Sometimes a dog that is frightened of people may have his targets fairly well delineated. For example, his fear may be of all men with white beards or men wearing boots. In other instances, dogs may respond to several different categories of fear-inducing people, including men with beards, men wearing hats, men of large stature, men with deep voicing, men smoking cigarettes, etc. You'll notice that men are profiled in all these examples: That's because men are most frequently the subjects of dogs' fears.
For more information, please read Fear of People