"相似性和传染性 "是一个统称，指的是将相似的物体视为形而上的联系（相似性），将接触过一次的物体视为一直在接触（传染性）的思维方式。 该术语在认知心理学和人类学中被用于人类推理中的这些普遍倾向。 人类学家对数百种文化进行了调查；包括几千年来基本没有变化的原始部落文化，并发现相似性和传染性是完全普遍的。

基于相似性和传染性的思维经常是禁忌、迷信、民间魔术和关于世界运作方式的民间理论的基础。 例如，为了诅咒某人，人们可能会得到目标人物的一绺头发，并隆重地玷污它。 因为头发来自这个人，甚至是从他们身上长出来的，所以它被认为在分离后还会保留这个人的本质。 头发所遭遇的将降临到头发的主人身上。 这一原则通过巫毒娃娃最明显地象征出来，但相似性和传染性的更微妙的变化存在于世界各地的文化中。 类似性和传染性可能是积极的，也可能是消极的，如果我们掌握了一个摇滚明星使用的吉他挑，我们可能认为它会提高我们的演奏能力。

In one experiment, graduate students refused to drink tea that had been stirred with a sanitized flyswatter, even when offered money to do so. This and similar experiments and observations show that similarity and contagion-based thinking is not just a cultural holdover from more primitive times, but an ingrained aspect of human cognitive processing that is coded into our genes. Similarity and contagion-based theories are heuristics – that is, rules of thumb – which helped our ancestors survive in the harsh world in which our species evolved. Most of the time, these heuristics were useful – for instance, it really is stupid to consume a fresh-looking piece of food if it is found in a pile of rotting food. But under different conditions, especially modern conditions, these heuristics no longer apply. Similarity and contagion-based thinking is not necessary scientific, and in many cases can get in the way of science.