Fragile X syndrome (FXS) is associated with executive functioning difficulties, including impairments in mental flexibility. While previous studies have primarily focused on behavioural performance, pupillometry provides a physiological measure of cognitive effort that may offer additional insight into the processes underlying task performance.This study examined cognitive effort allocation during a mental flexibility task in children with FXS, children with heightened autistic traits (hASD), and typically developing (TD) children. It was hypothesized that children with FXS would show a different temportal pattern of pupil dilation and a later peak pupil response than the other groups.

Data from 44 participants (9 FXS, 20 hASD, and 15 TD) were analysed. Generalized additive mixed models (GAMMs) were used to examine group differences in pupil dilation trajectories over time, and linear mixed models (LMMs) were used to analyse peak latency.

Although descriptive differences in pupil dilation trajectories and peak latency were observed, no statistically significant group differences were found. Sensitivity analyses yielded comparable results. These findings did not support the hypotheses that children with FXS differ from children with hASD or TD children in the temporal dynamics of cognitive effort allocation during task switching. The study demonstrated the value of pupillometry for investigating cognitive effort beyond behavioural performance and highlights the need for larger samples to detect potentially subtle group differences.