Result tables

Samples per run: 250,000; base columns pool 8 runs. Monte Carlo standard errors on class probabilities: Earth given 0.0007, typical, all time 0.0007, typical, to date 0.0006, typical observer 0.0019, SIA 0.0050 (largest class). The self-indication (SIA) column rests on far fewer effective samples and is good to about 5 points.

A0. The three answers

Answer	Prior	Earth given	typical, all time	typical, to date	typical observer	SIA
Nobody here, nobody coming (empty universe + none in our Galaxy)	0.469	0.726	0.707	0.722	0.537	0.000
They arise and never spread	0.097	0.164	0.169	0.162	0.452	0.697
Someone did spread (on the way, unnoticed, hidden, or evident)	0.434	0.110	0.123	0.116	0.010	0.303
Same three among non-evident explanations only: nobody	0.733	0.729	0.718	0.731	0.538	0.000
Same three among non-evident explanations only: never spread	0.152	0.165	0.172	0.164	0.453	0.697
Same three among non-evident explanations only: spread	0.115	0.106	0.110	0.105	0.009	0.303

A. Posterior by explanation and by convention

Explanation	Prior	Earth given	typical, all time	typical, to date	typical observer	SIA
Nobody else in the observable universe	0.262	0.394	0.397	0.403	0.240	0.000
Others exist, none ever in our Galaxy	0.207	0.332	0.310	0.319	0.297	0.000
Expanders hold >1% of the universe, not here yet	0.031	0.039	0.039	0.038	0.005	0.000
Civilisations arose in the Galaxy, none became a sustained expander	0.097	0.164	0.169	0.162	0.452	0.697
Galaxy settled by low-footprint expanders, not evident	0.031	0.043	0.046	0.043	0.003	0.191
Galaxy settled, deliberately hidden	0.012	0.022	0.023	0.022	0.001	0.112
Galaxy claimed by evident settlers (we are in a pocket)	0.360	0.005	0.015	0.012	0.001	0.001

B. Headline quantities and splits

Quantity	Prior	Earth given	typical, all time	typical, to date	typical observer	SIA
No other civilisation ever in the Galaxy	0.580	0.780	0.764	0.776	0.541	0.000
... of which: no life anywhere else in the Galaxy	0.343	0.471	0.498	0.497	0.311	0.000
... of which: life elsewhere, no other technology	0.237	0.309	0.267	0.279	0.230	0.000
Stuck: a universal barrier to expansion	0.063	0.118	0.122	0.116	0.436	0.695
Stuck: no barrier, 100+ civilisations, none expanded	0.010	0.014	0.015	0.015	0.011	0.001
Stuck: no barrier, fewer than 100 civilisations	0.024	0.032	0.032	0.031	0.005	0.000
Unnoticed: low footprint forced on everyone	0.020	0.035	0.036	0.034	0.002	0.190
Unnoticed: mixed lineages, all happen to be discreet	0.010	0.008	0.010	0.009	0.001	0.001
Nobody (three-answer sense): no life anywhere else in the Galaxy	0.324	0.459	0.482	0.483	0.311	0.000
Nobody (three-answer sense): life elsewhere, no other technology	0.145	0.267	0.226	0.239	0.227	0.000
Nobody and a universal barrier exists	0.087	0.140	0.135	0.138	0.482	0.000
No other ever arose here, but the Galaxy is settled from outside	0.111	0.054	0.057	0.054	0.003	0.000
Life starts easily and nobody	0.016	0.066	0.041	0.044	0.042	0.000
Life starts easily and never spread	0.033	0.090	0.082	0.078	0.247	0.675
A home-grown civilisation other than us exists in the Galaxy now	-	0.083	0.091	0.084	0.207	0.971
Anyone else (home-grown or settler) is in the Galaxy now	-	0.143	0.162	0.150	0.211	0.981
Nobody has yet arisen in the region reachable at 0.1c	-	0.523	0.520	0.529	0.318	0.000
Nobody has yet arisen in the region reachable at c	-	0.427	0.429	0.435	0.260	0.000
A universal barrier to expansion exists	0.150	0.258	0.257	0.254	0.918	0.695
P(p_exp < 1e-6)	0.166	0.278	0.276	0.273	0.971	0.702
Mean chance a civilisation becomes an evident sustained expander	-	0.049	0.049	0.05	1.1e-06	1.8e-07
Life starts easily (lambda >= 1)	0.200	0.199	0.165	0.162	0.293	0.970
No hard steps (gated)	0.348	0.338	0.426	0.391	0.485	0.899
Mean number of hard steps	1.15	1.58	1.28	1.38	1.07	0.10
Effective sample size	-	489,778	394,153	488,211	62,052	6,888

