Rigidity in autism spectrum disorder (ASD): A unified (evolutionary) account of linguistic and non-linguistic symptoms

Autism Spectrum Disorder (ASD) exhibits not only behavioral symptoms of rigidity, but also, as we argue, linguistic symptoms of rigidity, which include rigid application of morpho-syntactic rules (over-regularization), as well as a tendency to interpret metaphorical, non-literal language literally, i.e. rigidly. In addition, it exhibits higher levels of reactive aggression. In this paper we propose a common evolutionary cause for these symptoms of ASD, connecting them to an enhanced striatal function and to a reduced control of the striatum by cortical structures, considering that the striatum is associated not only with reactivity and impulsiveness, but also with rigid, ritual, automated responses. Our framework sees the evolution of human species, and of human language more specifically, as a result of an intense feedback loop between the emergence of (early) language and human self-domestication forces (in essence, a reduction in reactive aggression responses and an increase of prosocial behavior), the feedback loop which was responsible for the relatively recent evolution of a denser connectivity in these brain circuits, gradually enhancing the mechanism for the cortical structures to better control the striatum. In addition to gaining a clearer view of how our cognitive distinctiveness evolved, this view promises to achieve a better understanding of the ASD etiology and symptomatology.