The Dopamine Hypothesis of Schizophrenia: An Historical and ...

54 ■ PPP / Vol. 18, No. 1 / March 2011
and refutations, as newer and yet more falsifiable
theories were advanced.
How would the DHS fare on these Popperean
grounds? At its inception, the DHS was highly abstract
and nonspecific, postulating that the etiology
of a complex neuropsychiatric syndrome was due
to hyperactivity in one neurotransmitter system.
Even at the time of the articulation of the DHS,
neurotransmitter systems were known to be complex
and diverse, with the same neurotransmitter
being used in different pathways with distinct
functional roles. Control mechanisms existed at
many levels, both within and between cells. Overactivity
of the system could arise in many ways.
As scientific understanding of brain functioning
advanced, this problem became worse, making
it more and more difficult to articulate a single,
clear test of the DHS. Because of the complexity
of the systems involved, the DHS was elastic in
absorbing a wide range of results. The IT version
of the DHS predicted that levels of DA and HVA
should be increased in the brains of individuals
with schizophrenia. However, the IRF version of
the DHS could produce a decrease in DA and HVA
levels. If abnormalities were not found in global
measures of DA function, then it was because the
abnormalities in schizophrenia were found only
in specific brain regions or pathways.
In Popper’s terms, the DHS had a low degree
of falsifiability. As Popper might have predicted,
given its abstract formulation, if one prediction of
the DHS was not confirmed, another easily took
its place. This nonspecificity allowed the DHS to
take on the protean features that we described. In
this process, the DHS has become nearly immune
from falsification. Indeed, in its latest incarnation
(see quote above), it seems to be more of a general
framework than a specific theory.
There is a deep irony in the observation that the
DHS has fared poorly at the Popperean criterion
of falsifiability. The DHS emerged at a time when
the nascent field of biological psychiatry argued
strongly that psychoanalysis had to be abandoned.
One key argument was that psychoanalysis was
not a science that was illustrated by the protean
and non-falsifiable nature of key psychoanalytic
theories. Such theories, they argued, could explain
anything. At its inception, early biological psychiatrists
would have insisted that the DHS was the
antithesis of psychoanalytic theory. However, over
time, the nature of DHS as a scientific hypothesis
has come to resemble the psychodynamic theories
that it was meant to displace.
Scientists have frequently embraced the Popperean
philosophy and, on deeper reflection, later
became more critical of this approach (Katz 1994),
in part because the actual flexibility and practice
of science go beyond the Popperean model. It is to
such alternative approaches we now turn.
Thomas Kuhn
The work of Kuhn in his classic “The Structure
of Scientific Revolutions” (Kuhn 1996) can help
us to understand the endurance of the DHS. For
Kuhn, Popperean falsification did not accord with
scientific practice. Rather, Kuhn suggested that
scientists work within broad conceptual frameworks
or “paradigms.” Like most human beings,
working scientists find it difficult to think outside
the framework in which they “grew up.” Indeed,
Kuhn suggested that scientists typically strongly
resist purported falsifications of their paradigms,
and irrationally retain belief in them in the face
of increasing anomalies. However, at some critical
point, two things can occur together, which unleashes
what he termed a “scientific revolution.”
First, the accumulation of anomalies in a particular
research paradigm becomes unsustainable. Second,
another paradigm becomes available that
does a superior job at explaining these anomalies.
Then the scientific field undergoes a crisis, and experiences
a “conversion” as individual researchers
(more often the young than the old) switch to the
new paradigm.
The DHS probably does not rise to the level of
a Kuhnian paradigm, which might, for example,
be an appropriate description for the “biomedical
model” for psychiatric disorders. Nonetheless,
some of the insights of Kuhn on the historical process
of science can be usefully applied to the DHS.
In particular, no revolution has occurred in
theories about the etiology of schizophrenia that
has resulted in the wholesale abandonment of the
DHS. Why? Certainly, many empirical findings,
some outlined above, are poorly explained by
the DHS. However, one key ingredient has been