Evolution required us to react very quickly to stimuli in our environment. It's a "basic human survival skill," explains political scientist Arthur Lupia of the University of Michigan. We push threatening information away; we pull friendly information close. We apply fight-or-flight reflexes not only to predators, but to data itself.
Amanda Marcotte connects Mooney's article to the Trig question:
At its heart, I think the conspiracy theory reflects the belief that Sarah Palin is a phony. Some conspiracy theories are people rolling up what they know or suspect about a person or a system and making unproven links: 9/11 Truther theories are a manifestation of the suspicion that Bush is an opportunist and a liar. Global warming denialists believe that environmentalists are more anti-capitalism than pro-environment. Believing Sarah Palin made up an entire pregnancy is a way to "prove" to yourself your suspicions about a woman who has done a remarkable job of breezing through life by faking it. To my mind, the evidence against Palin on the record should be enough to satisfy a dislike of her–everything from her pretending she reads newspapers to using Alaska's proximity to Russia as evidence of her foreign policy qualifications to her embrace of reality television—but some people just need to believe something more dramatic.
Or maybe they just look at the countless pointless lies she has told and wonder if she couldn't also lie about something else. You have two moves here, not one. The first is to infer from Palin's general character that she's capable of almost anything; the second is to note that we have massive evidence that she has lied about countless things, even when there is empirical evidence that she is simply wrong (close to 50 odd lies in this category alone). So why not this as well?