[Digit-Al]

They don't have to have done something wrong.

Years ago I had a lurcher, raised from a pup. One day I noticed that if you spoke to him in a stern manner - even if he had done absolutely nothing wrong he would hang his head low and look up at you, and put his tail between his legs as if he was guilty of something bad and wanted to look contrite.

It seems to me that it may be a long ago learned, now instinctive, behaviour to react that way to a particular tone of human voice.

[kbenson]

It might have a simpler explanation. Maybe what you are seeing isn't guilt, but submission, and you are mistaking it for guilt. When spoken to a certain way, submission is the correct social response. This is true in people as well. If you have a work superior approach you in an angry manner, your best option is likely not to respond angrily or indifferently.