An essentialist view of mental illness states that mental illness is a state that exists independently from society and that it can be defined objectively and is caused by neurological issues or by lifetime experiences. Essentialists would argue that people have mental illness because they exhibit a concrete set of behaviors and that these behaviors define mental illness. They would also argue that if, for example, a drug cures or alleviates the symptoms of a mental illness, that illness exists in objective terms. The DSM, or Diagnostic and Statistical Manual used to diagnose mental disorders, follows this model. The "hard" essentialist model regards mental disorders as types of diseases, while the "soft" essentialist model regards mental disorders as a psychological state.
On the other hand, constructionism argues that mental illness was created by society to label people insane if they have behaviors that deviate from social norms. People in this camp have a broader view of how deviant behaviors are defined in society, and they believe that if people break the sociological standards made by society, those people are construed as insane. The subtypes of constructionism are causal construction, in which social factors help bring about a mental disorder, and constitutive construction, in which social factors help define it in the first place.
Both models likely have some validity in explaining many mental illnesses. For example, while the biochemistry of people with mental disorders might be different (supporting an essentialist approach), people with mental disorders might also act differently than others and be labeled as different (supporting constructionism).
Raphael van Riel. What Is Constructionism in Psychiatry? From Social Causes to Psychiatric Classification. Psychiatry. 2016; 7: 57. Published online 2016 Apr 18. doi: 10.3389/fpsyt.2016.00057 PMCID: PMC4834349.