A recent study (summarized here) examined how behavior develops in three different dog breeds, and found that the “timetable” of development was different in the three chosen breeds. Fear-related avoidance behavior began earlier in German Shepherd Dogs and Yorkshire Terriers than it did in Cavalier King Charles Spaniels. The results suggest that puppy socialization should begin earlier for those breeds which develop fear-related avoidance behavior soonest. Only the three breeds mentioned above were part of the study, it would be helpful to research the development of this behavior in a wider variety of dog breeds.
The study’s findings should not be a surprise – dog breeds have been selected for a variety of different behavioral traits over thousands of years. I would expect that breeds selected for their ability to herd, hunt or protect would develop fear-reactive behaviors earlier than those breeds selected solely as companions.