adult article 2 cooperation 2 female 2 human 2 male 2 transcranial direct current stimulation 2 ventromedial prefrontal cortex 2 controlled study 1 cooperation behavior 1 cooperation belief 1 decision making 1 framing bias 1 gains and losses 1 harm frame 1 help frame 1 human experiment 1 neuroscience 1 sex difference 1 sham procedure 1 social framing effect 1 victim 1