So it has come to my attention that there are a lot of LWers in and around Berkeley, so I thought it might be nice if we all got together and shot the breeze for a couple hours.  I think it would be good to meet at the Starbucks at 2224 Shattuck Avenue at 7 o'clock on September 5th.

This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.

Correlation does not imply causation.  Sometimes corr(X,Y) means X=>Y; sometimes it means Y=>X; sometimes it means W=>X, W=>Y.  And sometimes it's an artifact of people's beliefs about corr(X, Y).  With intelligent agents, perceived causation causes correlation.

Volvos are believed by many people to be safe.  Volvo has an excellent record of being concerned with safety; they introduced 3-point seat belts, crumple zones, laminated windshields, and safety cages, among other things.  But how would you evaluate the claim that Volvos are safer than other cars?

Presumably, you'd look at the accident rate for Volvos compared to the accident rate for similar cars driven by a similar demographic, as reflected, for instance in insurance rates.  (My google-fu did not find accident rates posted on the internet, but insurance rates don't come out especially pro-Volvo.)  But suppose the results showed that Volvos had only 3/4 as many accidents as similar cars driven by similar people.  Would that prove Volvos are safer?

Many thanks to Unknowns for inventing the scenario that led to this post, and to Wei Dai for helpful discussion.

Imagine you subscribe to the universal prior. Roughly, this means you assign credence 2^-k to each program of length k whose output matches your sensory inputs so far, and 0 to all programs that failed to match. Does this imply you should assign credence 2^-m to any statement about the universe ("hypothesis") that has length m? or maybe Kolmogorov complexity m?

The answer is no. Consider the following examples:

1. The complexity of "A and B and C and D" is roughly equal to the complexity of "A or B or C or D", but we know for certain that the former hypothesis can never be more probable than the latter, no matter what A, B, C and D are.

2. The hypothesis "the correct theory of everything is the lexicographically least algorithm with K-complexity 3^^^^3" is quite short, but the universal prior for it is astronomically low.

3. The hypothesis "if my brother's wife's first son's best friend flips a coin, it will fall heads" has quite high complexity, but should be assigned credence 0.5, just like its negation.

Instead, the right way to derive a prior over hypotheses from a prior over predictors should be to construct the set of all predictors (world-algorithms) that "match" the hypothesis, and see how "wide" or "narrow" that set is. There's no connection to the complexity of the hypothesis itself.

An exception is if the hypothesis gives an explicit way to construct a predictor that satisfies it. In that case the correct prior for the hypothesis is bounded from below by the "naive" prior implied by length, so it can't be too low. This isn't true for many interesting hypotheses, though. For example the words "Islam is true", even expanded into the complete meanings of these words as encoded in human minds, don't offer you a way to implement or predict an omnipotent Allah, so the correct prior value for the Islam hypothesis is not obvious.

This idea may or may not defuse Pascal's Mugging - I'm not sure yet. Sorry, I was wrong about that, see Spurlock's comment and my reply.

In celebration of the virtues of applied rationality, Less Wrong is going to Burning Man! And because Heinlein rationalists should win, Bayes Camp is going to be the most awesome place there.

A bunch of people from SingInst/Less Wrong will be descending upon the desert, bedecked as the members of the Bayesian Conspiracy. Kevin, Jasen, JustinShovelain, Peter de Blanc, Michael Vassar and Nick Tarleton, among others, will be there. If you'd like to stop by, say so in the comments! 

We'll be at 6:50, F, and should be there from Monday 30th.

Please note: Burning Man is serious stuff, and if you don’t think you’re up to the desert, you shouldn’t come. Either way, read the survival guide.

EDIT: updated location

There are many reasons I enjoy playing go: complex gameplay arises out of simple rules, single mistakes rarely decide games, games between between people of different skill can be handicapped without changing the dynamics of the game too much, there are no draws, and I just like the way it looks. The purpose of this article is to illustrate something else I like about playing go: the ways that it provides practice in basic habits of rationality, that is, the ways in which playing go helps me be less wrong.

