Scientists say that the entire vast diversity of life on Earth was caused by genetic mutation (which changes individuals) and natural selection (which distributes the most effective genes around a population). Whenever a population divides for a long enough time, they become genetically isolated and will thenceforth continue to be divided even if they share a territory; the two groups will go their own way, and over millions of years they can become astonishingly different. We have a sequence of fossils that demonstrates this gradual change throughout consistent strata of rock. We have several independent dating methods that corroborate one another, including tree-ring dating and many different types of radiometric dating, all of which support the notion that the fossils we've found are many millions of years old if not more. We've seen bacteria adapting through random mutation in laboratory settings, and, as we'd expect, the mutant bacteria thrive and take over the population.
Creationists, if they subscribe to the Intelligent Design hypothesis, say that, while evolution may happen in much the way scientists describe, there are some things that were too complex to have evolved naturally and required some sort of intervention from an outside force. Naturalistic evolutionists still posit that these complexities may still have arisen naturally, even if we have yet to puzzle out how: we say that a declaration that such and such cannot have possibly happened naturally is a failure of imagination. History has shown us that people who insist that we'll never be able to know something are generally proven wrong, if not in their lifetimes. Creationists who adhere to biblical literalism say that, for every bit of science that supports an old earth or common descent, either our data are wrong or we're misinterpreting them.
The problem we're left with is that either one of these groups is honestly mistaken or one of these groups is lying. How do we find out which? All right, well, there are two particular dilemmas we can pick out:
1. Is our science correct, at least in the broad picture? Scientists and the ID crowd say "yes", while the literalists say "no".
2. Are some things so complex that they definitely can't have evolved naturally? The ID crowd says "yes", scientists say "no", and literalists don't even need to address the question.
I think we can better approach an answer to which groups are right about these things if we consider motive, which, as you may have guessed, is the topic of this post. Only an extremely naive person would take every claim at face value; a more realistic individual should consider the possibility that somebody is either deliberately deceptive or at least misinformed somehow. It's possible that somebody's prior corpus of experience will inform the way they approach a particular question, such that they can receive the best information, come to an honest conclusion, and still be wrong. So we have to look at reasons why somebody would arrive at a particular conclusion about the world.
First, to simplify matters, I'd like to lump the ID folks and the literalists into a single group, the creationists, and I'll explain why it makes sense. Officially, the position of ID is that "something" intelligent intervened in the evolutionary process to make us the way we are. It could have been Sufficiently Advanced Aliens. It could have been a Flying Spaghetti Monster. It could have been one of many deities. However, in reality, we find that the only people who buy most of the science of evolution but fail to imagine the possibility that some organs could have arisen gradually are religious people. The leaders of the movement were rather specifically religious: as it was shown in the Dover case, the leading ID textbook was discovered to be essentially an edited version of a creationism textbook. ID is simply the necessarily scientific face they put on their position in order to try and make it fly in science classes; the concession that their hypothesis doesn't rule out non-God designers is a technicality that arose out of political necessity. So, for all intents and purposes, ID is just another manifestation of creationism: we can call it metaphorical creationism, which attempts to reconcile with science, in contrast to the literal creationism, which denies the science outright.
Right. So, metaphorical creationists assert that some features of organisms could not have happened naturally. Scientists assert that they can have done so -- we just might not know how in some cases. Literal creationists assert that the science we use to form a picture of the history of life on Earth is in some ways fundamentally flawed. Scientists, again, dissent. Why? What does anybody have to gain from the answers to these questions? Well, there are two major motives for each side: an individual motive and an institutional motive. The individual motive is what one person gains from believing a given answer; the institutional motive is what one person gains from other people believing a given answer.
