Section 3 of the WGA's guidelines for credit arbitration lay out the considerations that go into determining what the credits will look like for most movies you see. I've also copied part B.4, the guidelines for the "Screenplay by" credit, to a seperate blog post since I'm going to focus on that.
Other than some minor quibbles, it's a perfectly acceptable historical system of determining the writing credits. I have seen some comments that it's too subjective an account, which is definitely true. But certainly no-one was expecting to create some sort of algorithm into which you plug various drafts of screenplays and out of which comes a definitive answer. It's the sort of process that you really need a group of people to look at to evaluate correctly and the suggestions:
The percentage contribution made by writers to screenplay obviously cannot be determined by counting lines or even the number of pages to which a writer has contributed. Arbiters must take into consideration the following elements in determining whether a writer is entitled to screenplay credit:
* dramatic construction;
* original and different scenes;
* characterization or character relationships; and
strike me as something reasonable people could use to come to some sort of decision in what is necessarily a messy process.
Writing credits are not history; writing credits are a game, the goal of which is to gain as much credit as possible.
In a historical project, such as determining the causes of the Amercian Civil War, historians do research, write papers, and argue -- but none of what they do has any effect on the Civil War itself.
But the guidelines for determining writing credits have a huge impact on how people write and rewrite screenplays.
Given that financial rewards, such as residuals, are directly linked to the writing credits, and future work or feeling good about seeing your name in the titles is indirectly linked to the writing credits, there's a strong incentive to write so that one receives a "Screenplay by" credit.
This means that if I get a chance to do a rewrite of a script, I have a strong (self-interested) incentive to substantially change at least 33% or 50% of the material, even if the material doesn't need that much changing. In fact, it's an interesting little game by itself: I'm best off if I change enough to get as much credit as possible, while making the story something good, and retaining key elements from the earlier drafts that the producers, director, and actors aren't willing to sacrifice (otherwise I get removed from the project or the project dies).
This is markedly different from when I do a rewrite of my own script -- then I keep the good bits and eliminate the bad bits, and, hopefully, after a few iterations of this I have a screenplay composed mostly of good bits.
Interestingly, the 50% rule is intended to protect the first writer. Yet, like the "promote the passing game" rules the NFL pushed -- the effect on the writing credit game is just the opposite.
Of course, financial and self-interested reasons aren't the only motivating factors for writers (and rewriters). Writers are also motivated by aesthetic reasons -- write the best film, and by moral reasons -- show respect for the previous work done on the project. But the WGA is telling me when I'm rewriting that adhering to those sorts of scruples can prevent me from getting a writing credit. Also, as the original writer on a project, I have to be sympathetic when someone comes along later and makes drastic and unnecessary changes because the WGA has essentially ordered them to do so, if they want that vital "Screenplay by" credit.
And, a set of guidelines that punishes writers for doing the artisticly or morally right thing is a bad set of guidelines.