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Group-living species frequently pool individual information in order to reach consensus decisions, such as when and where to move, or whether a predator is present. Such opinion-pooling has been demonstrated empirically, and theoretical models proposed to explain why group decisions are more reliable than individual ones. In behavioural ecology theory frequently assumes all individuals have equal decision-making abilities, yet decision theory relaxes this assumption and has been tested in human groups. We summarise relevant theory and argue for its applicability to collective animal decisions. We consider selective pressure on confidence-weighting in groups of related and unrelated individuals. We also consider which species and behaviours may provide evidence of confidence-weighting, paying particular attention to the sophisticated vocal communication of cooperative breeders.