Background Different degenerative brain diseases result in distinct personality changes as a result of divergent patterns of brain damage; however, little is known about the natural history of these personality changes throughout the course of each disease.

Objective To investigate how interpersonal traits change as a function of degenerative brain disease type and severity.

Methods Using the Interpersonal Adjective Scales, informant ratings of retrospective premorbid and current scores for dominance, extraversion, warmth and ingenuousness were collected annually for 1 to 4 years on 188 patients (67 behavioural variant frontotemporal dementia (bvFTD), 40 semantic dementia (SemD), 81 Alzheimer's disease (AD)) and 65 older healthy controls. Using random coefficient models, interpersonal behaviour scores at very mild, mild or moderate-to-severe disease stages were compared within and between patient groups.

Results Group-level changes from premorbid personality occurred as a function of disease type and severity, and were apparent even at a very mild disease stage (Clinical Dementia Rating=0.5) for all three diseases. Decreases in interpersonal traits were associated with emotional affiliation (ie, extraversion, warmth and ingenuousness) and more rigid interpersonal behaviour differentiated bvFTD and SemD patients from AD patients.

Conclusions Specific changes in affiliative interpersonal traits differentiate degenerative brain diseases even at a very mild disease stage, and patterns of personality change differ across bvFTD, SemD and AD with advancing disease. This study describes the typical progression of change of interpersonal traits in each disease, improving the ability of clinicians and caregivers to predict and plan for symptom progression.