Tinbergen (1963) famously described a framework for understanding behavior in which he emphasized that in order to understand behavior we must appreciate not only its proximate but also its ultimate or evolutionary causes. While evolutionary theory has long been touted as critically important towards our understanding of human pathology (Williams & Nesse 1991; Nesse & Williams 1996; Nesse et al, 2010) the field of psychiatry has largely ignored evolutionary theory (Brüne 2012).

Instead, the predominant nosology of psychiatric diagnosis, the DSM 5, relies on a speculative constellation of symptoms, called disorders, which are deemed present or absent in a dynamic but otherwise dichotomous manner. This translation, from dimensional to categorical model, necessarily leads to a loss of information, in this instance limiting a more in depth understanding of the ailments affecting psychiatric patients. Further, the current model of psychiatric diagnosis has impeded advances of novel therapeutic and research models in psychiatric illnesses (Holtzheimer and Mayberg, 2011). Any further improvement in the efficacy of pharmacotherapy for psychiatric disorders is inherently limited because each disorder represents a heterogeneous group of syndromes (Wong et al., 2010).

Heterogeneity has also limited further determination of the underlying genetics of psychiatric disorder as the empirical genetic data suggest they are overlapping constructs (Cross-Disorder Group of the Psychiatric Genomics Consortium, 2013). Alarmingly, the DSM 5 has been criticized due to potential conflicts of interest, lack of scientific rigor and transparency (including a refusal for an independent review that was endorsed by many mental health organizations), which has led to “The New Crisis of Confidence in Psychiatric Diagnosis” (Frances, 2013). That is not to say that there is no benefit to the current system of diagnoses as it guides treatment and undoubtedly contributes to the improvement in quality of lives in our patients. However, psychiatric syndromes as they are currently defined, result from an incredibly complex and poorly understood interaction of genetics and environment and their etiology remains largely
elusive. Only now, with recent advancements in technology, and the broad understanding that our current definitions of disorders need to be modified, are we ready for a true paradigm shift in psychiatry.

The NIMH funded RDoC project represents a radical change as it attempts to utilize neurobiological measures to rethink psychiatric illnesses as disorders of brain neuro-circuitry along a dimensional model (Insel, 2014).  While the project is still in its early stages, and is currently lacking clinical applicability, the model fits the context of evolutionary theory as natural selection does not shape diseases, but rather selects for traits that render us vulnerable to disease (Nesse, 2011). Proximate and evolutionary understandings of behavior combine in a complimentary and synergistic way and, as Tinbergen suggested, both must be appreciated to truly understand complex behavioral phenomena including psychiatric syndromes. Evolutionary theory allows us to understand why behavior exists and provides the metatheory for the generation of testable hypotheses to guide both research and interventions.

Life History Theory and Evolutionary Mismatch
Life history theory explores how and why organisms utilize time and energy for activities that comprise the life cycle and differential allocation of resources by individuals comprises different life history strategies (Del Giudice & Ellis, 2014).  Life history helps us understand individual differences in behavior and its concepts provide an organizing framework for understanding the development of mental health disorders (Brune, 2014). Life history theory predicts that harsh environmental conditions reduce the amount of parental investment in individual offspring, which in turn biases the offspring towards immediate resource extraction and opportunistic behavior, accelerating biological maturation of offspring, and fostering short-term pair bonds and risky sexual behavior (Belsky, 1991).  This is one explanation that helps us understand why early adverse life events are associated with the development of psychiatric disorders later in life. These events are suggestive of a harsh environment and cue an individual towards a faster life history strategy which has a number of significant consequences including more cognitive resources being devoted towards riskier, emotional behavior as opposed to behavior being guided by higher level cognitive processes; thereby, rendering these individuals more vulnerable to the emotional symptoms that underlie some mental health problems.

This concept is supported by data from humans which suggest that the two most profound structural changes in the brain as a result of early life stress are increases in amygdala volume and decreases in the prefrontal cortex volume (Davidson & McEwen 2012). However, what may have been adaptive at one time in human history may no longer be so due to a number of reasons including evolutionary mismatch (Durisko, 2016). The human environment of evolutionary adaptedness (EEA) is meant to explore the selection pressures in the environment that led to the development of a given adaptation. However it is important to note that EEA “is not a place or a habitat, or even a time period. Rather, it is a statistical composite of the adaptation-relevant properties of the ancestral environments encountered by members of ancestral populations, weighted by their frequency and fitness consequences” (Tooby and Cosmides 1990, 386–7).Each different psychological adaptation will have different evolutionary histories and consequently different EEAs. The majority of human psychological adaptations are thought to have evolved in hunter-gather societies in sub-Saharan Africa as this is where our species is believed to have spent the majority of its ancestral history. As our cultural evolution (technology, medicine) progresses at astounding speeds we are in a constant and dynamic state of evolutionary mismatch. Not only is the way we interact with the world markedly different now than in any other time in human history, but the way in which we interact with each other continues to rapidly change. Perhaps, it is no wonder that the rate of psychiatric disorders in modern society continues to rise as the demands that society places on individuals in no way resembles the life that we have been adapted to live.


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