This chapter argues that vaccine denialism need not result from especially blameworthy forms of parental ignorance or irrationality. People who talk and write about vaccine denialism, in both popular media and academic research. Popular depictions of vaccine denialism also often explicitly invoke cognitive biases. The affect bias likely plays only a positive role when it comes to the ways physicians form beliefs about vaccines. The availability heuristic may contribute to vaccine denialism when someone's most vivid and accessible ideas about vaccines are consistent with the belief that vaccines are unsafe or ineffective. Many pediatricians likely have easily recallable memories of treating children who suffered from vaccine-preventable diseases. Groupishness may contribute to vaccine denialism even though vaccine denialism does not correlate with religiosity or political ideology. A framing bias could contribute to vaccine refusal if vaccine refusers were more likely than vaccinating parents to reflect on information about vaccine safety that emphasized the possible negative outcomes of vaccination.