Chart 8 in the Second Special Abridged Edition of ‘Race, Evolution, and Behavior.’

Twin Studies

J. Philippe Rushton

Genes, Environment, or Both?

A number of studies show that race differences are caused by both genes and environment. Heritabilities, cross-race adoptions, genetic weights, and regression to the average all tell the same story. Cross-race adoptions give some of the best proof that the genes cause rare differences in IQ. Growing up in a middle-class White home does not lower the average IQ for Orientals nor raise it for Blacks.

Can any environmental factor explain all the data on speed of dental development, age of sexual maturity, brain size, IQ, testosterone level, and the number of multiple births? Genes seem to be involved. But how can we know for sure?

Some traits are clearly inherited. For example, we know that the race differences in twinning rate are due to heredity and not to the environment. Studies of Oriental. White, and Mixed-Race children in Hawaii and of White. Black, and Mixed-Race children in Brazil show that it is the mother’s race, and not the father’s, that is the determining factor. But the role of racial heredity is found for other traits as well.

Heritability Studies

Heritability is the amount of variation in a trait due to the genes. A heritability of 1.00 means that the differences are inborn and the environment has no effect. A heritability of zero (0.00) means the trait is controlled by the environment and not at all by the genes. A heritability of 0.50 means that the differences come from both the genes and the environment.

Heritability is useful for animal breeders. They like to know how much genes influence things like milk yields and beefiness in cattle or determine which dogs can hunt, and which are good with children. The higher the heritability, the more the offspring will resemble their parents. On the other hand, low heritabilities mean that environmental factors like diet and health are more important.

For people, we measure heritability by comparing family members, especially identical with fraternal twins, and adopted children with ordinary brothers and sisters. Identical twins share 100% of their genes, while fraternal twins share only 50%. Ordinary brothers and sisters also share 50% of their genes, while adopted children share no genes. If genes are important, identical twins should be twice as similar to each other as are fraternal twins or ordinary siblings – and so they are.

Some identical twins are separated early in life and grow up apart. The famous Minnesota Twin Study by Thomas J. Bouchard and others compared many of these. (See Chart 8).

Even though they grew up in different homes, identical twins grow to be very similar to each other. They are similar both in physical traits (like height and fingerprints) and in behavioral traits (like IQ and personality).

Identical twins who grow up in different homes share all their genes but do not share the effects of upbringing. As you can see in Chart 8, heredity accounted for 97% of the difference for fingerprints, and the environment only 3% Social attitudes were 40% heredity, 60% environment. IQ was 70% heredity, 30% environment.

Identical twins are often so alike that even close friends cannot tell them apart. Although the twins in the Minnesota Project lived separate lives, they shared many likes and dislikes. They often had the same hobbies and enjoyed the same music, food, and clothes. Their manners and gestures were often the same. The twins were very alike in when they got married (and sometimes divorced) and in the jobs they held. They even gave similar names to their children and pets.

One of these pairs, the “Jim twins,” were adopted as infants by two different working-class families. But they marked their lives with a trail of similar names. Both named their childhood pet “Toy.” Both married and divorced women named Linda and then married women named Betty. One twin named his son James Allen, the other named his son James Alan.

Another pair of separated twins were helpless gigglers. Each twin said her adoptive parents were reserved and serious. Each one said she never met anyone who laughed as easily as she did – until she met her twin! Heredity also affects the sex drive. The age of our first sexual experience, how often we have sex, and our total number of sexual partners all have heritabilities of about 50%. So do the odds that we will get divorced. Several studies find that homosexuality, lesbianism, and other sexual orientations are about 50% genetic.

Twin studies show that even social attitudes are partly genetic in origin. One Australian study of 4,000 twin pairs found there was a genetic influence on specific political beliefs like capital punishment, abortion, and immigration. It turns out that criminal tendency is also heritable. About 50% of identical twins with criminal records have twins with criminal records, while only about 25% of fraternal twins do.

Genes influence helping behavior and aggression. A large study of British twins found that the desire to help or hurt others has a heritability of around 50%. For men, fighting, carrying a weapon, and struggling with a police officer are all about 50% heritable.

My article in the 1989 Behavioral and Brain Sciences shows that who we marry and who we choose as friends is also partly genetic. When the blood groups and heritabilities of friends and spouses are compared, we find that people chose partners who are genetically similar to themselves. The tendency for like to attract like is rooted in the genes.

Adoption Studies

A good check on the results of twin studies comes from adoption studies. A Danish study (in the 1984 issue of Science) examined 14,427 children separated from their birth parents as infants Boys were more likely to have a criminal record if their birth parents had a criminal record than if their adoptive parents did. Even though they were brought up in different homes, 20% of the full brothers and 13% of the half-brothers had similar criminal records. Only 9% of the unrelated boys brought up in the same home both had criminal records.

The Colorado Adoption Project found that genes increase in influence as we age. Between age 3 and 16, adopted children grew to be more like their birth parents in height, weight, and IQ. By age 16 the adopted children did not resemble the people who had reared them. The heritability of height, weight, and IQ in infancy are all about 30%. By the teenage years, they are about 50% and by adulthood, they are about 80%. Thus, as children grow older, their home environments have less impact and their genes have more impact, just the opposite of what culture theory predicts.

From Chapter 5 of the Second Special Abridged Edition of Race, Evolution, and Behavior, Charles Darwin Research Institute (2000).

About the author: J. Philippe Rushton is a professor of psychology at the University of Western Ontario, London, Ontario, Canada. Rushton holds two doctorates from the University of London (Ph.D. and D.Sc.) and is a Fellow of the John Simon Guggenheim Foundation, the American Association for the Advancement of Science, and the American, British, and Canadian Psychological Associations. He is also a member of the Behavior Genetics Associations, the Human Behavior and Evolution Society, and the Society for Neuroscience. Rushton has published six books and nearly 200 articles. In 1992 the Institute for Scientific Information ranked him the 22nd most published psychologist and the 11th most cited.

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