  Idiosyncratic tendency to choose one alternative over others in the absence
of an identified reason, is a common observation in two-alternative
forced-choice experiments. It is tempting to account for it as resulting from
the (unknown) participant-specific history and thus treat it as a measurement
noise. Indeed, idiosyncratic choice biases are typically considered as
nuisance. Care is taken to account for them by adding an ad-hoc bias parameter
or by counterbalancing the choices to average them out. Here we quantify
idiosyncratic choice biases in a perceptual discrimination task and a motor
task. We report substantial and significant biases in both cases. Then, we
present theoretical evidence that even in idealized experiments, in which the
settings are symmetric, idiosyncratic choice bias is expected to emerge from
the dynamics of competing neuronal networks. We thus argue that idiosyncratic
choice bias reflects the microscopic dynamics of choice and therefore is
virtually inevitable in any comparison or decision task.
