The puppy socialisation period between 3-12 weeks of age is the single most important developmental window for long-term behavioural welfare, and inadequate early experience causes lifelong welfare impairment.
Dogs that miss adequate socialisation during the critical window live with chronic fear and anxiety that significantly impairs their quality of life and the safety of those around them. Fear-based behaviours including reactive barking, aggression, and phobias are direct welfare harms that are largely preventable through appropriate early experience. The puppies most at risk are those from commercial breeders who minimise human contact for biosecurity reasons, internet-purchased puppies with unknown early conditions, and those kept isolated after purchase awaiting vaccination completion. The evidence strongly supports gentle, positive exposure to diverse stimuli during the sensitive period as the most important single welfare investment in a dog's life.