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The Formation of Belief 309

Subsumption
Negation
Figure 9.3. Dissonance reduction via the creation of an exception. A belief is contra-
dicted by new information. in response, the memory system creates two new subnodes
to the corresponding node, and re-attaches the contradicting information to one of the
subnodes.

B′ applies more generally. Festinger laid out one example from an anthropo-
logical study: The members of the ifaluk tribe believed that people are basically
good-hearted and wish others well. At the same time, they were in a position
to observe aggressive behaviors by the young men in their own villages. Taken
at face value, these observations constitute evidence that people are not all
good, creating dissonance. However, the ifaluk also believed that evil spirits
can take possession of individual human beings and make them perform evil
acts. This belief reduces the dissonance between the principle that people are
naturally good and the recurring observations of evil behaviors. The accuracy
of this description of ifaluk culture is not the issue here, only the nature of the
response to the contradiction: Add a belief that explains how the belief and the
seemingly contradictory fact can both be true. 42
Festinger did not name this mechanism, but in the philosophy of science,
an inference from an observation to a possible explanation of that observation
43
is called abductive reasoning. Abduction was first codified as a special form
of inference by C. s. Pierce in the late 19th and early 20th centuries, but his
concept languished in relative obscurity until highlighted by norwood russell
Hanson in his 1965 book Patterns of Discovery. The added belief B′ is in this
context thought of as a new explanatory hypothesis or auxiliary assumption.
A famous example of abduction occurred in astronomy when the French
19th-century mathematician Urbain Jean Joseph Le verrier studied pertur-
bations in the observed orbit of the planet Uranus that did not follow from
newton’s theory of gravitation. To explain these perturbations, Le verrier
44
assumed the existence of an as yet unobserved planet with a gravitational pull
that accounted for the perturbations. At the time of his proposal, there was
no other reason to believe in the existence of this planet, but the truth of this