Read Online Welcome to Your Child's Brain: How the Mind Grows From Conception to College by Sam Wang, Sandra Aamodt - Free Book Online
Authors: Sam Wang, Sandra Aamodt
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brain function, as we would expect from the substantial advantages provided by brain talents like language and learning ability. Somewhat curiously, a third category is genes that drive physical appearance, such as skin color, hair color and thickness, and eye color. Selection for these genes may be a product of culturally driven sexual preferences as well as environmental drivers such as sun exposure.
Cultural changes can also protect human populations from the selection pressures imposed by new environments. When people migrated to cold places, they learned to build fires and dress in fur coats, rather than developing the fur and insulating layers of fat that protect other animals from freezing temperatures. Some researchers have speculated that the ability to adapt to new environments through learned behaviors could free our species from some of the constraints imposed on other animals by natural selection, thus allowing us to maintain an unusually large variety of genetic traits in our population—another possible contributor to our famous behavioral flexibility.
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For example, in middle-class populations, about 60 percent of the individual variability in IQ is attributed to genetic differences and almost none to the environmental circumstances shared by children within a family. In contrast, among people living in poverty (see chapter 30 ), about 60 percent of individual variability in IQ is due to shared environment, while genes account for less than 10 percent. In other words, what genes tell you about a child’s potential for intelligence is limited to the particular circumstances in which he or she is growing up.
The second problem is even more serious. A developmental outcome that occurs only when a child with a particular set of genes encounters a certain environment is known as a gene-environment interaction . Another way to put this idea is that certain genetic characteristics can make a child sensitive to aspects of her experience that wouldn’t have any effect on a child of a different genetic background, a theme we will return to later in the book. Such interactions explain the otherwise paradoxical findings that many highly heritable characteristics have increased in the population much faster than biological evolution could explain over the past few decades. Examples range from obesity to intelligence (see chapter 22 ) to nearsightedness (see Practical tip: Outdoor play improves vision ).
Gene-environment interactions are a problem in this context because researchers assume that the two factors act independently when they calculate those percentages of genetic versus environmental influence that you’ve seen in the newspaper. But, as we’ve said, that’s rarely the case. Worse still, any interactions that do occur are included in the “genetic” percentage—making the effects of the environment look less important than they really are.
To illustrate these points, let’s look at a study of petty criminality in 862 adopted Swedish boys. In this study, either genetics (a criminal parent) or a bad environment (unstable early placement or poor adoptive family) increased the risk of criminality in a child. We would not expect geneticists to identify a “lawbreaking gene,” but traits like impulsiveness and aggression are influenced by heredity and can substantially affect a person’s odds of breaking the law. Compared to the baseline crime incidence of 2.9 percent for children born to noncriminals and raised in a good environment, the incidence was 12.1 percent for biological children of criminals raised in a good environment and 6.7 percent for biological children of noncriminals raised in a bad environment.
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From an individual neuron’s perspective, it would be hard to distinguish between “genetic” and “environmental” influences.
----
Imagine that genetic and environmental influences were independent of one another. In that case, you could guess the likelihood that a child born
Cultural changes can also protect human populations from the selection pressures imposed by new environments. When people migrated to cold places, they learned to build fires and dress in fur coats, rather than developing the fur and insulating layers of fat that protect other animals from freezing temperatures. Some researchers have speculated that the ability to adapt to new environments through learned behaviors could free our species from some of the constraints imposed on other animals by natural selection, thus allowing us to maintain an unusually large variety of genetic traits in our population—another possible contributor to our famous behavioral flexibility.
----
For example, in middle-class populations, about 60 percent of the individual variability in IQ is attributed to genetic differences and almost none to the environmental circumstances shared by children within a family. In contrast, among people living in poverty (see chapter 30 ), about 60 percent of individual variability in IQ is due to shared environment, while genes account for less than 10 percent. In other words, what genes tell you about a child’s potential for intelligence is limited to the particular circumstances in which he or she is growing up.
The second problem is even more serious. A developmental outcome that occurs only when a child with a particular set of genes encounters a certain environment is known as a gene-environment interaction . Another way to put this idea is that certain genetic characteristics can make a child sensitive to aspects of her experience that wouldn’t have any effect on a child of a different genetic background, a theme we will return to later in the book. Such interactions explain the otherwise paradoxical findings that many highly heritable characteristics have increased in the population much faster than biological evolution could explain over the past few decades. Examples range from obesity to intelligence (see chapter 22 ) to nearsightedness (see Practical tip: Outdoor play improves vision ).
Gene-environment interactions are a problem in this context because researchers assume that the two factors act independently when they calculate those percentages of genetic versus environmental influence that you’ve seen in the newspaper. But, as we’ve said, that’s rarely the case. Worse still, any interactions that do occur are included in the “genetic” percentage—making the effects of the environment look less important than they really are.
To illustrate these points, let’s look at a study of petty criminality in 862 adopted Swedish boys. In this study, either genetics (a criminal parent) or a bad environment (unstable early placement or poor adoptive family) increased the risk of criminality in a child. We would not expect geneticists to identify a “lawbreaking gene,” but traits like impulsiveness and aggression are influenced by heredity and can substantially affect a person’s odds of breaking the law. Compared to the baseline crime incidence of 2.9 percent for children born to noncriminals and raised in a good environment, the incidence was 12.1 percent for biological children of criminals raised in a good environment and 6.7 percent for biological children of noncriminals raised in a bad environment.
----
From an individual neuron’s perspective, it would be hard to distinguish between “genetic” and “environmental” influences.
----
Imagine that genetic and environmental influences were independent of one another. In that case, you could guess the likelihood that a child born
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