Such harmful policies are often born out of arrogance and stupidity.

stupidity 是名词，由形容词 stupid（愚蠢的）加上名词后缀 -ity 构成，表示"愚蠢的状态或性质"。这个词缀 -ity 常用于将形容词转化为抽象名词，表示某种性质或状态。