Race, Evolution and Behavior; A Life History Perspective by J. Philippe Rushton

Author:J. Philippe Rushton [Rushton, J. Philippe]
Language: eng
Format: epub
Published: 2016-05-19T04:00:00+00:00

5

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 race 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.

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