Negative symptoms in schizophrenia have been grouped into the two factors of apathy and diminished expression, which might be caused by separable pathophysiological mechanisms.
Recently, it has been proposed that apathy could be due to dysfunctional integration of reward and effort during decision making.
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From http://schizophreniabulletin.oxfordjournals.org/content/early/2014/07/22/schbul.sbu102.abstract.html?papetoc