2.4 Nature and Nurture Connected

We close this chapter with a detailed look at two phenomena—alcoholism and nearsightedness—that illustrate the connections among genes, prenatal care, developmental age, and culture. The interaction is dynamic, not simple, but understanding it has many practical applications for parents, professionals, and everyone else.

Alcoholism

At various times, people have considered alcohol and drug abuse to be a moral weakness, a social scourge, or a personality defect. Attention has been on alcohol, in part because fermentation is natural and universal, present in every culture and era, and in part because alcoholism is a far more common addiction than any other.

In various times and places, alcoholics were locked up, doused with cold water, or burned at the stake. Alcohol has been declared illegal (as in Ontario from 1916 to 1927 and in Alberta from 1916 to 1924), deemed sinful (in Islam, Mormonism, and many other religions), and considered sacred (as in many Jewish and Catholic rituals). Science has now learned that the human reaction to alcohol is affected by dozens of alleles as well as by diverse cultural practices, so any universal prohibition or veneration will affect individuals in opposite ways. Differential sensitivity again!

As you might expect, alcoholism begins with genes that create an addictive pull that can be overpowering, extremely weak, or somewhere in between. To be specific, each person’s biochemistry reacts to alcohol, causing sleep, nausea, aggression, joy, relaxation, forgetfulness, sexual urges, or tears.

How bodies metabolize alcohol allows some people to “hold their liquor” and therefore drink too much, whereas others (including many East Asians) sweat and become red-faced after just a few sips, an embarrassing response that may lead to abstinence. Candidate genes and alleles for alcoholism have been identified on every chromosome except the Y chromosome (ironic, since, internationally, more men than women are alcoholics) (Epps & Holt, 2011).

Inherited psychological traits affect alcoholism as much as biological ones (Macgregor et al., 2009). A quick temper, sensation seeking, or high anxiety encourage drinking. It is impossible to specify how much alcoholism is genetic or cultural because these influences are “inexorably intertwined” (Dick, 2011); the relationship between nature and nurture varies by age and context (Young-Wolff et al., 2011). For example, some contexts (such as fraternity parties) make it hard to avoid alcohol; other contexts (a church social in a “dry” county) make it difficult to swallow anything stronger than lemonade. Age is also pivotal. Adolescents experience more pleasure, and less pain, from being drunk than people older or younger (Spear, 2011). Consequently, they get drunk more often—and have higher rates of car accidents, temper tantrums, and unprotected sex while drinking.

Biological sex (XX or XY) and gender (cultural) also affect the risk of alcoholism. For biological reasons (body size, fat composition, metabolism), women become drunk on less alcohol than men, but how much a woman drinks depends on her social context.

For example, in Japan, both sexes inherit the same genes for metabolizing alcohol, yet Japanese men—not women—drink more alcohol than their peers elsewhere. When Japanese women immigrate to the United States, their alcohol consumption is said to increase about fivefold (Higuchi et al., 1996; Makimoto, 1998). Their alcoholism increases as well. For all immigrants, alcohol consumption is related to the original culture, the stress of immigration, and norms of the country to which they immigrate—increasing or decreasing depending on specifics (Szaflarski et al., 2011)

As you have read, prenatal exposure to alcohol may seriously impair the fetus, but it also is more likely that a baby born to a drinking mother will become an alcoholic later on. Is that a genetic, prenatal, or childhood effect, or all three?

Nearsightedness

Age, genes, and culture affect vision as well. First consider age. Newborns focus only on things within 60 centimetres of their eyes; vision improves steadily until about age 10. At puberty, the eyeball changes shape, which increases nearsightedness (myopia); eyeball shape changes again in middle age, decreasing nearsightedness but increasing farsightedness (hyperopia).

Now consider genes. A study of British twins found that the gene that governs eye formation (Pax6) has many alleles that increase nearsightedness (Hammond et al., 2004). This research found heritability of almost 90 percent, which means that if one monozygotic twin was nearsighted, the other twin was almost always nearsighted, too.

However, heritability is a statistic that indicates only how much of the variation in a particular trait within a particular population in a particular context and era can be traced to genes. For example, the heritability of height is very high (about 95 percent) when children receive good medical care and ample nourishment, but it is low (about 20 percent) when children are malnourished. Thus, the 90 percent heritability of nearsightedness among the British children may not apply elsewhere.

Instead, visual problems may be caused by the environment. In some African nations, heritability of vision is close to zero: Severe vitamin A deficiency is the main reason some children see less well than others. Scientists are working to develop a strain of maize (the local staple) high in vitamin A. If they succeed, heritability will increase and overall vision will improve (Harjes et al., 2008).