Mean likelihood of the observations by explanation (Earth-given column): Nobody else in the observable universe 0.436, Others exist 0.463, Expanders hold >1% of the universe 0.366, Civilisations arose in the Galaxy 0.491, Galaxy settled by low-footprint expand 0.415, Galaxy settled 0.535, Galaxy claimed by evident settlers (we 0.004.

Implied priors (5%, 25%, 50%, 75%, 95%): log10 P(technological civilisation per rocky habitable-zone planet of a Sun-like star) -54.4, -21.7, -11.7, -6.9, -3.5; log10 p_exp barrier, -3.6, -1.5, -0.7, -0.1; log10 expected civilisations ever in the Galaxy -44.5, -11.8, -1.8, 3.0, 6.5.

C. What each observation contributes (Shapley values over all orderings)

P(no other civilisation ever arose in the Galaxy), Earth given	Shapley share (points)	lowest over orders	highest over orders	odds multiplied by
Before the evidence	0.580
Life started early on Earth	-0.073	-0.096	-0.049	0.70
We arrived late in Earth's window	+0.030	+0.018	+0.042	1.16
Nobody is visibly here	+0.219	+0.181	+0.257	2.83
Galaxy surveys and radio searches	+0.023	+0.007	+0.041	1.11
After all the evidence	0.780			2.57 in total

P(nobody here, nobody coming), Earth given	Shapley share (points)	lowest over orders	highest over orders	odds multiplied by
Before the evidence	0.469
Life started early on Earth	-0.067	-0.083	-0.049	0.74
We arrived late in Earth's window	+0.027	+0.017	+0.034	1.13
Nobody is visibly here	+0.268	+0.236	+0.299	3.18
Galaxy surveys and radio searches	+0.029	+0.013	+0.045	1.13
After all the evidence	0.726			3.01 in total

Early life as evidence for an easy origin, as a likelihood ratio (easy : hard): 2.43 alone, 2.80 given our late arrival, 2.75 given everything else. P(life starts easily): prior 0.200, posterior 0.199.

Sequential (one ordering) and single observations, Earth given:

Observations included	no other ever	nobody	never spread	evident settlement
prior	0.580	0.469	0.097	0.360
early	0.484	0.386	0.112	0.427
early+late	0.525	0.420	0.105	0.400
early+late+local	0.770	0.711	0.172	0.005
early+late+local+surveys	0.780	0.726	0.164	0.005
local	0.802	0.744	0.148	0.005
surveys	0.616	0.511	0.099	0.317
early+late+surveys	0.565	0.464	0.108	0.352

