APMM: Automated Parlay Market Maker

Abstract

Parlays - joint contracts on the simultaneous resolution of several events - are among the most heavily traded products in betting markets, but prediction markets have struggled to offer them natively. In this paper, we offer the full combinatorial family of parlays on top of $M$ binary events, as liquid markets, bounding the market maker loss for subsidizing the markets to $O(M^2)$. Any single parlay attracts few traders, so each is an inherently thin market, and the logarithmic market scoring rule (LMSR) is the natural mechanism for thin markets. But running a separate LMSR for each of the exponentially many parlays forces the market maker to pay for the same information many times over. We show that a market maker which automatically propagates information across related parlays avoids this redundancy. We make three contributions. First, we introduce the automated parlay market maker (APMM), which uses a \emph{hierarchical parameterization}: the state of each low-leg parlay is shared into every higher-leg parlay that contains it, so after pricing one, the new information propagates. Second, we show that when informed trading is concentrated in parlays with few legs, the market maker's worst-case loss is bounded by $O(M^2)$, and the expected loss is bounded by $O(M)$. Third, we validate these bounds in simulation and on historical Kalshi order flow, confirming that real belief updates are dominated by low-leg changes and that APMM's advantage persists under real trading patterns.