Wednesday, January 23, 2019

Intelligence and Heredity: Nurturing Our Children Is Critical


We are in an age where the human genome can be quickly and cheaply evaluated.  This allows genetic evaluations to be made to determine where genetic defects might occur that could cause serious illnesses or physical deficiencies.  There has been some success at pinpointing mutations that can lead to harmful effects.  We have also seen technology improve to the point where sections of the genome can be replaced by modified sections.  This capability has the potential to be tremendously beneficial or terribly dangerous, depending on the wisdom with which it is used.  One potential application that is often discussed is to use genetic modification to increase intelligence.  Such a goal could pinpoint abnormalities that are related to subnormal intelligence, or to boost the intelligence of a child to be born.  The first application seems beneficial, the latter, perhaps not.  It seems the best way to increase intelligence is to provide the young, developing brain a healthy environment in which it can flourish.

Carl Zimmer provides a wealth of material on the topic of human intelligence in his book She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity.  What his discussion makes abundantly clear is that while a component of intelligence can be traced to heredity (nature), development environment, health, and nourishment (nurture), play at least as big a role. 

“While test scores are unquestionably heritable, their heritability is not 100 percent.  It sits instead somewhere near the middle of the range of possibilities.  While identical twins often end up with similar test scores, sometimes they don’t.  If you get average scores on intelligence tests, it’s entirely possible your children will turn out to be geniuses.  And if you’re a genius, you should be smart enough to recognize your children may not follow suit.  Intelligence is not a thing to will to your descendants like a crown.”

“One reason for this complexity is that intelligence, like height, develops.  In an embryo, it does not yet exist.  Children need a few years of growth and experience before they can get a meaningful, predictive score on an intelligence test.  All along that path, experiences can influence how intelligence develops, and different experiences can lead to different intelligence test scores.”

Influences on how intelligence develops in the brain begin in the womb and continue at least through the years of schooling.  A mother subject to various forms of pollution can affect the development of her fetus’s brain.  That is why we should be concerned about the various environmental issues that drive the need for regulations.

“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.”

It has long been known that a lack of iodine can cause the thyroid gland to perform poorly.  Iodine is plentiful in sea water and can be acquired from exposure to it or from food products from the sea.  Consequently, its availability varies considerably from location to location.  Iodine deficiency was known to cause goiter, a swelling of the neck, and cretinism, a form of mental disability.  For that reason, beginning early in the 1900s, iodine was added to table salt in developed countries to guard against those conditions.  It was only in the past decade that iodine deficiency in pregnant women was also linked to the development of intelligence. 

“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.”

Zimmer provides notable examples.

“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 to 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 out 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 timing of the introduction of iodine in salt meant that the US soldiers from World War I did not benefit from the iodine, while those of World War II did.  Intelligence test data was not made available for researchers to use, but the test results had been used by the military to assign recruits to various activities.  James Feyrer, an economist at Dartmouth College, decided to determine the effect of iodine introduction on intelligence from the data available. 

“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 leapt 15 points.”

Environmental factors have proved to be important for intelligence even when introduced after birth.

“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.”

It is not just the chemicals that enter our bodies that can affect intelligence.  The social environment can also alter one’s measured intelligence.  It has long been known that a change in a child’s social or family situation can produce significant changes.  As far back as the 1920s, at a time when eugenics was popular and heredity was assumed to be the dominant factor, studies were being performed to counter this assumption.

“Helen Barrett and Helen Koch of the University of Chicago studied a group of children who were moved from an orphanage into preschool, where they were no longer neglected.  After six months, Barrett and Koch claimed, their test scores jumped far beyond those of the children left behind in the orphanage.”

George Stoddard and his colleagues ran a similar study in the 1930s.

“Stoddard and his team tracked 275 children who were put into foster care.  Their parents were poor, badly educated, and scored below average on intelligence tests.  After being placed ‘in better than average homes,’ Time [Magazine] reported, they scored an average IQ of 116—'equal to the average for children of university professors’.”

We now know that health and intelligence in adults can by affected by a number of adverse childhood experiences.  Poverty does not just mean a lack of access to good nourishment, it is often accompanied by a dysfunctional home life that increases the stress encountered as the brain and body are trying to develop.  This correlation was first detected in a study by the healthcare organization Kaiser Permanente.  It began surveying members with a focus on gathering information on traumatic childhood experiences.  The information collected could then be correlated with the organization’s medical records for the survey participants.  The results were tabulated for a large group that was representative of the middle class rather than having any particular association with issues of poverty.  The results were published in a paper written by Vincent Felitti and Robert Anda under the title The Relationship of Adverse Childhood Experiences to Adult Health: Turning Gold into Lead.  This work is commonly referred to as the ACE study with the acronym representing “adverse childhood experiences.”  The data showed a clear relationship between poor health as an adult and the degree of adversity encountered in childhood.  Follow up studies by other researchers confirmed that the stress induced by these adverse experiences can diminish the capability to learn and thus lower scores on IQ tests.

Zimmer describes studies with twins that further illustrate the variability of intelligence. Identical twins have as close to the identical genetic composition as it is possible to get.

“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 to not 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.”

But we know from Kaiser Permanente’s ACE study that poverty is not a simple socioeconomic quantity.  Such a study as that of Turkheimer’s should have also tallied childhood adversity as a defining characteristic rather than merely income.  Zimmer tells us that Turkheimer’s findings have sometimes been duplicated, and sometimes not.  The inadequacy of “socioeconomic class” as a control parameter may explain the inconsistencies.  Zimmer also provides an intriguing conclusion based on studies of European twins versus US twins. 

“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.”

Perhaps national social policies can be important.

Scientists have been sequencing DNA for several years now with little success in discovering significant genetic markers associated with intelligence.  Similar studies were performed attempting to determine the genetic basis for human height.  What was learned was that over a million markers were found throughout the entire genome and all of them had a very small effect.  It is expected that many more will be found as searches become more detailed, but there will be no one or few locations where a dominant contribution will be found.  Zimmer expects a similar result when it comes to intelligence.  Thus far the research is consistent with that expectation.

“In hindsight, searching for candidate genes was a strategy pretty much guaranteed to fail.  Our brains use 84 percent of our twenty-thousand-odd protein-coding genes.  Each type of neuron uses a distinctive combination of those genes, and it turns out the brain is made up of hundreds of cell types—so many that scientists will not finish its catalog for a long time.  To think we could just reach into this jumble and pluck out a single gene that had a clear-cut role in intelligence was to pretend we know more about the brain than we really do.”

What is striking about Zimmer’s section on intelligence is that nearly all the research presented studying the social and physical environmental factors affecting the development of human intelligence were reported in the twenty-first century.  This is mainly new research that is providing what are often alarming results.  We cannot yet claim to know all the chemicals or social influences that can alter the development of the brain and intelligence.  We should be funding biological and environmental research at an ever-higher level, yet the current administration is headed in the opposite direction.  We—and particularly our children—are being placed in danger so that a few individuals and corporations can earn more money.  That must stop.


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