Valid use of the Tonic Immobility Scale with consideration of peritraumatic substance use: Analysis of item differences and measurement invariance.

OBJECTIVE

Tonic immobility (TI) is an involuntary freeze response sometimes experienced during trauma. The role of perceived substance use in influencing the experience or severity of TI is poorly understood, in part due to measurement difficulties. The goal of this study is to contrast four scoring methodologies and test the measurement invariance of the Tonic Immobility Scale (TIS) comparing sexual assaults that are or are not perceived to be associated with substance use.

METHOD: A total of 179 young women with sexual assault histories completed the TIS, including an added item querying their belief of whether alcohol or substances were related to their TI symptoms during past assaults. Configural, scalar, metric, and residual invariance models were tested to examine whether the TIS functioned equivalently between substance belief-endorsing and nongroups.

RESULTS

Between-group t tests found that those who perceived substances influenced their TI symptoms (n = 63) reported significantly more paralysis, inability to move, detachment from self, and detachment from environment than no-TI peers (n = 116). We tested measurement invariance of the TIS; only Bados and Peró's Modified TIS-Brief Form model demonstrated excellent fit for configural invariance. The Modified TIS-Brief Form also retained excellent metrics following metric, scalar, and residual invariance testing.

CONCLUSIONS

There were differences in the experiences of TI between participants who believed substances impacted their experience of TI and those who did not. Our research suggests that, of the models tested, the Modified TIS-Brief Form may be superior for measuring TI. (PsycInfo Database Record (c) 2026 APA, all rights reserved).