A decision maker acquires and processes information about an uncertain state of nature by an inquiry: a contingent sequence of questions to be asked before a decision is reached. Inquiry is a costly activity, with the cost proportional to its length. We characterize optimal inquiries and uncover two behavioural implications associated with costly inquiry: attention span reduction (i.e., favouring shorter inquiries by focusing on a subset of decisions and assigning them different priorities) and confirmation bias (i.e., seeking evidence through inquiry to confirm a prior guess of which decisions are optimal). This framework can be used to understand prominent cognitive biases, such as framing and search satisficing in healthcare and tunnel vision in criminal investigation.