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Posted on March 16, by Scott Alexander [Epistemic status: Heritable mostly means genes. Shared environmental means anything that two twins have in common — usually parents, siblings, household, and neighborhood.
Non-shared environmental is everything else. That summary is wrong. The underwhelming performance of shared environment in twin studies torpedoed that whole area of study. But at least these scholars of social behavior could tell themselves it provided a consolation prize.
That means things like your friends, your schoolteachers, and even that time you and your twin got sent away to separate camps must be really important. More than enough there to continue worrying about how society is Ruining The Children, right? Measurement error is neither genetics nor family, so it ends up in the non-shared environmental term.
IQ tests measure intelligence, but not perfectly. For example, someone who makes a lucky guess on a multiple choice IQ test will get a higher score even though they are not more intelligent than someone who makes an unlucky guess.
Imagine a world where intelligence is entirely genetic. Two identical twins take an IQ test, one makes some lucky guesses, the other is tired, and they end up with a score difference of 5 points.
Then some random unrelated people take the test and they get the 5 point difference plus an extra 20 point difference from genuinely having different IQs. This gets even harder when trying to measure fuzzier constructs like criminality.
Suppose someone does a twin study on criminality and their outcome is whether a twin was ever convicted of a felony.
This depends partly on whether the twin is actually the sort of person with criminal tendencies — but also partly on whether a policeman happened to be in the area to catch them, whether their lawyer happened to be good enough to get them off, whether their judge was feeling merciful that day, et cetera.
Imagine a world where criminality is entirely genetic. Identical Twin A becomes a small-time cocaine dealer in a back alley in West Philly, sells to an undercover cop, and ends up in jail. Riemann and Kandler obsessively collect every possible measurement of personality — self-report, other-report, multiple different tests — and average them out to get an unusually accurate and low-noise estimate of the personality of the twins in their study.
So it looks like much of the non-shared environmental variation in traditional studies of personality was just error.
Luck of the draw. Bob becomes a junior advertising executive at Coca-Cola, where he designs a new ad targeting young female consumers.
His identical twin Rob becomes a junior advertising executive at Pepsi-Cola, where he designs his own new ad targeting young female consumers. Both ads are very successful — in fact, exactly equally successful.
Now a scientist comes along, does a twin study on them, and finds that they have very different levels of income. But actually, Bob and Rob are completely identical in every way, no incident in their past did anything to separate them, and Bob just ended up working for a crappy CEO.
In this scenario, inherent predisposition to earning money is exactly the same in both twins, they just have different amounts of luck at it. Instead, it specifies high-level processes which create lower-level processes which create those cells.
But this gives the lower-level processes a lot of leeway, meaning that there can be significant biological differences between identical twins. Consider by analogy The Postmodernism Generator. We can imagine hundreds of different programmers all designing their own postmodernism generators.
Some would be really brilliantly designed and consistently come up with plausible looking essays. But there would also be variation within the results of each generator. There might be a generator that is mostly terrible but occasionally by coincidence comes up with a really funny essay, or vice versa.
In this analogy, the genes are the code for the generator, and the person is an individual essay produced by that generator. Thus, identical twins have different fingerprints, different freckles, and different birthmarks.
And twins even look different enough that their friends and parents eventually learn to tell them apart. Immunology is still poorly understood, but it seems very important. Immune reactions and neuroinflammation have been implicated to one degree or another in a lot of psychiatric diseases.
A functional immune system can protect good health; a dysfunctional immune system can make someone constantly tired and miserable.
There seems to be more of an element of chance to the immune system than to a lot of other bodily processes.
Part of it is the input — one child in a twin pair might inhale a particle of cat dander at a critical time; another might get some unknown adenovirus with no immediate effects but which contributes to obesity twenty years later.The Rogerian Style of Argument - One of the elements of the Rogerian argument is cooperation.
There will always be disagreements among people, especially within the United States Government and between politicians, this is more apparent now than ever.
It was late-afternoon. Forty-nine of us, forty-eight men and one woman, lay on the green waiting for the spike to open. We were too tired to talk much.
The Moments That Make Us Who We Are. Life provides turning points of many kinds, but the most powerful of all may be character-revealing moments. Mar 09, · A curated playlist featuring Future, Kelela, Adele and more, with essays by Margo Jefferson, Wesley Morris, Angela Flournoy and others.
Mar 10, · The second single from “Purpose,” Justin Bieber’s fourth studio album, “Sorry” is an infectious confection — a Dorito for your ears. Guard Towers at Walmart. This feels very creepy and police-state-like. What on earth could Walmart be worried about?
EDITED TO ADD (1/4): A reader points out that they're increasingly common in parking lots to deter automobile crimes.