General disease factor: evidence of a unifying dimension across mental and physical illness in children and adolescents.

Background: Understanding the relationship between mental and physical health conditions is crucial for developing comprehensive healthcare strategies. The putative existence of a general disease factor (d-factor) that underlies the vulnerability to both physical and mental conditions could have important implications for our approach to health assessment and treatment.

Objective: To investigate the presence and characteristics of a general d-factor in children and adolescents.

Methods: This Swedish registry-based cross-sectional study included children and adolescents born between 1996 and 2003 with follow-up until 2013. We extracted data on 25 mental and physical health conditions according to the ICD-10 system. To determine the optimal dimensional structure of these conditions, several competing measurement models were tested, including correlated factors, one factor, various bifactor specifications and bifactor exploratory structural equation modelling (ESEM).

Findings: The study cohort included 776 667 individuals (mean age 13.96 years, IQR=11.96-16.04; 51% male). The bifactor ESEM model, including a general d-factor and specific mental and physical health factors, provided the best fit to the data compared to alternative models (Comparative Fit Index=0.971, Tucker-Lewis Index=0.962, root mean square error of approximation=0.007 (0.007-0.007)). The d-factor accounted for substantial variance (ω_h=0.582, explained common variance (ECV)=0.498), while specific mental (ω_hs=0.377, ECV=0.373) and physical (ω_hs=0.423; ECV=0.130) factors also indicated additional significant unique contributions.

Conclusions: This study provided evidence for a multidimensional structure of health in children and adolescents, characterised by a general d-factor underlying both mental and physical conditions, alongside distinct domain-specific factors. These findings have important implications for clinical practice, providing evidence that suggests the need for more integrated approaches to health assessment and treatment that consider the interconnectedness of mental and physical health.