Objectives. Construct overlap among important variables in health psychology was studied. Competing theories related to the reporting of medically unexplained symptoms (somatization) and illness severity were examined. These objectives were studied in the setting of insulin‐dependent diabetes mellitus (IDDM).Design. A correlational design was employed. Sufficient numbers of participants were sought in order to test theories of symptom reporting using structural equation modelling and other multivariate techniques.Methods. Two hundred and two people with IDDM attending a diabetes out‐patient clinic acted as participants. Personality traits, alexithymia and illness‐related coping factors were the key psychological constructs studied. Experience of medically unexplained symptoms (somatization) and diabetes severity were measured.Results. Neuroticism, alexithymia and negative emotion coping were all significantly related to somatization, and there was significant construct overlap. Competing models of symptom reports—transactional versus negative affectivity—were formulated and tested using structural equation modelling. However, the best fitting model was one that integrated features of both. Constructs showed considerable overlap, with evidence for a very general factor related to the reporting of negative affects and bodily disturbances. However, there is also evidence of unique variance in constructs, especially that of negative emotion coping.Conclusions. Important constructs used in health psychology show significant overlap, and this general source of variance warrants further study. Despite overlap, constructs contributed unique variance to health outcomes. Transactional and negative affectivity models of symptom reporting may be reconciled in an integrated model. Generalization of the models tested here to other illness samples and healthy individuals is recommended. Suggestions are made for further research to refine constructs in health psychology and to limit their present