• Measurement development and validation emerged as a core methodological pillar, with standardized inventories and self-report scales shaping depression assessment across decades [1], [3], [9], [10], [18].

• Biochemical and neuroendocrine explorations framed depression as a disorder of brain chemistry, leveraging monoamine metabolites, serotonin hypotheses, and HPA axis activity to explain symptoms and pharmacology [5], [11], [12], [14], [19], [20].

• Diagnostic classifications and theoretical framing shifted toward syndrome-based, etiological, and comorbidity perspectives, guiding clinical psychiatry and research organization [2], [7], [8], [13], [16].

• Epidemiology and demographic modifiers such as sex differences were foregrounded as critical for prevalence, risk factors, and treatment planning, enabling cross-study synthesis [4], [6], [16].

• Integrated treatment paradigms combined pharmacotherapy with psychotherapy and outcome measurement, reflecting a pragmatic approach to depression care and the role of standardized scales [2], [3], [7], [15].