SACHIN
SARIN MD
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.
References
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