The following sentence by J. M. Berger has been widely quoted: “If you believe that only ‘the other guys’ can produce extremists and that your own identity group cannot, you may be an extremist yourself.”
The original context of the quote provides more nuance:
“In the United States, the term extremist is frequently hurled, shorn of context, across racial and partisan divides. Many in the wider West contend that the entire religion of Islam is inherently extreme, arguing for policies that range from the curtailment of civil rights to mass internment. Within Islam itself, furious debates rage about which sect, movement, or nation is normative and which is extremist. These debates influence the study of extremism. There are perhaps three times as many academic studies referencing jihadism as there are referencing white nationalism. Pseudo-intellectuals, some in positions of political power, have argued that white nationalism is far less important than jihadism, despite the fact that white nationalism has a far longer and more deadly history. And they have shaped policies accordingly. If you believe that only ‘the other guys’ can produce extremists and that your own identity group cannot, you may be an extremist yourself. History provides ample evidence that extremism is part of the human condition and not the exclusive province of any single race, religion, or nation. Not all violence is extremism, nor are all of humanity’s countless wars, conflicts, and atrocities. Many cases are ambiguous, but some clearly align with our modern understanding of the word. The diversity and ubiquity of the problem can be seen in a review of historical outbreaks of significant violence driven by ideological belief.”
— from the book Extremism by J. M. Berger (Cambridge, Mass.: MIT Press, 2018), p. 2.