Lack of measurement invariance in mental health assessment across intelligence levels: Investigation into nonlinearity reveals a broader issue

The notion of individuals with extremely high intelligence experiencing specific psychological difficulties has been brought up many times by researchers, with no consensus being reached on this matter. Modeling the relationship between intelligence and mental health nonlinearly allows for revealing the possibility for extremely intelligent individuals to experience specific difficulties affecting their mental health while still having better mental health on average. Moreover, with gender roles ingrained in the social world, men and women are likely to have different experiences with how their social environments react to their high intelligence. This study used data from two cohort studies: the National Longitudinal Survey of Youth 1979 (NLSY79; n = 8474) and 1997 (NLSY97; n = 6472). Although, the results of polynomial and piecewise regression suggested that the relationship of intelligence with depression and distress is nonlinear, the results of local structural equation models showed the lack of measurement invariance of mental health measures across intelligence values for both men and women. The factor loadings for the measures decreased, as the intelligence values increased. As such, values of mental health are not comparable across different levels of intelligence. If this effect is also present on a larger number of psychometric instruments, the implications on psychological research could be substantial. The lack of measurement invariance of psychometric instruments across intelligence can compromise the results of studies, not only those testing for nonlinear relationships of intelligence, but also those testing for linear ones.