President Obama seems to be the focus of a disproportionate number of conspiracy theories: he was born in Africa, he was involved in domestic terrorist bombings during his university years at Columbia, Bill Ayers wrote Dreams of My Father, and so on. Why? Why do people believe this nonsense, or the hundreds of other "urban myths" that make their way through the Internet?
As Francis Bacon put it four centuries ago: "What a man would like to be true, he preferentially believes."
Several years ago, Michael Shermer, who writes the Skeptic column for Scientific American magazine, published a book called Why People Believe Weird Things. He lists fallacies common to most conspiracy theories, belief in the paranormal, and assorted superstitions, including: 1) anecdotes are accepted as evidence; 2) coincidence is mistaken for causality; 3) heresy is assumed to imply correctness ("They laughed at Copernicus, too."); and 4) the unexplained is assumed to be conventionally unexplainable.
Science, too, is not immune to these fallacies, which is why organized doubt is built into the process. Scientific method is a highly evolved b-s detector that relies on double-blind experiments, quantitative observations, reproducibility, and rigorous citation of all other relevant work, pro or con. In science, it is as important to show something is wrong -- even a favored theory -- as to show it is right.
Is the scientific b-s detector perfect? Of course not. But it's the best tool the human race has yet evolved for choosing between various weird theories. What could be more weird than the big bang, say, or the four-letter DNA code of all life?
Of course, none of the above is relevant to conspiracy theorists. They tailor evidence to belief, rather than the other way around.