# Functional Decision Theory: A New Theory of Instrumental Rationality

arXiv: https://arxiv.org/abs/1710.05060 · HTML: https://mlsanity.com/paper/1710.05060/

## Key facts

- Cited by 7 papers in the map (data through 14 Oct 2026).
- First posted on arXiv 13 Oct 2017.

## Built on by (most cited first, 7 of 7)

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- [Characterising Decision Theories with Mechanised Causal Graphs](https://mlsanity.com/paper/2307.10987/) (2023)
- [Dynamic Normativity: Necessary and Sufficient Conditions for Value Alignment](https://mlsanity.com/paper/2406.11039/) (2024)
- [Can CDT rationalise the ex ante optimal policy via modified anthropics?](https://mlsanity.com/paper/2411.04462/) (2024)
- [A game theory for foundation models shows new paths to rational cooperation through similarity inference](https://mlsanity.com/paper/2608.03958/) (2026)
- [Arguments about Highly Reliable Agent Designs as a Useful Path to Artificial Intelligence Safety](https://mlsanity.com/paper/2201.02950/) (2022)
- [Do LLMs Take Care of Their Own? Similarity Signals Can Induce Cooperation](https://mlsanity.com/paper/2608.12125/) (2026)

## Abstract (arXiv)

This paper describes and motivates a new decision theory known as functional decision theory (FDT), as distinct from causal decision theory and evidential decision theory. Functional decision theorists hold that the normative principle for action is to treat one's decision as the output of a fixed mathematical function that answers the question, "Which output of this very function would yield the best outcome?" Adhering to this principle delivers a number of benefits, including the ability to maximize wealth in an array of traditional decision-theoretic and game-theoretic problems where CDT and EDT perform poorly. Using one simple and coherent decision rule, functional decision theorists (for example) achieve more utility than CDT on Newcomb's problem, more utility than EDT on the smoking lesion problem, and more utility than both in Parfit's hitchhiker problem. In this paper, we define FDT, explore its prescriptions in a number of different decision problems, compare it to CDT and EDT, and give philosophical justifications for FDT as a normative theory of decision-making.