From scorpion to spider: tracing the origin of fear of spiders and other chelicerates through perceptual fear generalization gradients across chelicerates

Spiders are among the most feared animals worldwide, with arachnophobia being one of the most common phobias. Biological preparedness theory proposes that such fear may reflect evolutionary adaptations to ancestral threats. Yet, the high prevalence of spider fear is puzzling, as spiders have posed minimal danger to humans compared to several other animals. One hypothesis is that fear of spiders reflects a generalization from fear of scorpions, given their morphological similarity and scorpions' higher lethality. In the current research, using fear generalization gradients, we primarily investigated whether fear of scorpions generalizes to spiders and other Chelicerates, or whether spider fear is specifically evolved to deal with spiders. A total of 374 Iranian participants completed an online questionnaire in which they rated their fear of multiple Chelicerate species and provided absolute and relative similarity judgments between them. Scorpions were rated as the most fear-evoking Chelicerates. Both absolute and relative similarity of Chelicerates to scorpions significantly predicted fear ratings in Chelicerates, and the spider's relative similarity to the scorpion was associated with higher spider fear. By contrast, the scorpion's similarity to the spider was not predictive of scorpion fear. Additionally, Chelicerates' relative similarity to the spider did not predict Chelicerates' fear ratings. However, absolute similarity to the spider indicated context-dependent significance across Chelicerates. The existence and the degree of similarity of shared traits with scorpions also predicted fear across Chelicerates. These findings support the hypothesis that scorpions serve as the primary generalization stimulus, with spider and Chelicerate fear emerging largely (if not exclusively) as a byproduct of their perceptual similarity to scorpions.