Temperament and mental illnesses are considered to be varying degrees along the same continuum of neurotransmitter imbalance in neurophysiological systems of behavioural regulation. Three studies investigated relationships between temperament traits and clinical scales measuring five groups of symptoms of mental illness: Depression, Anxiety, Antisocial behaviour, Dominance-Mania and Borderline/Schizophrenia.

The intake records of 395 (Study 1), 331 (Study 2), and 214 (Study 3) clients of a private psychological practice were examined, including their scores on the Structure of Temperament Questionnaire (STQ-77); Study 3 used the Personality Assessment Inventory (PAI). The sample was divided into four age groups (17-24, 25-45, 46-65, and 66-84).

Study 1 and 2 found changes in the dynamical aspects of behaviour, such as endurance, plasticity and impulsivity in patients suffering from anxiety and depression. Study 3: Scores on the Depression- and Anxiety-related scales of PAI correlated with temperament scales of the STQ in a pattern similar to Studies 1 and 2. Anti-social scales of PAI correlated positively with Sensation Seeking and Impulsivity of the STQ-77. Higher Anxiety and Anti-social symptoms appeared to be more prominent in the younger age (unlike Depression symptoms), and declined with age. Dominance-Mania scores of PAI correlated positively with the Social-verbal Endurance and Tempo, Empathy and Self-confidence.

The analysis of age differences in association between temperament and mental illness showed age-sensitive patterns. Strong concurrent validity between the STQ-77 and the PAI scales suggests links between temperament traits and disposition for the main group of mental illness.