She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (42 page)

If a mother drinks heavily during pregnancy, the alcohol can interfere with the growth of neurons, leading to
fetal alcohol syndrome. After birth, a child's brain continues growing swiftly, and along the way it stays
vulnerable
to toxins such as lead paint. Sometimes the enemies of intelligence work together to wreak havoc. In 1999,
Brenda Eskenazi and her colleagues at the University of California, Berkeley, went to the farming communities of the Salinas Valley to see how intelligence is influenced by the pesticides sprayed on the fields. They followed 601 women through their pregnancies and then tracked the development of their children. The children of mothers with the highest levels of pesticide in their blood scored low on intelligence tests they took at age seven. And Eskenazi also found that poverty, abuse, and other kinds of adversity worsened the effects of the pesticides.

The environment's power isn't limited to lowering intelligence test scores, however. It can—under certain circumstances—lift them up. One of the simplest ways to do so, it turns out, is to
give people iodine.

Iodine is essential for making hormones in the thyroid gland. A lack of iodine can lead to a number of diseases, including a neck swelling called goiter.
It can also lead to cretinism, which leads to both dwarfism and severe intellectual disability. Normally, a pregnant mother's thyroid hormones travel into the brain of her fetus, where they help
neurons crawl to their proper location in the brain. If she has a deficiency of iodine, she makes fewer hormones, leaving the fetal brain to fail to develop properly.

To keep our iodine levels high, we depend on our food. Seafood is a good source of iodine, because the element is abundant at sea. Meat and vegetables and milk can be good sources, too, but only if they come from places where the soil is rich in iodine.
A third of the world's population lives in places that put them at risk of iodine deficiency. Adding it to salt is all that's necessary to give people healthy levels of iodine. When the United States and other countries established this policy in the early 1900s, both goiter and cretinism started to disappear.

Another century would pass before scientists began uncovering evidence that iodine deficiency may have a much wider impact on intelligence.
Sarah Bath of the University of Surrey and her colleagues documented this effect in a survey of children growing up in southwestern England. England has never required iodine be added to salt, in the belief that people could get enough of it in milk. That turns out to have been wrong. Bath and her
colleagues found that two-thirds of the pregnant women they studied had a mild iodine deficiency. And the children of these women, Bath found, got significantly lower verbal IQ scores at age eight and scored lower at age nine on tests for reading accuracy and comprehension.

The growing appreciation for iodine's importance for intelligence led
James Feyrer, an economist at Dartmouth College, to take a fresh look at its history. He took advantage of the fact that the introduction of iodine in the United States fell squarely between the two world wars. Millions of young American men who served in World War I lacked the benefit of iodized salt. Thanks to their iodine deficiency, twelve thousand recruits had goiter, a third of whom couldn't button a military tunic around their neck and were judged unfit for service. But by the time the military inspected
recruits for World War II, the rate of goiters had dropped 60 percent.

Feyrer wondered if this shift also affected the intelligence of the recruits. He was not allowed to look at their individual IQ scores, but he and his colleagues found a way to infer them: The highest-scoring recruits were put into the air force instead of the ground forces. Reviewing the records of two million recruits, Feyrer and his colleagues also checked the natural iodine levels in their hometowns. Nationwide, the researchers found, the introduction of iodine raised the average IQ by an estimated 3.5 points. And in the parts of the country where natural iodine levels were lowest, Feyrer and his colleagues estimated that scores leaped 15 points.

It may be hard to believe that such a straightforward change in people's diets could have such a tremendous effect on intelligence. But as public health workers continue to bring iodine to more of the world, the same jumps happen. In 1990,
Robert DeLong, an expert on iodine at Duke University, traveled to the Taklamakan Desert in western China. The region has extremely low levels of iodine in the soil, and the people in the region have resisted attempts to introduce iodized salt. It didn't help that the people of the region, the Uyghurs, distrusted the government in Beijing. Rumors spread that government-issued iodized salt had contraceptives in it, as a way to wipe out the community.

DeLong and his Chinese medical colleagues approached local officials
with a different idea: They would put iodine in the irrigation canals. Crops would absorb it in their water, and people in the Taklamakan region would eat it in their food. The officials agreed to the plan, and when DeLong later gave children from the region IQ tests, their average score jumped 16 points.

Chemically altering people's brains isn't the only way to change their scores on intelligence tests. James Flynn, a social scientist at the University of Otago in New Zealand, has discovered that across the world, IQ test scores have been steadily increasing. Flynn's first inkling of this shift came in 1984. He had asked a Dutch colleague to send him results of IQ tests administered to eighteen-year-olds in the Netherlands. When the scores arrived in the mail, he settled down to peruse them. A puzzling discrepancy jumped out: Dutch students in the 1980s did substantially better than students in the 1950s.

Flynn found a similar trend in nearly thirty developed countries. In Britain and the United States, for example, test scores increased 0.3 points a year. If the average score in 2000 was 100, then it would have been 70 in 1900. “
We are driven to the absurd conclusion that a majority of our ancestors were mentally retarded,” Flynn wrote in his 2007 book,
What Is Intelligence?

Yet this trend—now known as the Flynn effect—has been confirmed many times over. As we've gotten taller, we've gotten smarter. Now the challenge is to figure out what's driving this increase.

As in the case of height, the Flynn effect has been too big and quick to pin on genetic change. For that to be the case, people who scored high on intelligence tests would have to have much bigger families than everyone else to spread their genes, and that hasn't happened. It's possible that what's been happening to intelligence test scores is similar to what's been happening to height. The global height boom has been brought about in part by better food, sanitation, medicine, and—in some places—greater economic equality. Some of the same factors may be at play in the Flynn effect. Better childhood health and nutrition makes the body grow quickly and the brain develop well.