D. Camps: the same evidence under each school's priors

Camp	nobody before	nobody	never spread	spread	no other ever, before	no other ever	nobody (typical, all time)	nobody (typical observer)	P(easy life) before	after
Base (modeller's judgement)	0.47	0.72	0.16	0.11	0.58	0.78	0.70	0.53	0.20	0.20
Life starts easily (chemistry optimist)	0.27	0.53	0.31	0.17	0.36	0.58	0.50	0.30	0.60	0.60
Life starts easily, 90% sure	0.13	0.38	0.41	0.21	0.19	0.44	0.31	0.18	0.90	0.90
Life starts easily, as in model 3 (post-data value)	0.37	0.63	0.24	0.14	0.47	0.68	0.60	0.41	0.40	0.40
Life starts easily and no later step is improbable (was: Biology optimist)	0.05	0.13	0.60	0.27	0.06	0.14	0.12	0.04	0.90	0.86
Gradual evolution, no hard steps (Mills et al. 2025)	0.18	0.37	0.43	0.20	0.21	0.39	0.36	0.16	0.60	0.51
Classic SETI: life common, expansion doubtful	0.19	0.23	0.68	0.09	0.21	0.24	0.23	0.17	0.60	0.64
Origin of life no rarer than once per observable universe	0.28	0.56	0.27	0.17	0.41	0.64	0.52	0.38	0.28	0.33
Early life at the timing study's strength	0.47	0.69	0.19	0.12	0.58	0.74	0.67	0.48	0.20	0.28
Early life credited at 6 to 1	0.47	0.65	0.22	0.13	0.58	0.71	0.63	0.45	0.20	0.34
Early life credited at 9 to 1	0.47	0.61	0.25	0.14	0.58	0.66	0.59	0.39	0.20	0.44
Early life credited at 13 to 1	0.47	0.56	0.28	0.16	0.58	0.62	0.54	0.34	0.20	0.53
One or two shallow steps	0.49	0.74	0.15	0.11	0.61	0.80	0.72	0.58	0.20	0.21
Flat over 0 to 4 steps	0.54	0.78	0.12	0.10	0.66	0.84	0.76	0.63	0.20	0.22
Carter / Snyder-Beattie hard steps	0.85	0.94	0.02	0.04	0.95	0.98	0.93	0.95	0.10	0.14
Strict ignorance about rates	0.80	0.93	0.02	0.04	0.91	0.97	0.92	0.92	0.10	0.16
Rare Earth	0.55	0.77	0.13	0.10	0.69	0.83	0.75	0.62	0.20	0.22
Life dated from LUCA (very early)	0.47	0.67	0.20	0.13	0.58	0.72	0.65	0.46	0.20	0.31
Life dated from accepted fossils only	0.47	0.75	0.15	0.10	0.58	0.80	0.72	0.56	0.20	0.16
Expansion is easy (it only takes one)	0.44	0.79	0.06	0.15	0.58	0.87	0.77	0.51	0.20	0.15
Expansion sceptic	0.52	0.63	0.31	0.06	0.58	0.65	0.61	0.54	0.20	0.28
Settlement would be obvious (Hart)	0.47	0.77	0.17	0.05	0.58	0.81	0.75	0.53	0.20	0.18
Hart, crossed: easy and obvious	0.44	0.87	0.06	0.07	0.58	0.93	0.84	0.51	0.20	0.12
Expansionist (fast, intergalactic)	0.33	0.75	0.04	0.22	0.58	0.87	0.73	0.51	0.20	0.13
Expansionist with Carter steps	0.69	0.90	0.00	0.10	0.95	0.99	0.89	0.95	0.10	0.11
Expansionist, Carter steps, long-lived planets	0.69	0.90	0.00	0.10	0.94	0.99	0.53	0.73	0.10	0.12
Low-footprint settlement likely	0.47	0.61	0.14	0.25	0.58	0.69	0.60	0.53	0.20	0.26
Most lineages are discreet	0.47	0.69	0.16	0.15	0.58	0.76	0.68	0.53	0.20	0.21
Zoo sympathiser	0.47	0.64	0.15	0.21	0.58	0.71	0.62	0.53	0.20	0.24
Planets stay habitable for tens of Gyr	0.47	0.72	0.17	0.11	0.58	0.78	0.69	0.51	0.20	0.20
Long-lived hosts count fully	0.46	0.71	0.17	0.11	0.57	0.77	0.37	0.18	0.20	0.22
Stable planets are a fixed fraction	0.47	0.72	0.17	0.11	0.58	0.77	0.68	0.50	0.20	0.22
Persistence is a constant hazard	0.46	0.73	0.16	0.11	0.57	0.78	0.72	0.56	0.20	0.18
No ultraviolet penalty on K and M dwarfs	0.46	0.72	0.17	0.11	0.57	0.77	0.65	0.46	0.20	0.20
Loud beacons impossible (radio searches see nothing)	0.47	0.72	0.17	0.11	0.58	0.77	0.70	0.52	0.20	0.21

Columns 2 to 7 and the last two are the Earth-given convention.

