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By Root 977 0

reared apart and 27 pairs of fraternal twins reared apart. Many of these twins’ very first reunions have taken place in the Minnesota laboratory. The stories of spooky similarity are repeated over and over (e.g., two identicals who each divorced a Linda to marry a Betty; another pair who named their sons James Allen and James Alan, respectively). Could it just be coincidence? Unlikely: These “coincidences” do not seem to occur in the lives of fraternal twins reared apart.

To scientists, the degree of heritability and the range of personality traits that are heritable are more impressive than the anecdotes. All of the following are strongly related in identical twins reared apart, and are much less related in fraternal twins reared apart:20

• IQ • Job satisfaction • Neuroticism

• Mental speed • Actual choice of jobs • Amount of television

• Perceptual speed • Cheerfulness viewing

and accuracy (positive affectivity) • Well-being

• Religiosity • Depression (negative • Self-acceptance

• Traditionalism affectivity) • Self-control

• Alcohol and drug • Danger-seeking • Dominance

abuse • Authoritarianism

• Crime and conduct • Extraversion

These findings have been duplicated in another massive study carried out with five hundred pairs of Swedish twins, identical and fraternal, reared apart and reared together, and now middle-aged.21 The results are similar, but you can add to the list:

Optimism

Pessimism

Hostility

Cynicism

Adopted children. In addition to studying identical twins reared apart, there is a second way to separate the effects of genes from those of child rearing: by comparing adopted children to their biological parents versus comparing them to their adoptive parents. Hundreds of adoption studies have been done: Denmark keeps complete records of adoptions (and complete criminal records as well), so the Danish Population Register is a gold mine for untangling childrearing from biology. The criminal records (or their absence) for the fathers, both biological and adoptive, of all the adopted boys born in Copenhagen in 1953 and the criminal records of the sons have been scrutinized.

If neither the natural nor the adopted father had ever been convicted of a crime, 10.5 percent of the sons turned out to be criminals. If the adopted father was a criminal, but the natural father was not, 11.5 percent of the sons were criminals, an insignificant difference. So having a criminal rearing a child does not increase the child’s risk of himself becoming a criminal.

If the natural father (whom the child had not seen since he was, at most, six months old) was a criminal, but the adopted father was not, 22 percent of the sons were criminals. Crime rate is doubled by having “criminal genes.” If both natural and adopted fathers were criminals, the sons’ crime rate was 36.2 percent—more than triple the rate of the sons of upstanding fathers.

This means that there is a biological predisposition to commit crime (and to get caught). If it is present and you are reared by a criminal father, you are at very high risk. Merely having a criminal father rear you, without the biological predisposition, does not increase your risk.22

So crime, astonishingly, is heritable. Similar adoptive studies strongly confirm the findings of the twin studies: Most of human personality has a strong genetic component.

The other major finding of the adoption studies is that two children raised in the same family are almost as different from each other as any two random kids—on almost every measure of personality and intelligence—once you take genes into account. There is no similarity between two children adopted into the same family; everyone who has raised two adopted kids knows this, but others who have only ideology to guide them greatly overestimate the importance of the family environment. This revolutionary finding—which suggests that many of our labors in childrearing are simply irrelevant—will be discussed later.

But for every one of these heritable traits, the degree of heritability is much less than 1.00. Generally, it