Summary:  We tested the fixity of political preferences of 136 healthy males during the 2011 U.S. presidential election season by administering synthetic testosterone or placebo to participants who had identified the strength of their political affiliation. Before the testosterone treatment, we found that weakly affiliated Democrats had 19% higher basal testosterone than those who identified strongly with the party (p=0.015). When weakly affiliated Democrats received additional testosterone, the strength of their party fell by 12% (p=.01) and they reported 45% warmer feelings towards Republican candidates for president .  Our results demonstrate that testosterone induces a “red shift" among weakly-affiliated Democrats.  This effect was associated with improved mood.  No effects were found of testosterone administration for strongly affiliated Democrats or strong or weak Republicans.  Our findings provide evidence that neuroactive hormones affect political preferences.