E. One prior at a time

P(life starts easily)	nobody before	nobody	never spread before	never spread	unnoticed before	unnoticed	hidden	no other ever
0.05	0.54	0.80	0.08	0.11	0.03	0.03	0.02	0.85
0.1	0.52	0.77	0.09	0.13	0.03	0.03	0.02	0.83
0.2	0.47	0.72	0.10	0.17	0.03	0.04	0.02	0.78
0.3	0.42	0.68	0.10	0.20	0.03	0.05	0.03	0.73
0.4	0.37	0.63	0.11	0.24	0.03	0.06	0.03	0.68
0.6	0.27	0.53	0.13	0.31	0.04	0.08	0.04	0.58
0.8	0.18	0.43	0.15	0.38	0.04	0.09	0.05	0.48
0.9	0.13	0.38	0.16	0.42	0.04	0.10	0.06	0.43
0.95	0.10	0.35	0.16	0.43	0.05	0.11	0.06	0.41

P(universal barrier to expansion)	nobody before	nobody	never spread before	never spread	unnoticed before	unnoticed	hidden	no other ever
0	0.45	0.79	0.04	0.06	0.04	0.06	0.03	0.86
0.05	0.45	0.77	0.06	0.10	0.03	0.05	0.03	0.83
0.15	0.47	0.72	0.10	0.17	0.03	0.04	0.02	0.78
0.3	0.49	0.67	0.15	0.25	0.02	0.03	0.02	0.71
0.5	0.51	0.62	0.23	0.33	0.02	0.02	0.01	0.65

P(low footprint forced on everyone)	nobody before	nobody	never spread before	never spread	unnoticed before	unnoticed	hidden	no other ever
0	0.47	0.75	0.10	0.17	0.01	0.01	0.02	0.80
0.02	0.47	0.74	0.10	0.17	0.02	0.02	0.02	0.79
0.05	0.47	0.72	0.10	0.17	0.03	0.04	0.02	0.78
0.1	0.47	0.70	0.10	0.16	0.05	0.08	0.02	0.76
0.25	0.47	0.63	0.10	0.15	0.11	0.16	0.02	0.70
0.5	0.47	0.55	0.10	0.13	0.21	0.27	0.02	0.64

P(everyone hides)	nobody before	nobody	never spread before	never spread	unnoticed before	unnoticed	hidden	no other ever
0.003	0.47	0.74	0.10	0.17	0.03	0.05	0.00	0.79
0.03	0.47	0.72	0.10	0.17	0.03	0.04	0.02	0.78
0.1	0.47	0.69	0.10	0.16	0.03	0.04	0.07	0.75
0.2	0.47	0.64	0.10	0.15	0.03	0.04	0.13	0.71

observers per expander (typical-observer column)	nobody before	nobody	never spread before	never spread	unnoticed before	unnoticed	hidden	no other ever
1000	0.47	0.60	0.10	0.35	0.03	0.02	0.01	0.62
1e+06	0.47	0.53	0.10	0.46	0.03	0.00	0.00	0.53
1e+10	0.47	0.51	0.10	0.49	0.03	0.00	0.00	0.51

P(expansion can cross between galaxies)	nobody before	nobody	never spread before	never spread	unnoticed before	unnoticed	hidden	no other ever
0.2	0.54	0.75	0.13	0.19	0.03	0.03	0.02	0.76
0.6	0.47	0.72	0.10	0.17	0.03	0.04	0.02	0.78
0.9	0.41	0.70	0.07	0.15	0.03	0.05	0.03	0.79

P(no other civilisation ever in the Galaxy), Earth-given, against the two late-stage priors (rows: barrier; columns: forced low footprint):

barrier \ low footprint	0	0.02	0.05	0.1	0.25	0.5
0	0.90	0.88	0.86	0.83	0.75	0.66
0.05	0.86	0.85	0.83	0.80	0.74	0.65
0.15	0.80	0.79	0.78	0.76	0.70	0.64
0.3	0.73	0.72	0.71	0.70	0.66	0.61
0.5	0.66	0.65	0.65	0.64	0.62	0.59

