A study tying autism to vaccinations has worried parents for 12 years, fueling a fierce debate. On Tuesday, the medical journal Lancet formally and fully retracted the study that started the controversy. Yet many think the fight will continue, regardless, and Arthur Allen in Slate explains why.

It's not just irrationality, he insists. Anti-vaccine activists may be irrational--opponents have called them "true believers" and "religious zealots"--but Allen insists they are irrational for a reason. "Blaming vaccines," he points out, "can promise benefits. Victory in a lawsuit is an obvious one, especially for middle-class parents struggling to care for and educate their unruly and unresponsive kids." But it's not merely a matter of profit. There are strong parental instincts at work. Many parents believe that "if vaccines are the cause, the damage can be repaired, the child made whole." Allen quotes Jim Laidler, scientist and parent of an autistic child, who observes that "hope is a powerful drug."

Moreover, Allen argues, anti-vaccine parents are no more irrational than some religious believers, or adherents of "even scientific paradigms." Confirmation biases, where anecdotes are seized upon as evidence, are ubiquitous no matter the cause. Here, events have conspired to make a confirmation bias almost unavoidable. The contrarian anti-vaccine angle is irresistible to journalists and environmentalists alike. There's also an even trickier trend:

In the pre-Internet days, the parents of an autistic child living in a small city might have found a handful of other parents in their predicament. Now, they instantly find thousands online. The denominator--healthy children--has disappeared. This is a good thing if you're looking for answers. But the answers may not be good ones.