Miners allocate limited block space to transactions that maximize their expected fee revenue, using simple economic rules and practical heuristics. Blocks are constrained by Bitcoin’s block weight limit, so miners favor transactions with the highest fee rate measured in satoshis per virtual byte. This approach is described in technical literature and educational sources such as Arvind Narayanan of Princeton University, who explains how fee markets emerge when block space is scarce. In practice, that means crowding out low-fee or dust transactions during peaks of demand.
Fee rate, package selection, and child-parent dynamics
Beyond single-transaction ranking, miners consider ancestor feerate and package selection. Transactions that are part of chains—parents that are unconfirmed and children that pay higher fees—are evaluated together because including a low-fee parent can enable collecting the child’s higher fee. This is the principle behind Child Pays for Parent CPFP and is handled in mining software and node mempool policies. Implementation details and policy trade-offs have been worked on by Bitcoin Core contributors including Pieter Wuille of Blockstream. Miners therefore often run algorithms that compute effective feerates for packages instead of naively sorting by individual fee.
Causes, consequences, and broader implications
Mempool congestion arises from sudden demand spikes, application-layer batching patterns, or fee estimation failures by wallets; it can also be intentionally induced by fee-sniping or spam. Ittay Eyal and Emin Gün Sirer of Cornell University have shown that miner incentives can create strategic behavior beyond naive revenue maximization, with potential systemic effects on confirmation fairness. The immediate consequence of prioritizing high-feerate transactions is longer waits and higher costs for low-fee users, which has social and territorial implications where connectivity and income limit users’ ability to pay dynamic fees. At a network level, persistent high-fee environments shift user behavior toward batching transactions, second-layer solutions, or alternative chains, influencing ecosystem architecture and software development priorities.
Miners may also exercise non-economic discretion: honoring custodial clients, including zero-fee transactions for partners, or following community norms encoded in node policies. Over time, software updates and research-driven policy changes—documented by protocol developers and academic authors—shape how miners translate fee signals into block templates, balancing short-term revenue with long-term network health.