They found that participants with high maths anxiety made correct responses more quickly and, after the test, showed lower levels of cortisol, an indicator of stress. On the other hand, individuals with low maths anxiety performed worse after tDCS.
“It is hard to believe that all people would benefit similarly [from] brain stimulation,” says Cohen Kadosh. He says that further research could shed light on how to optimise the technology and help to discover who is most likely to benefit from stimulation.

via Personality affects maths-enhancing brain-zap method – life – 09 December 2014 – New Scientist.
Link to full paper: Cognitive Enhancement or Cognitive Cost: Trait-Specific Outcomes of Brain Stimulation in the Case of Mathematics Anxiety