There's actually a great deal of individual motive to believe in a creationist viewpoint. It's obvious that some degree of divine intervention is necessary for any of the major religious belief systems to be true. If you espouse a completely scientific worldview, then there's no cause to ever suspect a supernatural explanation (since being able to be described by science is part of the definition of "natural"), so it's necessary to concede at least some part of our origins to the inscrutability of God if you're going to believe in God at all. And belief in God is where the motive lies. If God is real, then not only do you no longer have to worry about the oblivion of death, you get at least a chance at eternal paradise. For such social animals, humans are remarkably egotistical: we're willing to sacrifice a lot for the promise of heaven. So, even though this is a logical fallacy -- the argumentum ad consequentiam, or argument from consequence -- it's easy to see why people would choose to believe in this chance at heaven. But there's more. As I've mentioned in an earlier post, it's often extremely difficult for people to go against the traditional religion of their families. They risk being disowned or at least very uncomfortable around them. What's more, they're often forced to accept a particular religion long before they start learning the science necessary to adequately describe evolutionary theory.
Contrast the individual motive for a scientific worldview. It is virtually nil. Now, mind you, I'm only talking about motive beyond and independent of truth value. But if we're judging a scientific perspective on what it brings to the table besides truth, it turns out to be very little indeed. Certainly, many scientists are deeply and emotionally awed by some of the most mind-boggling implications of their work. Richard Dawkins says, of the notion that matter is constantly flowing into and out of our bodies such that we don't have any of the atoms in our body that we had at birth, "Some people find that thought disturbing. I find the reality thrilling." Yes, oftentimes reality is much stranger and more wondrous than what we can imagine. But do you really think we couldn't have done better? If scientists were inventing a false belief, as creationists suppose, why wouldn't they invent one with some more rhetorical punch to it? Something that actively favors those who follow it? As it is, even the most amazing things about science are staggeringly neutral.
Next, let us look at institutional motives. Religious leaders are certainly the people who benefit the most from other people believing in religious worldviews. Most cynically, they make money based on their rhetorical ability to maintain a congregation. At a slightly more sophisticated level, they receive large amounts of admiration merely by being qualified as a cleric. If nobody else believed what they were preaching, people with these particular skills would have to go elsewhere -- probably into politics, where their particular skills would be most relevant. So the religious hierarchy obviously has something at stake in the battle for the minds of the people. What's more, for people who genuinely believe in a particular religious worldview, it will often benefit them at a personal level for friends and family to follow suit, such as when a religion promotes proselytizing. It will be a personal emotional benefit for a person if he or she doesn't have to worry that his or her relatives are doomed to eternal hellfire.
Set against that is the question of what sort of motives scientists would have for other people to believe their worldview. Money, yes. Scientists get funded based on doing credible research, and they make money off of books that sell well enough. Somebody would have to do a study, but I'd wager that the median scientist probably makes a lot less money for how much hard work he does compared to the median cleric. I think it's fairly universal that people become scientists not out of a desire to strike it rich but out of love for the field. In any case, you'd have to upset very large amounts of extremely well-established scientific knowledge in order to render any scientists obsolete. Most could merely redirect their research if evidence for some creationist hypothesis were found. I'd say that scientists certainly do receive admiration for their position, but I estimate that it's rather less than comparable clerics, at least in total number of people.
So, ultimately, I conclude based on these things that in both individual and institutional cases (though far more strongly in the former), there is far more cause to deceive in favor of a creationist worldview than a scientific one. Scientists gain little for themselves by believing in their fields -- aside from the wonder at how amazing their fields truly are -- and don't risk their entire jobs being rendered obsolete by a potential discovery of a truth in creationism. Why would they bother to fight so hard against the vocal creationists just to put forth some meaningless science-fiction as fact? Creationists, on the other hand, have rather more to gain by being disingenuous.
What's important here is looking at why your opponent would argue a certain way, and not just dismissing his entire argument out of hand because it nominally opposes yours. If believing in religion X makes you happy, it's in your interest to do logical gymnastics in order to hold that belief. Believing in science, however, doesn't always make use feel good inherently. I'm sure many if not most atheists would like to have eternal paradise, depending on how, exactly, it's defined. We don't oppose religious folks because we hate God. We do it because we're free from the psychic tyranny of a rationally untenable worldview, and we think you'd appreciate the rationality if you allowed yourself to think about it.