Something I've thought a lot about is the root of Marge's fears: whether they can be accounted for mainly by nature (biochemical imbalances, genetics) or nuture (bad early-life experiences or simply a lack of good ones).
Now, I know that neither of these are mutually exclusive, and that there is an awful lot of overlap. But, it is well-known that there are dogs who aren't "wired right," no matter how uncommon extreme circumstances of this mis-wiring may be, and that there are dogs whose outward temperament is a product of their very bad life experiences and, should they have been raised correctly, they would not be fearful.
We can also throw a curveball in to this by saying that the reaction each dog has to its "nuture" is, in part, dictated by "nature:" that two dogs, of different genetic background, reared in the same negative environment, wouldn't necessarily both turn out to be fearful. Similarly, dogs of the same genetic background, reared in different environments, both won't necessarily turn out to be fearful.
In fact, that's where I really think the answer lies.
Have you ever heard of the diathesis-stress model? No?
Okay. Imagine a cup.. an ordinary plastic cup. The cup is labeled "FEAR." (Don't mind my crappy drawings.)
The cup is filled part of the way with the dog's genetic endowment. In the case of fearfulness, a dog with a low genetic predisposition for being fearful would have only a tiny bit of their cup filled. A dog with a high genetic predisposition for being fearful would have a LOT of their cup filled.
Obviously, in a dog with a lot of genetic predisposition towards a fearful personality, it would not take a lot of different stressors to reach that point. In a dog with a very small genetic predisposition towards fear, it would take an awful lot of those stressors - but, it is still possible, if the dog encounters very bad life experiences.
Whatever the cause, there is something about Marge that has lead to a lot of her fears being reversible or at least partially reversible. She is extremely responsive to classical conditioning, whereas a lot of other fearful dogs aren't. That leads me to believe that it was her upbringing that caused her to be fearful, rather than a biological reason (which may have required medication to reverse). But I might just be pulling that out of thin air.
What do you think? How do you think a dog's outward behavior comes to be - from genetic endowment or environmental stimulation?