Anthropic Bias Observation Selection Effects in Science and Philosophy Nick Bostrom

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Anthropic Bias

Observation Selection Effects in Science and Philosophy
Nick Bostrom


This book explores how to reason when you suspect that your evidence is biased by observation selection effects. An explanation of what observation selection effects are has to await chapter 1. Suffice it to say here that the topic is intellectually fun, difficult, and important. We will be discussing many interesting applications: philosophical thought experiments and paradoxes aside, we will use our results to address several juicy bits of contemporary science: cosmology (how many universes are there?), evolution theory (how improbable was the evolution of intelligent life on our planet?), the problem of time’s arrow (can it be given a thermodynamic explanation?), game theoretic problems with imperfect recall (how to model them?), traffic analysis (why is the “next lane” faster?) and a lot more – the sort of stuff that intellectually active people like to think about…


Preface 2


Acknowledgements 6


Observation selection effects 7

A brief history of anthropic reasoning 10

Synopsis of this book 12


Does fine-tuning need explaining? 17

No “Inverse Gambler’s Fallacy” 20

Roger White and Phil Dowe’s analysis 21

Surprising vs. unsurprising improbable events 26

Observation selection effects 34

Conclusions 41

CHAPTER 3: Anthropic Principles, the motley family 44

The anthropic principle as expressing an observation selection effect 44

Anthropic hodgepodge 49

Freak observers and why earlier formulations are inadequate 56

The Self-Sampling Assumption 62

CHAPTER 4: thought experiments supporting the SELF-SAMPLING ASSUMPTION 64

The Dungeon 64

Two thought experiments by John Leslie 67

Incubator 69

The reference class problem 74

chapter 5: The Self-Sampling Assumption in science 78

SSA in cosmology 78

SSA in thermodynamics 80

SSA in evolutionary biology 82

SSA in traffic analysis 85

SSA in quantum physics 86

Summary of the case for SSA 89


Introduction 92

Doomsday à la Gott 93

The incorrectness of Gott’s argument 96

Doomsday à la Leslie 99

The assumptions used in DA, and the Old evidence problem 100

Leslie on the problem with the reference class 108

Alternative conclusions of the Doomsday argument 111

CHAPTER 7: invalid objections against the DOOMSDAY ARGUMENT 113

Objection One (Korb and Oliver) 113

Objection Two (Korb and Oliver) 114

Objection Three (Korb and Oliver) 120

Objection Four (Korb and Oliver) 121

Objection Five (Korb and Oliver) 123

Couldn’t a Cro-Magnon man have used the Doomsday argument? (Various) 125

Aren’t we necessarily alive now? (Mark Greenberg) 125

Sliding reference of “soon” and “late”? (Mark Greenberg) 126

How could I have been a 16th century human? (Mark Greenberg) 126

Doesn’t your theory presuppose that the content of causally disconnected regions affects what happens here? (Ken Olum) 127

But we know so much more about ourselves than our birth ranks! (Various) 127

Safety in numbers? Why the Self-Indication Assumption should be rejected (several) 128


Leslie’s argument, and why it fails 132

Observer-relative chances: another go 144

Discussion 150

Conclusion 160

Appendix 160


The Adam & Eve experiments 173

Analysis of Lazy Adam: predictions and counterfactuals 178

The UN++ gedanken: reasons, abilities, and decision theory 186

Quantum Joe: SSA and the Principal Principle 189

Conclusion 190

Appendix: The Meta-Newcomb problem 192

CHAPTER 10: Observation theory: A methodology for anthropic reasoning 194

Building blocks, theory constraints and desiderata 194

Outline of the solution 195

SSSA: Taking account of indexical information of observer-moments 196

Reassessing Incubator 198

How the reference class may be observer-moment relative 201

Formalizing the theory: the Observation Equation 204

A quantum generalization of OE 205

Non-triviality of the reference class: why must be rejected 206

Final thought on the reference class problem 210

Chapter 11: Observation selection theory applied to cosmological fine-tuning 212

Chapter 12: The Sleeping-Beauty problem: modelling imperfect recall 219

The Sleeping Beauty Problem 219

The case of no outsiders 220

The case with outsiders 221

Synthesis 221

General summary: How the theory measures up against desiderata 222



This work has benefited from copious feedback generated by bits and pieces of it that have been published earlier. Over the years, I must have corresponded with several hundreds of people about these issues. In addition, I’ve received comments from conference audiences, journal referees, students, and authors of replies to parts of this research that have already been published. For all this, I am most grateful!

There is a website associated with the book,, where there is a preprint archive collecting relevant writings that are available online, an updated bibliography, primers on various topics, and other resources that will make it easier for a scholar or interested layperson to get up to speed with the latest research on observation selection effects.

Although I cannot name everybody who has helped me in some way with this manuscript, there are some persons who must be singled out for my special thanks: Paul Bartha, Darren Bradley, John Broome, Jeremy Butterfield, Erik Carlson, Douglas Chamberlain, Pierre Cruse, Wei Dai, J-P Delahaye, Jean-Michel Delhotel, Dennis Dieks, William Eckhardt, Ellery Eells, Adam Elga, Hal Finney, Paul Franceschi, Mark Greenberg, Robin Hanson, Daniel Hill, Christopher Hitchcock, Richard Jeffrey, Bill Jefferys, Vassiliki Kambourelli, Loren A. King, Kevin Korb, Eugene Kusmiak, Jacques Mallah, Neil Manson, Peter Milne, Bradley Monton, Floss Morgan, Jonathan Oliver, Ken Olum, Don N. Page, David Pearce, Elliott Sober, Richard Swinburne, Max Tegmark, Alexander Vilenkin, Saar Wilf, and Roger White. I am so grateful to all those friends, named and unnamed, without whose input this book could not have been written. (The faults that it contains, however, I was perfectly capable of producing all by myself!)

I want to especially thank John Leslie for his exceedingly helpful guidance, Colin Howson and Craig Callender for long assistance and advice, Nancy Cartwright for removing a seemingly insurmountable administrative obstacle, and Milan C. Ćirković for managing to keep up collaboration with me on a paper whilst bombs were falling all around him in Belgrade. Finally, I want to thank Robert Nozick for encouraging rapid publication.

I gratefully acknowledge a generous research grant from the John Templeton Foundation that helped fund large parts of the research. I’m thankful to Synthese, Mind, Analysis, and Erkenntnis for permitting texts to be republished here. The book is dedicated to my father – tack pappa!

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