But what about children who are well nourished? Is their vision entirely inherited? Cross-cultural research suggests not. One report claims that “myopia is increasing at an ‘epidemic’ rate, particularly in East Asia” (Park & Congdon, 2004). The first published research on this phenomenon appeared in 1992, when scholars noticed that, in army-mandated medical exams of all 17-year-old males in Singapore, 26 percent were nearsighted in 1980 but 43 percent were nearsighted in 1990 (Tay et al., 1992).

Further studies found nearsightedness increasing from 12 to 84 percent between ages 6 and 17 in Taiwan, with increases in myopia during middle childhood also in Singapore and Hong Kong (cited in Grosvenor, 2003). One author claims “very strong environmental impacts” on Asian children’s vision (Morgan, 2003). Why could that be? One clue is that, unlike earlier generations or children in other nations, since 1980 East Asian children have become amazingly proficient in math and science because they study intensely, in school and after school. As their developing eyes focus on the print in front of them, those with a genetic vulnerability to myopia may lose acuity for objects far away—which is exactly what nearsightedness means.

A study of Singaporean 10- to 12-year-olds found a correlation between nearsightedness (measured by optometric exams) and high achievement, especially in language (presumably reflecting more reading). Correlation is not causation, but statistics (odds ratio of 2.5, significance of 0.001) strongly suggest a link (Saw et al., 2007).

A First Pair of Glasses This girl from South Korea may require glasses because she spends a lot of time studying and less time playing outdoors.

JONGBEOM KIM/TONGRO IMAGES/CORBIS

Ophthalmologists believe that the underlying cause is not time spent studying but inadequate time spent in daylight. An editorial in a leading U.S. journal for ophthalmologists explains:

Between the early 1970s and the early 2000s, nearsightedness in the U.S. population increased from 25 to 42 percent (Vitale et al., 2009). Urbanization, video games, homework, and fear of strangers have kept many contemporary North American children indoors doing close work much of the time, unlike earlier generations who played outside for hours each day. The correlation between nearsightedness and lack of outside play is striking (see Figure 2.13).

FIGURE 2.13 Go Out and Play! If both your parents are nearsighted, chances are you will be, too—but not if you play sports at least six hours a week. The dramatic correlation between childhood myopia and playing sports does not prove causation: Some children who wear glasses choose to avoid sports. Nonetheless, parents and schools should encourage children to play outside every day. In 2010, only about 64 percent of all children did so, a 10 percent decline over the course of 10 years.

A vision scientist from Ohio State University says: “We’re kind of a dim indoors people nowadays… If you ask me, I would say modern society is missing the protect effect of being outdoors” [Mutti, quoted in Holden, 2010].

However, correlation is not cause. To prove a causal link, a longitudinal experiment would require some children to stay indoors while others from the same families (to control for genes) play outside. Since that is hypothesized to harm those who stay inside, that research would be unethical (see Chapter 1) as well as impossible. Nonetheless, many applications arise from this apparent connection and from many other ideas from this chapter.

Practical Applications

Since genes affect every disorder, no one should be blamed or punished for inherited problems. However, knowing that genes do not act in isolation can lead to preventive measures, before, during, and after prenatal development.

For instance, if alcoholism is in their genes, women can avoid alcohol before, during, and after pregnancy. Further, parents can keep alcohol out of their home, hoping their children become cognitively and socially mature before drinking. (If alcohol is available at home, most children taste it before age 10.) Similarly, if nearsightedness runs in the family, parents can make sure that children play outdoors every day.

Of course, outdoor play and abstention from alcohol are recommended for every child, as are dozens of other behaviours, such as flossing the teeth, saying “please,” getting enough sleep, eating vegetables, and writing thank-you notes. However, no parent can enforce every recommendation.

Awareness of genetic risks alerts parents to set priorities and act on them, and helps professionals advise pregnant women. Some recommendations should be routine (e.g., prenatal vitamins including folic acid) because it is impossible to know who is at risk. Others are tailored to the individual, such as weight gain for underweight women.

Care must be taken to keep pregnancy and birth from being an anxious time, filled with restrictions and fears about diet, diseases, drugs, and other possible dangers. Anxiety itself may reduce sleep, impair digestion, and raise blood pressure—all of which hinder development—and then may make birth complicated and postpartum depression likely.

Indeed, stress reduces the chances of conception, increases the chance of prenatal damage, and slows down the birth process. A conclusion from every page of this chapter is that risks are apparent in every moment of conception, pregnancy, and birth but those risks can be minimal if everyone, fathers as well as mothers, professionals as well as community members, does what is needed to ensure that newborns begin life eager and able to live 80 more healthy years.