive pup of a litter is likely to become extremely dominant as an adult. If you choose a pet that displays any temperamental extremes, be prepared for life-long challenges. This may be particularly true for dogs. Testing a puppy's temperament during the first few months of life can be misleading.
Pups go through phases of development that are largely influenced by their environment.
A pet's earliest experiences influence it throughout its lifetime. This can be more important for pets adopted as adults or even older than 4 months of age. With proper training from the start, even difficult pups can grow into exceptional pets. Look for the pet that has the traits you desire, but in moderation. This does not mean that the average pup or kitten will not have behavioral or medical problems, but it will have a better chance of developing only average, and not extreme, problems.