judith rich harris no two alike

Judith Rich Harris: No Two Alike

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Chris Hibbert

Judith Rich Harris's book No Two Alike is a followup to her previous

work, The Nurture Assumption. In her first book, Harris explained that

most of the non-genetic affects on the personalities of adults are a

result of their interactions with peers rather than with their

parents. She pointed out that many people want to believe (and prove)

that parents are the major source of their children's personalities.

According to The Nurture Assumption, the field of sociology has been

confused for several decades and has been trying to distinguish nature

and nurture, when they needed to be either distinguishing the effects

of heredity from environment, or disentangling the environmental

influences which include both parents and peers.

The best tool, according to Harris, for distinguishing the effects of

heredity and environment consists of studies of twins. Comparing twins

raised together and twins raised apart controls for genetic affects

and allows us to see the effect of gross differences in environment.

Comparing fraternal twins raised together with identical twins raised

together holds the gross environment constant and makes it easier to

see what differences are purely genetic.

In the new book, Harris focuses on why twins are so different, in

order to isolate the causes of differences that aren't explained by

other results. The existing literature says that some proportion of

personality differences are due to genetics, and some proportion by

each of various environmental causes: parents, wealth, neighborhood,

etc. But a significant amount of variation remains that isn't

apparently caused by any of these. Her focal example is that even

siamese twins have different personalities, and they share all of

their genes, and all of the environmental influences that anyone could

hope to treat as responsible.

Harris' conclusion (skipping over most of her argument for the moment)

is that there must be something driving each of us to be unique, and

that means we have to find a distinction to enhance. The bottom line

is that a significant part of personality (who we are) isn't

determined by factors that we can examine or control. Each individual

starts out with an endowment of heredity, and occupies an environment

that isn't fully under their control, but the developing personality

is still a negotiation between that individual and their context. If

the strongest part of their innate tendencies is best suited for a

niche that is already filled, they will look for a second best. When

two identical individuals struggle to fill the same niche, some factor

(random or not) will eventually determine a winner in each particular

event, and at some point the effects of competition, if nothing else,

will drive them to exploit different strengths. The different choices

and different results in competition will magnify any differences, and

over a reasonable lifetime, they will become recognizably different

people.

Along the way, Harris spends a good deal of effort (successfully)

demolishing other possible explanations (differences in environment,

combination of nature and nurture, gene-environment interactions,

environmental differences within the family, gene-environment

correlations, and transferability of learning between situations). At

the end she argues that she has demolished all the other possibilities

and provided an argument for the one remaining theory (an innate drive

for status), and so it must be true. But her argument for the specific

mechanism is a little too weak, and it seems plausible that some

variation or related description will fit the data a little better.

I'm reasonably convinced that something drives us to differentiate,

but it may not be purely a status drive. Two possible variations on

her theory include drives for attention or to master something.

I found the style of Harris' presentation sometimes compelling, and

sometimes distracting. She fit the presentation into the framework of

a detective story. The presentation is salted liberally with examples

from popular detective stories to show how attentive the detective has

to be to details that have distracted other investigators. This worked

for me when I was familiar with the detective in question (Sherlock

Holmes, Kinsey Milhone), and didn't work when I hadn't read the

stories (Alan Grant). I suspect that Holmes is the only one of these

that is widely enough known that other readers would feel that they

should get the references even when they don't.

On the whole, I think the book was successful in explaining that

fundamental differences in personality are effectively the result of

an innate drive that causes us to differentiate. The drive makes use

of arbitrary differences in the material it has to work with (genes

and environment). Parents do make a difference in the lives their

children lead, but the ultimate person each child becomes isn't