Excerpt:

[...Past research has often relied on scales of political conservatism as a proxy for extremism (by considering those who self-report as ‘strongly liberal’ and ‘strongly conservative’ as ‘extreme’; e.g. [21,22]). This is potentially misleading because it can confound partisan direction with extremity, dogmatism, or allegiance to established doctrines; for instance, a self-professed ‘strongly liberal’ participant may be ‘extreme’ or could simply possess liberal views without necessarily being partisan or radical.

In a recent study of over 700 US citizens, Zmigrod et al. [17••] measured the strength of partisan identities using a continuous pictorial measure which allowed participants to indicate the level of identity fusion between their personal identities and the two prevailing political parties (Democratic and Republican Parties). Using three independent behavioural measures of cognitive rigidity from the neuropsychological literature, the analyses revealed a clear inverted-U shaped relationship between partisan intensity and flexibility: participants on the extreme left and extreme right displayed reduced cognitive flexibility on these three tests relative to moderates and those with only weak personal attachments to the political parties (see Figure 1). Consequently, when partisan strength is measured directly, rather than through measures that were designed to quantify political conservatism, it is possible to observe the elusive ‘rigidity-of-the-left’ [....]