This paper investigates a problem where the solver must
firstly determine which of two possible causes are the source
of an effect where one cause has a historically higher
propensity to cause that effect. Secondly, they must update the
propensity of the two causes to produce the effect in light of
the observation. Firstly, we find an error commensurate with
the ‘double updating’ error observed within the polarisation
literature: individuals appear to first use their prior beliefs to
interpret the evidence, then use the interpreted form of the
evidence, rather than the raw form, when updating. Secondly,
we find an error where individuals convert from a
probabilistic representation of the evidence to a categorical
one and use this representation when updating. Both errors
have the effect of exaggerating the evidence in favour of the
solver’s prior belief and could lead to confirmation bias and
polarisation