Beyond Surface Performance: Analyzing Grammatical Production and Judgment Abilities in Boys With Fragile X Syndrome

Purpose: Boys with fragile X syndrome (FXS) have relative weakness in morphosyntax, specifically finiteness markers. However, most research has focused on grammatical production as compared to receptive grammatical judgment. Thus, the current study examined both grammatical production and judgment through pattern analysis and individual differences. Method: Thirty-three boys with FXS aged 9–18 years completed the third-person singular, BE/DO , and grammatical judgment probes from the Test of Early Grammatical Impairment. Unscorable responses were categorized and coded. To compare grammatical judgment to grammatical production, participants were categorized using a median split approach. Results: Grammatical production and grammatical judgment performance showed variability across participants, with mean scores below ceiling in both domains. Comparison of production and judgment revealed substantial heterogeneity. Specifically, over half of the participants demonstrated discrepant performance between the grammatical production and judgment domains, whereas the remaining participants showed consistent performance across both domains. Differences in grammatical profiles were not explained by chronological age or nonverbal cognitive ability. Conclusions: Findings revealed persistent difficulties in the grammatical production and judgment of finiteness marking (particularly BE forms) and highlighted the heterogeneity of linguistic profiles. These findings emphasize the importance of comprehensively assessing both grammatical production and judgment skills to inform intervention approaches in FXS.