A diversity of neurochemical hypotheses related to Major Depression (MD) and Generalized Anxiety (GAD) suggests that further investigation is needed for uncovering the neurophysiological nature of MD. Since both temperament and mental illness have been linked to varying degrees of the same neurotransmitter imbalances in neurophysiological systems of behavioural regulation, the analysis of temperament traits as weak forms of neurochemical imbalances might give new insights into symptoms and nature of MD and GAD. This study analyzed the predictions of the opioid receptor hypothesis within the neurochemical model of Functional Ensemble of Temperament proposing that a dysregulation of the opioid receptors acting on monoaminergic systems gives rise to the symptoms of MD and GAD. The study investigated the coupling of sex, age and temperament traits with MD across four age groups (17-24, 25-45, 46-65, 66-85) by reviewing the records of testing in patients and volunteers with and without mental illness. In line with the hypothesis, patients with MD had significantly lower Physical Endurance, Social-verbal Endurance, Mental Endurance, Plasticity, Physical Tempo, Sensation Seeking and Satisfaction, and higher Impulsivity and Neuroticism than non-depressed individuals. Other sex- and age-related effects are reported.