Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Self-fulfilling correlations, published by Self-fulfilling correlations by PhilGoetzon the LessWrong. 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? Perceived causation causes correlation No. Besides having a reputation for safety, Volvos also have a reputation for being overpriced and ugly. Mostly people who are concerned about safety buy Volvos. Once the reputation exists, even if it's not true, a cycle begins that feeds on itself: Cautious drivers buy Volvos, have fewer accidents, resulting in better statistics, leading more cautious drivers to buy Volvos. Do Montessori schools or home-schooling result in better scores on standardized tests? I'd bet that they do. Again, my google-fu is not strong enough to find any actual reports on, say, average SAT-score increases for students in Montessori schools vs. public schools. But the largest observable factor determining student test scores, last I heard, is participation by the parents. Any new education method will show increases in student test scores if people believe it results in increases in student test scores, because only interested parents will sign up for that method. The crazier, more-expensive, and more-difficult the method is, the more improvement it should show; craziness should filter out less-committed parents. Are vegetarian diets or yoga healthy for you? Does using the phone while driving increase accident rates? Yes, probably; but there is a self-fulfilling component in the data that is difficult to factor out. Conditions under which this occurs If you believe X helps you achieve Y, and so you use X when you are most-motivated to achieve Y and your motivation has some bearing on the outcome, you will observe a correlation between X and Y. This won't happen if your motivation or attitude has no bearing on the outcome (beyond your choice of X). If passengers prefer one airline based on their perception of its safety, that won't make its safety record improve. However, this is different from either confidence or the placebo effect. I'm not talking about the PUA mantra that "if you believe a pickup line will work, it will work". And I'm not talking about feeling better when you take a pill that you think will help you feel better. This is a sample-selection bias. A person is more likely to choose X when they are motivated to achieve Y relative to other possible positive outcomes of X, and hence more inclined to make many other little trade-offs to achieve Y which will not be visible in the data set. It's also not the effect people are guarding against with double-blind experiments. That's guarding against the experimenter favoring one method over another. This is, rather, an effect guarded against with random assignment to different groups. Nor should it happen in cases where the outcome being studied is the only outcome people consider. If a Montessori s...