There is a game studied in Philosophy of Science and Probably Approximately Correct (machine) learning. It's a cousin to the Looney Labs game "Zendo", but less fun to play with your friends. http://en.wikipedia.org/wiki/Zendo_(game) (By the way, playing this kind of game is excellent practice at avoiding confirmation bias.) The game has two players, who are asymmetric. One player plays Nature, and the other player plays Science. First Nature makes up a law, a specific Grand Unified Theory, and then Science tries to guess it. Nature provides some information about the law, and then Science can change their guess, if they want to. Science wins if it converges to the rule that Nature made up.

A common trope on Less Wrong is the idea that governments and the academic establishment have neglected to consider, study and work against existential risk on account of their shortsightedness. This idea is undoubtedly true in large measure. In my opinion and in the opinion of many Less Wrong posters, it would be very desirable to get more people thinking seriously about existential risk. The question then arises: is it possible to get more people thinking seriously about existential risk? A first approximation to an answer to this question is "yes, by talking about it." But this answer requires substantial qualification: if the speaker or the speaker's claims have low credibility in the eyes of the audience then the speaker will be almost entirely unsuccessful in persuading his or her audience to think seriously about existential risk. Speakers who have low credibility in the eyes of an audience member decrease the audience member's receptiveness to thinking about existential risk. Rather perversely, speakers who have low credibility in the eyes of a sufficiently large fraction of their audience systematically raise existential risk by decreasing people's inclination to think about existential risk. This is true whether or not the speakers' claims are valid.

Evolutionary theories get mentioned a lot on this site, and I frequently feel that they are given far more weight than would be warranted. In particular, evolutionary theories about sex differences seem to get mentioned and appealed to as if they had an iron-cast certainty. People also don't hesitate to make up their own evolutionary psychological explanations. To counterbalance this, I present a list of evolutionary psychology-related problems, divided into four rough categories.

Problems in hypothesis generation

Rationalization bias. We know that human minds are very prone to first deciding on a desired outcome, then coming up with a plausible-sounding story of why it must be so. In general, our minds have difficulty noticing faulty reasoning if it leads to the right conclusion. It's easy and tempting to come up with an ad-hoc evolutionary explanation for any behavior, regardless of whether or not it actually has any biological roots.

Over-attributing meaning. Humans also have a strong tendency to attribute meaning to random chance. We might easily come up with explanations that are unnecessarily complex, and try to make everything into an evolved adaptation. For instance, humans tend to avoid thinking about unpleasant thoughts about themselves. A contrived evpsych explanation might be that this is evolved self-deception: by not acknowledging our own faults, it makes it easier for us to deceive others about them. But mental unpleasantness tends to be correlated with harmful experiences: we avoid situations where we'd be afraid, and fear is correlated with danger. It could just as well be that the mechanism for avoiding mental unpleasantness evolved from the mechanism for avoiding physical unpleasantness, and we avoid thinking unpleasant thoughts of ourselves for the same reason why we avoid poking our fingers at hot stoves. (Example courtesy of Anna Salamon.)

Alternative ways of reaching the goal. Eliezer previously gave us the example of the scientists who thought insects would under the right circumstances limit their breeding, but the insects ended up eating their competitors' offspring instead. We can only cover a limited part of the space of all possible routes evolution could take. While ”but another hypothesis might explain it better” is admittedly a problem all scientific disciplines face, it is especially acute here, since we have very little knowledge of what life in the EEA was actually like.

Problems in background assumptions

Did a genetic path to the adaptation exist? Evolution works by the rule of immediate advantage: for mutation X to reach fixation, it has to provide an immediate advantage. It's well and good to propose that under specific circumstances, organisms that developed a specific behavior would have gained a fitness advantage. But that, by itself, tells us nothing about how many mutations reaching such a behavior would have required. Nor does it tell us anything about whether all of those intermediate stages actually conferred the organism a fitness benefit, making it possible for the final form of the adaptation to actually be reached.