F. Possible future observations

Observation	forecast (Earth given)	of which true	forecast (typical observer)	forecast (SIA)	nobody after	never spread after	spread after	no other ever, after	other error rates
Independent origin of life confirmed in the Solar System	0.31	0.19	0.38	about 0.95	0.52	0.31	0.17	0.58	sg_fp=0.05: nobody 0.42, never spread 0.39; sg_fp=0.3: nobody 0.61, never spread 0.25
Thorough search finds no second origin in the Solar System	0.69	-	0.62	about 0.05	0.82	0.10	0.08	0.87	-
HWO: 25 habitable-zone planets, at least one biosignature	0.15	0.06	0.19	about 0.80	0.57	0.28	0.15	0.62	hwo_fp=0.03: nobody 0.42, never spread 0.39; hwo_fp=0.2: nobody 0.64, never spread 0.23
HWO: 25 habitable-zone planets, no biosignature	0.85	-	0.81	about 0.20	0.75	0.14	0.10	0.81	-
Earth-level technosignatures excluded for the nearest million stars	1.00	-	0.99	about 0.85	0.73	0.16	0.11	0.78	-
Earth-level technosignatures excluded for the whole Galaxy	0.92	-	0.79	about 0.05	0.79	0.12	0.09	0.85	-
Thorough Solar System artefact survey finds nothing	0.97	-	1.00	about 0.90	0.75	0.17	0.09	0.79	art_d=0.1: nobody 0.73, never spread 0.16; art_d=0.3: nobody 0.74, never spread 0.17; art_d=0.8: nobody 0.76, never spread 0.17; art_d=0.95: nobody 0.77, never spread 0.17
Sustained interstellar expansion shown feasible for us	0.74	-	0.08	about 0.30	0.79	0.06	0.15	0.86	-
Sustained interstellar expansion shown to be common (over 1 civilisation in 1000)	0.59	-	0.00	about 0.25	0.81	0.04	0.15	0.88	-
Sustained interstellar expansion shown infeasible	0.26	-	0.92	about 0.70	0.54	0.46	0.00	0.54	-
M-dwarf planets shown habitable	0.30	-	0.25	about 0.30	0.73	0.16	0.11	0.78	-
M-dwarf planets shown dead	0.70	-	0.75	about 0.70	0.73	0.16	0.11	0.78	-
Rocky planets shown to stay habitable beyond 15 Gyr	0.30	-	0.23	about 0.30	0.72	0.17	0.11	0.78	-
Rocky planets shown to lose habitability within 15 Gyr	0.70	-	0.77	about 0.70	0.73	0.16	0.11	0.78	-

Now (Earth given): nobody 0.73, never spread 0.16, spread 0.11; no other ever 0.78.

G. Joint scan: 400 complete alternative sets of priors

Convention	nobody 5/50/95%	never spread	spread	no other ever	anyone else here now	draws where nobody leads	draws where nobody > 50%	min ESS
Earth given	0.26/0.56/0.78	0.09/0.24/0.55	0.06/0.17/0.39	0.29/0.63/0.85	0.10/0.26/0.53	78%	60%	276
typical, all time	0.13/0.46/0.73	0.11/0.29/0.65	0.08/0.21/0.46	0.16/0.52/0.81	0.14/0.35/0.64	62%	41%	114
typical, to date	0.19/0.51/0.77	0.10/0.26/0.60	0.07/0.19/0.43	0.22/0.58/0.83	0.12/0.31/0.58	70%	51%	171
typical observer	0.07/0.29/0.59	0.38/0.68/0.91	0.00/0.01/0.11	0.07/0.29/0.59	0.17/0.39/0.64	15%	12%	15

Prior P(nobody) across the scan: 0.10/0.28/0.53. Posterior minus prior (Earth given): 0.11/0.25/0.39; the observations raise P(nobody) in 100% of draws. Correlation of posterior with prior P(nobody): 0.85. Correlation of posterior P(nobody) with the drawn prior on easy life: -0.63.