[ Shown above: Glow-in-the-Dark Zombie Devil Rubber Ducky. Available from the always-tasteful Archie McPhee (mcphee.com)]
Storytelling in the 20th century was strongly influenced by Freud. An obvious consequence was the self-conscious deployment of symbols. (And even though Freud famously said that sometimes a cigar was just a cigar, Bill Clinton went far towards proving him wrong.) But I think a less obvious and more pervasive effect of the growth of psychoanalysis was the idea that some small event in the past--the memory of which was probably repressed--could provide motivation for even the most improbable character behaviors.
Playwright Paddy Chayefsky famously derided this as the Rubber Ducky school of character depth: when the villain was eight, someone took away his rubber ducky--and that is why he grew up to be a savage serial killer who dresses in his mother's outfits when he attacks his victims. (Hey, it's no sillier than some of the things you can find in early psychoanalytical case studies.)
Now, I'm not arguing that traumatic past events can't shape a character, or even that a character's primary reason for living can't result from a single event--especially if what we're writing is more of a tale and less of a modernist piece. For example, in William Goldman's The Princess Bride, I'm perfectly content to have The World's Greatest Swordsman roam from country to country searching for the Man With Six Fingers, muttering all the while under his breath, "My name is Inigo Montoya. You killed my father. Prepare to die!" It's a comprehensible motivation--it explains why Inigo was driven to become such a great swordsman--and provides just as much depth as the character requires (i.e. not much). Someone might argue it's a Rubber Ducky, but it's a solid one, and one that is matched perfectly to Inigo's character and quest.
On the other hand, one of the most disappointing books I've read is Thomas Harris' Hannibal. I think Red Dragon and Silence of the Lambs are examples of the best the genre can do, but Hannibal had a number of problems. The biggest error that Harris made, though, was when he tried to Rubber-Ducky the great Hannibal Lecter. Harris is too astute a writer to come up with a simple and arbitrary event in Lecter's past; instead, he crafts a prolonged situation in Lecter's childhood that in many ways matches Lecter's preoccupations. As Rubber Duckies go, it's a bath toy of Olympian scale, but it's still just a Rubber Ducky.
Harris did this to himself. Hannibal Lecter is a great character, full of subtlety and depth, and after we've watched him on the page for two fine novels, we don't want him "explained." A few hints would have been enough. A simple, heroic character like Inigo Montoya can survive a good explanation, but I resented the explanation of Lecter in the same way I'd resent someone summing up the whole of my own life and personality by referring to a single event in my childhood.
And why am I going on about this? Ah. In my work-in-progress (it's always about me, isn't it?), three of the characters have huge backstories--ones that will result in prolonged scenes in the past, which is always tricky to manage. And at least one of the backstories runs the risk of being a Rubber Ducky. So I'm fretting. How do you test to see if your story concept is a Rubber Ducky? Do they squeak when you squeeze them?
Or, as with witches, do you just toss them into the pond and hope they don't float?