Government regulations have also helped. Feyrer has argued that the
push to give people iodine played a part in the worldwide Flynn effect. Exposure to lead can be toxic for the brain, and up until the 1970s, American children were exposed to high levels of lead in paint and gasoline. In 2014,
Alan Kaufman, an intelligence expert at Yale, and his colleagues published a study on intelligence tests they gave to hundreds of Americans who were exposed to high lead levels before the 1970s and to hundreds more Americans who were born afterward. They estimated that lowering lead levels in children gave them a boost of 4 to 5 IQ points.

But scientists are also investigating other possible causes, because they're keenly aware that intelligence isn't just affected by molecules that flow through the brain.
Our behaviors are shaped by our experiences, particularly the ones we have with other people. As our parents talk to us, they help build our vocabularies, for example. The world's fertility rate dropped drastically over the past century. In 1950, it was 5 children per woman; in 2010, it was 2.5. In a smaller family, the children have the opportunity to listen more to their parents.

Going to school can also raise intelligence test scores. To measure schooling's effect, two statisticians,
Christian Brinch and Taryn Ann Galloway, took advantage of reforms that Norway put into place starting in the 1950s. By reorganizing their school system, the Norwegians increased the time students had to spend in school from seven years to nine. Different towns made the switch at different times between 1955 and 1972. Brinch and Galloway looked at how the extra schooling affected the IQ tests that nineteen-year-old men took as part of Norway's universal draft. In 2012, they reported an extra year of education raised scores by 3.7 IQ points.

This natural experiment takes on greater importance when you consider how much more schooling children get now than in previous centuries. In the United States,
the enrollment rate in the early 1900s was 50 percent. By 1960, it reached 90 percent. American students went from an average schooling of 6.5 years to 12.

To Flynn himself, the Flynn effect doesn't mean that people in the nineteenth century were intellectually disabled, nor does it mean that people today have neurons that fire signals to each other in a fundamentally new
way. Our forerunners relied on ways of thinking suited to their age. In the early 1900s, intelligence tests included questions like “What do dogs and rabbits have in common?” The answer that the test givers wanted was that they were both mammals. But often the answer they got instead was, “You use dogs to hunt rabbits.” To people who spent their time hunting rather than learning taxonomy, that fact was the one that mattered.

Twentieth-century schooling began to train students more in thinking in terms of classification, logic, and hypotheses. To get a job, people had to understand how to operate machines, and then computers. Rather than hunting rabbits with dogs, children today are more likely to pass their free time on a smartphone. Flynn's argument is also bolstered by the way the Flynn effect spread over the world. It started in the United States and Europe, but as developing countries became more modernized, their intelligence test scores started their own upward trend.

As in the case of height, intelligence forces us to hold two contradictory ideas in our heads at once. Over the past century the world has gotten taller and smarter, but these increases were not brought about by a shift in our genetic variants. The change is so dramatic that it can be hard to see how genes matter at all. And yet heredity has not stopped mattering. Height was a strongly heritable trait in the early 1900s, when scientists first began to measure it. Intelligence was as well. Today, both remain heritable. Under similar conditions, people will grow to different heights and get different scores on intelligence tests in part because of the genes they inherited.

It's also becoming clear that we can't treat genes and the environment as two distinct forces that act independently of each other. Each one influences the other. In 2003,
Eric Turkheimer of the University of Virginia and his colleagues gave a twist to the standard studies on twins. To calculate the heritability of intelligence, they decided not to just look at the typical middle-class families who were the subject of earlier studies. They looked for twins from poorer families, too. Turkheimer and his colleagues found that the socioeconomic class determined how heritable intelligence was. Among children who grew up in affluent families, the heritability was about
60 percent. But twins from poorer families showed no greater correlation than other siblings. Their heritability was close to zero.

It may seem weird that the environment itself can change heritability. We tend to think of genes as rigid purveyors of destiny, the inescapable agents of heredity. But biologists have always known that the two are intimately linked together. If you raise corn in uniformly healthy soil, with the same level of abundant sunlight and water, the variation in their height will largely be the product of the variation in their genes. But if you plant them in a bad soil, where they may or may not get enough of some vital nutrient, the environment will be responsible for more of their differences.

Turkheimer's study hints that something similar happens to intelligence. By focusing their research on affluent families—or on countries such as Norway, where people get universal health care—intelligence researchers may end up giving too much credit to heredity. Poverty may be powerful enough to swamp the influence of variants in our DNA.

In the years since Turkheimer's study, some researchers have gotten the same results,
although others have not. It's possible that the effect is milder than once thought. A 2016 study pointed to another possibility, however. It showed that poverty reduced the heritability of intelligence in the United States, but not in Europe. Perhaps Europe just doesn't impoverish the soil of its children enough to see the effect.

Yet there's another paradox in the relationship between genes and the environment. Over time, genes can mold the environment in which our intelligence develops. In 2010, Robert Plomin led a study on eleven thousand twins from four countries,
measuring their heritability at different ages. At age nine, the heritability of intelligence was 42 percent. By age twelve, it had risen to 54 percent. And at age seventeen, it was at 68 percent. In other words, the genetic variants we inherit assert themselves more strongly as we get older.

Plomin has argued that this shift happens as people with different variants end up in different environments. A child who has trouble reading due to inherited variants may shy away from books, and not get the benefits that come from reading them. A child who learns quickly how to do math may
get encouraged by teachers to do more. As children get older, they gain more power to choose those environments, which can shape their intelligence even more. We think of the environment as our physical surroundings—of heat and cold, of chemicals and food. But as humans, we also build an environment around ourselves of words and numbers.