Why can it be difficult to draw a clear line between mental health and mental disorder?

Drawing a clear line between mental health and mental disorder is difficult because there is no universally satisfactory definition for either state. Several factors contribute to this ambiguity:1

• Cultural Relativity: Mental states or behaviors considered abnormal in one culture may be viewed as normal or acceptable in another.1
• Lack of Biological Markers: Unlike many physical illnesses that can be identified through specific biological indicators, the causes of many mental disorders remain unknown. Consequently, diagnostic systems often rely on symptom patterns rather than underlying biological or structural brain disease.123
• Subjectivity and Overlap: Diagnostic categories are often poorly defined, and many patients present with intermediate cases that do not fit neatly into a single category. Furthermore, the same symptoms can appear across different, unrelated disorders, and patients frequently exhibit a mix of symptoms from multiple conditions.2
• Broad Definitions: Definitions of mental illness range from narrow (requiring organic brain disease) to broad (defining it simply as the absence of mental health). This lack of consensus makes it challenging to demarcate the boundary between healthy functioning and disorder.1