Adequate socialisation during the sensitive period from 3-14 weeks shapes a dog's lifelong emotional responses; failures in socialisation cause lasting welfare harm through fearfulness.
The neurological sensitivity of puppies during the socialisation window makes what they experience in weeks 3-14 disproportionately influential on their lifelong emotional responses. A puppy that is well-socialised to diverse environments, handling, sounds, and species during this period becomes a confident adult dog that recovers quickly from novel situations. A puppy that spends this critical period in an impoverished environment — as is typical of puppy farm breeding facilities — develops chronic fearfulness and anxiety that cannot be fully resolved through later training. The welfare harm of poor socialisation is lifelong and represents one of the strongest arguments against puppy farming.