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« October 13, 2009 | Main | October 19, 2009 »

14 October 2009

The Fundamental Regret of Causal Inference

Tim Kreider at the New York Times has a short piece on what he dubs "The Referendum" and how it plagues us:

The Referendum is a phenomenon typical of (but not limited to) midlife, whereby people, increasingly aware of the finiteness of their time in the world, the limitations placed on them by their choices so far, and the narrowing options remaining to them, start judging their peers' differing choices with reactions ranging from envy to contempt. ...Friends who seemed pretty much indistinguishable from you in your 20s make different choices about family or career, and after a decade or two these initial differences yield such radically divergent trajectories that when you get together again you can only regard each other's lives with bemused incomprehension.

Those familiar with casual inference will recognize this as stemming from the Fundamental Problem of Causal Inference: we cannot observe, for one individual, both their response to treatment and control. The article is an elegant look at how we grow to worry about those mysterious missing potential outcomes--the paths we didn't choose--and how we use our friends' lives to impute those missing missing outcomes. Kreider goes on to make this point exactly, with a beautiful quote from a novel:

The problem is, we only get one chance at this, with no do-overs. Life is, in effect, a non-repeatable experiment with no control. In his novel about marriage, "Light Years," James Salter writes: "For whatever we do, even whatever we do not do prevents us from doing its opposite. Acts demolish their alternatives, that is the paradox." Watching our peers' lives is the closest we can come to a glimpse of the parallel universes in which we didn't ruin that relationship years ago, or got that job we applied for, or got on that plane after all. It's tempting to read other people's lives as cautionary fables or repudiations of our own.

Perhaps the only response is that, while so close to us in so many respects, friends may be poor matches for gauging these kinds of effects. In any case, "Acts demolish their alternatives, that is the paradox" is the best description of the problem of causal inference that I have seen.

Posted by Matt Blackwell at 4:19 PM