In 1961 Freedman, King and Elliot identified the age of three weeks as the start of a puppy’s critical period, in terms of social/environmental interaction and the commencement of their capacity to develop social relationships. Significantly, this is the point in time when the puppy becomes truly mobile and can hear and coincides with increased electrical activity in the brain (Fox 1971a).
Michael Fox (1971a), a behavioural researcher, found that three week old Chihuahua puppies fostered individually in litters of four week old kittens would, at twelve weeks, prefer the company of cats over the company of their litter mates that had not been fostered. ...Michael Fox (1971a) carried out a subtler but even more revealing experiment. Litters of puppies were split into three groups: one group of puppies were hand-reared from birth and received no canine contact; the second group were given an equal amount of canine and human contact; and the third group only experienced the company of other puppies and their dam. When these three groups of puppies were reunited those that had only experienced human interaction preferred the company of those who had received the same rearing experience. Similarly, those puppies who had been exposed to both human and canine company preferred the company of puppies of the same upbringing, as did the puppies only used to canine company.