Sometimes, a person wants to do something that could have adverse impacts on well-being which they are not in the best position to judge. In these circumstances, the decision-making required of social workers is complex. Using a novel vignette-based methodology, we conducted four studies with registered social workers in England to estimate their evaluations of information, potential actions and relevant legal principles. Study One, where 75 practitioners completed a decision-making task for two vignettes, showed that evaluation breadth was limited, perhaps reflecting the inherent difficulty of this decision-making, and not consistently determined by identifying relevant legal principles. Study Two, where 84 practitioners completed the task for two different vignettes, replicated these findings, despite enhanced instructions. Study Three, where eight practitioners ‘thought-aloud’ whilst considering a vignette, revealed limitations in information processing, with fixed explanatory frames, an apparent default to prioritising the person’s decision, and confirmation bias. Study Four, where Study One and Two data were reanalysed, showed that the more information social workers referred to, the better their evaluations. We are now developing tools to support social workers balance core professional values, attend to more information, apply strategies to counteract confirmation bias and confidently apply relevant legal principles in their decisionmaking.