Shall we get the COVID-19 vaccine? Is it okay to lie to benefit others? People are inundated with decisions in everyday life. Although traditional economic theories assume that individuals should act as if balancing costs against benefits to reach a decision that maximizes personal advantage, in reality, people often make suboptimal decisions. Such systematic ‘errors’ are known as decision biases. We use novel behavioral paradigms combined with neuroimaging methods such as EEG/fMRI to study why people fall prey to biases, such as the default bias. We show that the coding of cost/benefit information in the brain is heavily susceptible to contextual information rather than determined by objective values, and suggest that the neuroscience approach may help better understand decision biases.