# Negative and Positive Selection

_(Originally [posted](http://rationalconspiracy.com/2012/06/19/negative-and-positive-selection/) to my blog, [The Rationalist Conspiracy](http://www.rationalconspiracy.com); cross-posted here on request of Lukeprog.)_

You’re the captain of a team, and you want to select really good players. How do you do it?

One way is through what I call _positive selection_. You devise a test – say, who can run the fastest – and pick the people who do best. If you want to be really strict, like if you’re selecting for the Olympics, you only pick the top fraction of a percent. If you’re a player, and you want to get selected, you have to train to do better on the test.

The opposite method is _negative selection._ Instead of one test to pick out winners, you design many tests to pick out losers. You test, say, who can’t run very well when it’s hot out, and get rid of them. Then you test who can’t run very well when it’s cold out, and get rid of them. Then you test running in the rain, and get rid of the losers there. And so on and so forth. When you’re strict with negative selection, you have lots and lots of tests, so that it’s very hard for any one person to pass through all the filters.

I think a big part of where American society’s gone wrong over the last hundred years is the ubiquitous use of _negative selection_ over _positiv_e _selection_. (Athletics is one of the only exceptions. It’s apparently so important that people really care about performance – as opposed to, say, in medicine, where we exclude brilliant doctors if they don’t have the stamina to work [ninety hours a week](http://en.wikipedia.org/wiki/Medical_resident_work_hours).) A single test can always be flawed; for example, IQ tests and SATs have many flaws. However, with negative selection, how badly you do is determined by the failure rate of every test _combined._ If you have twenty tests, and even one of them is so flawed it excludes good players, then your team will suck.

Elite [college admissions](http://www.paulgraham.com/colleges.html) is an example of a negative selection test. There’s no one way you can do really, really well, and thereby be admitted to Harvard. Instead, you have to pass a bunch of different selection filters: Are your SATs good enough? Are your grades good enough? Is your essay good enough? Are your extracurriculars good enough? Are your recommendations good enough? Failure on any one step usually means not getting admitted. And as competition has intensified, colleges have added more and more filters, like the supplemental applications top schools now require (in addition to the Common Application). It wasn’t always this way – Harvard used to admit primarily based on an entrance exam – until they discovered this let too many Jews in (no, [seriously](http://www.gladwell.com/2005/2005_10_10_a_admissions.html)). More recently, the negative selection has been intensified by eliminating the SAT’s high ceiling.

Academia is another example of negative selection. To get tenure, first you have to get into a top PhD program. Then you have to graduate. Then you have to get a good recommendation from your advisor. Then you have to get a good postdoc. Then you have to get another good postdoc. Then you have to get a good assistant professorship. Then you have to get approved by the tenure committee. For the most part, if even one of those steps goes wrong – if you went to a second-tier PhD program, say – there’s no way to recover. Once you’re off the “track”, you’re off, and there’s no getting back on. It’s fail once, fail forever.

Grades are another example – A is a good grade, but there’s no _excellent_ grade. There’s no grade that you only get if you’re in the top 0.1%. Hence, getting a really good GPA doesn’t mean excelling, so much as it means never failing. If you’re in high school and are taking six classes, if you fail one, your GPA is now 3.3 or less, regardless of how good you are otherwise.

In any field, at the top end, you tend to get a lot of variance. (Insert tales of the mad artist and mad mathematician.) Negative selection suppresses variance, by eliminating many of the dimensions on which people vary. Students at Yale are, for the most part, all strikingly similar – same socioeconomic class, same interests, same pursuits, same life goals, even the same style of dress. A lot of people tend to assume performance follows a bell curve, but in some cases, it’s more like a Pareto distribution: the [top people](http://paulgraham.com/wealth.html) do hundreds or thousands of times better than average. Hence, if you eliminate the small fraction of people at the very top, your performance is [hosed](http://www.paulgraham.com/yahoo.html). Fortunately for VC funds, the startup world is still [positive selection](http://grasshopper.com/blog/2011/11/the-early-failures-of-famous-entrepreneurs-and-what-they-learned-3/).

Less obviously, a world with lots of negative selection might be a nasty one to live in. If you think of yourself as trying to _eliminate bad_, rather than _encourage good_, you start operating on the [purity vs. contamination](http://www.nationalreview.com/corner/149137/wired-morality/jonah-goldberg) moral axis. Any tiny amount of bad, anywhere, must be gotten rid of, and that can lead to all sorts of [nastiness](/lw/lz/guardians_of_the_truth/). “When you are a Guardian of the Truth, all you can do is try to stave off the inevitable slide into entropy by zapping anything that departs from the Truth.  If there’s some way to pump against entropy, generate new true beliefs along with a little waste heat, that same pump can keep the truth alive without secret police.”