As concern on the effect of fake news and misinformation on voting grows, a literature has developed over the last few years on the relationship between conspiratorial thinking and populism. According to this approach, both phenomena go hand in hand, as they express preference for simplistic solutions to complex problems (Moffitt 2016; Castanho Silva et al. 2017). Most of these works agree that right-wing populist citizens are especially prone to believe and share conspiracy theories (Oliver and Rahn 2016; Rydgren, 2004; Betz, 2013), although some contend that populists on both extremes of the left-right axis are equally prone to a conspiratorial mind-set (Van Prooijen et al. 2015; Oliver and Rahn 2016).

This small yet growing research field presents a series of limitations. First, an excessive focus in the western, Anglo-Saxon world, despite the scarcity of left-wing populist parties in this region, which may lead to an overstatement of the relationship between right-wing populism and conspiratorial thinking. Second, the omission of the ideological dimension within conspiratorial beliefs --that is, the notion that some conspiracies have a more right-wing nuance and others may be more left-wing leaning--  has been systematically neglected. Third, the omission of a potential endogenous relationship between populism and the emergence of conspiracy theories. Forth, and last, the failure to provide causal mechanisms to understand how the two phenomena are interrelated.

This research tries to overcome these limitations by relying in a case study that provides a full range of variation for populist attitudes, covering all the left-right spectrum: Spain. Furthermore, and since populism is a new element in the Spanish electoral competition (with new populist parties fore independent to populist attitudes, the former affecting the later. Third, besides exploring the heterogeneous effects of a conspiratorial thinking on populism (for both the right and the left), we test four Our results confirm a link between conspiracy beliefs and populism in the Spanish case, which -contrarily to previous findings- is particularly strong among left-wing individuals and even stronger among those who place themselves to the extreme of the left-wing pole.