The existence of a critical period for the establishment of primary social relationships is a well-established phenomenon in social animals. This paper has dealt with the physical and hereditary basis of the critical period in the dog.
Normal social development in the puppy can be divided into several periods based on changes in social relationships. Several of these may be critical, but the most important is that of primary socialization, beginning about 3 weeks of age.
The beginning of this period is accompanied by certain anatomical changes: the eruption of the teeth and the opening of the ears. Eyes open at an earlier date.
Its beginning is also closely associated with the appearance of the function of the ears. The senses of touch and taste (including tasting gases) are present at birth. The eye is sensitive to light before it opens, but not completely functional until some time after.
Changes in the heart rate appear which correspond to changes in the EEG and ability to be conditioned.
At 3 weeks there is a change in the rate of growth, which may be attributed to both additional food and to a psychological change in food motivation.
Experimental evidence indicates that feeding is an important factor in socialization, but not the only factor.
There is a marked change in the fingersucking response of weaned puppies at an age corresponding to the beginning of the critical period.
Hereditary variability of the exact time of onset of the critical period exists both between individuals and between breeds. However, the functional variability appears to be smaller than that in the accompanying anatomical changes.
In the period immediately following birth the puppy is strongly protected from psychological influences. During the critical period it becomes highly sensitive, at a time when the sense organs and nervous system are still not completely developed. The exact effects of experience during this time are still to be determined.
These data suggest facts which must be ascertained in order to establish the existence and duration of a similar critical period in human infants. They also suggest ways in which important clinical theories can be experimentally tested.