In Cardano’s proof-of-stake protocol, the reward sharing scheme is designed to balance incentives for stake pool operators (SPOs) and delegators, while promoting decentralisation. One key parameter influencing this balance is a0, a parameter that determines how much influence a stake pool's pledge has on its overall rewards. Understanding a0 helps SPOs and delegators make informed decisions about pool setup and delegation strategy.
What is a0?
a0 controls the weight of pledge in the reward calculation formula. Pledge is the amount of ADA a pool operator commits to their own pool, signalling “skin in the game.” When a0 is higher, pledged ADA earns a higher proportion of rewards, increasing the incentive for pool operators to pledge more of their own funds. Conversely, a low a0 diminishes the reward benefit of pledge and flattens the reward landscape between high-pledge and low-pledge pools (1).
Why should pledge matter?
In theory, a well-designed pledge mechanism helps prevent Sybil attacks — where a single entity creates multiple pools to gain an unfair share of rewards — by requiring operators to put more capital at stake. The a0 parameter ties pledge to rewards, making such attacks economically costly if a0 is sufficiently high. As such, a0 plays a role in Cardano’s approach to Sybil resistance and decentralisation (2).
The current value and real-world effects
As of June 2025, Cardano’s a0 is set to 0.3. This level is considered low-to-moderate in terms of its impact on the reward curve. A fully pledge-saturated pool (~75M ADA pledged, with current parameters) might gain a rewards advantage relative to a pool with minimal pledge, assuming all else is equal.
However, data shows that pledge leverage — the ratio of stake controlled versus pledged — remains high across much of the network. Many pools control tens of millions of ADA with little to no pledge. This suggests that a0 is currently too weak to significantly influence operator behaviour or deter large multipool operators, who divide their stake across multiple pools to maximize rewards (3).
This concern has been echoed by IOG Chief Scientist Prof. Aggelos Kiayias, who noted that multipool dominance is “contrary to the intended incentives of the reward scheme” and can harm the decentralisation of the network (4).
CIP-50 and misconceptions
Discussion around a0 resurfaced with CIP-50, a proposal aiming to rebalance incentives through the introduction of L (pledge leverage). Contrary to some interpretations, the authors of CIP-50 did not advocate for removing a0 entirely. Rather, they proposed setting it to 0 under a revised formula as a starting point, with room for further tuning later (5). This distinction is important when evaluating the intent and design goals of the proposal.
Why a0 and its impact need addressing
For SPOs who pledge substantial amounts, a0 is the only mechanism that gives better returns for higher commitment. Without a meaningful a0, these operators are on par with those who pledge little to nothing.
With current network parametrisation, a0 has little to no effect except for pools that are near full pledge saturation. It is therefore fair to say that a0 in the current RSS has failed to provide its intended purposes, both for mitigating pool-splitting behaviour and incentivising high "skin-in-the-game".
See it in action
Discover the effect of changes to a0 and other parameters with our public RSS Tool.
References
- Corduan et al. (2019). Design Specification for Shelley Incentives.
- Brünjes et al. (2020). Reward Sharing Schemes for Stake Pools.
- Pledge data charts available at the Donut Shop by Balance Analytics.
- Kiayias, A. (2020). General perspective on staking in Cardano.
- CIP-50 (2023). Reward Sharing Scheme Updates.