The Twins Paradox

According to behavioral genetics, identical twins raised together are about as similar as identical twins raised separately.

Identical twins (raised separately or together) have personalities that are much more similar than two people chosen at random and much more similar than fraternal twins (raised together or separately).  However, they do not have identical personalities.  Far from it.  Behavioral geneticists say that identical twins’ personalities correlate about 55% (without going into the technical details of how exactly these correlations are defined).

Identical twins share all of their genes.  Identical twins raised apart share all of their genes and half of their environment; identical twins raised together share all of their genes and all of their environment.  Behavioral geneticists conclude that 50% of personality comes from the genes, 0% to 10% comes from the environment, and 40% to 50% comes from...somewhere else.

(Note that behavioral geneticists also study fraternal twins and non-twin siblings, and the data from those studies tends to support these conclusions.  For more details, see Chapter 19 of “The Blank Slate” by Steven Pinker.)

Somewhere else?  But where else?  Where else could personality come from, besides heredity and environment?

The simplest way to resolve The Twins Paradox is, of course, to conclude that the difference in personality comes from the soul.  You might point out that identical twins raised together have the same genes and the same environment, but different souls.  Maybe about half of your personality is from your genes and half is from your soul.

However, many modern thinkers are reluctant to embrace the hypothesis that the soul is a metaphysical entity with an independent existence.

Some have proposed that the hidden variable in The Twins Paradox is birth order, or the peer group.  These solutions don’t make any sense to me.  Identical twins raised together share both their birth order and their peer group, so how could these variables possibly account for the variation?

Two interesting solutions are those proposed by Steven Pinker and Daniel Dennett.

Steven Pinker proposes that the computational process that builds our personalities relies, like many computational processes in man-made digital computers, on random (quantum random or pseudo-random, it doesn’t matter) branches.  If this is the case, then the finished “Personality” product can look very different, even if the original plan was the same, and even if there was no systematic input from the environment.

Daniel Dennett suggests that each human being spin a Self like a spider spins a web.  The choices that each human being makes in his developmental years, as he is constructing himself, has profound consequences for the Self he turns out to be.

There’s a big difference between these two suggestions.  Pinker’s suggestion is an attempt at explaining an unexpected phenomenon.  Dennett’s was a prediction.  It can be found in his book “Elbow Room”, published in 1984.  He makes no mention of the data from behavioral genetics.  Usually, Dennett loves data.  He’s always quoting some experiment or another.  So either the data was not available in 1984, or Dennett wasn’t aware of it.

(OK, wise guy, I don’t know what year the behavioral genetics data are from, or what year they became well-known.  I tried to figure it out with my usual method: I searched Wikipedia for 5 minutes and then gave up.  If the behavioral genetics data are from after 1984, that would show definitively that Dennett’s theory was a prediction.  If the behavioral genetics data are from before 1984, we wouldn’t really know.  I can tell you that Pinker’s “Blank Slate” is from 2003, and Judith Rich Harris’s “Nurture Assumption”, which relies on the same data, is from 1998.)

If I’m right, and Dennett’s theory was really a prediction of a experimental results that shocked everyone else, it doesn’t mean he’s right.  It’s just one of those things that make you go “hmmm...”