Optimism bias and positive attention bias are important features of healthy information processing. Recent findings suggest dynamic bidirectional optimism-attention interactions, but the underlying neural mechanisms remain to be identified. The current functional magnetic resonance imaging (fMRI) study, therefore, investigated the neural mechanisms underlying causal effects of optimistic expectancies on attention. We hypothesized that expectancies guide attention to confirmatory evidence in the environment, with enhanced salience and executive control network (SN/ECN) activity for unexpected information. Moreover, based on previous findings, we anticipated optimistic expectancies to more strongly impact attention and SN/ECN activity than pessimistic expectancies. Expectancies were induced with visual cues in 50 participants; subsequent attention to reward and punishment was assessed in a visual search task. As hypothesized, cues shortened reaction times to expected information, and unexpected information enhanced SN/ECN activity. Notably, these effects were stronger for optimistic than pessimistic expectancy cues. Our findings suggest that optimistic expectancies involve particularly strong predictions of reward, causing automatic guidance of attention to reward and great surprise about unexpected punishment. Such great surprise may be counteracted by visual avoidance of the punishing evidence, as revealed by prior evidence, thereby reducing the need to update (over)optimistic reward expectancies.Most of us are overly optimistic about our future 1,2 . This optimism bias and similar cognitive phenomena such as positive attention bias (preferably attending to positive vs. neutral information in our environment 3 ) influence the way we see the world. Despite the fact that both biases are considered as important features of healthy information processing 1,4,5 and share an association with positive health outcomes 6,7 , optimism bias and positive attention bias have mostly been examined separately.The combined cognitive biases hypothesis suggests that cognitive biases usually interact and mutually enforce each other and, together, have a greater impact than each bias on its own 8,9 . The combined cognitive biases hypothesis was originally proposed based on findings of a reciprocal relationship between negative self-imagery and biased interpretation in social phobia but has soon been extended to other psychological disorders such as depression (e.g. interacting biases in attention, interpretation, memory, cognitive control) 10 and anxiety (e.g., expectancy and attention bias interactions) 8 .Moreover, we 11 have argued that cognitive bias interactions are not restricted to negative biases that maintain psychological dysfunction. Instead, they may well translate to positivity biases promoting psychological function. Specifically, we proposed a bidirectional interplay, namely (a) optimistic expectancies guide attention to positive information in the environment, and (b) directing attention to positive information enhanc...