I've been present at times when a dog reacted aggressively - or perhaps, warningly? - when someone it didn't know passed by. Both of the dogs I'm thinking of at the moment are friendly dogs who consider everybody their friend.

This one time I was sitting with a friend beside a trail on the grass when two men walked by. My friend's dog's hackles rose high and an ugly snarling sound erupted. If he hadn't been on leash I really think he'd have chased them. Mind you, other people were passing by regularly acknowledged by a friendly wave of a tail.

Then the other day my Cairn terrier who is a mush bag did the same thing when a boy, maybe 16, passed by us on the street. The only thing I know about the boy, really, is that he and his family are afraid of dogs.

Anyway. My point is perhaps our reactions to strangers are akin to those of dogs; we still feel the visceral response but civilisation has taught us so well to cover it up that we no longer understand it.