What's Driving the Decline in IQ Scores?

Story at-a-glance

  • Intelligence quotient (IQ) scores, which had been rising for decades throughout the 20th century, are now on the decline
  • It was long believed that people with higher IQs would have children with higher IQs and also that people with lower IQs would have more children, leading to an eventual lowering of IQ scores across the population
  • A new Norwegian study challenges both of these assumptions, suggesting instead that declining IQ scores have little to do with genetics and everything to do with the environment
  • In the study, brothers born to the same parents often had significantly different IQ scores, suggesting an environmental factor — not a genetic one — was causing the disparity
  • Changes in nutrition and exposure to environmental toxins like fluoride, flame retardants and pesticides could all be playing a role in lowering IQ levels


This is an older article that may not reflect Dr. Mercola’s current view on this topic. Use our search engine to find Dr. Mercola’s latest position on any health topic.

By Dr. Mercola

Intelligence quotient (IQ) scores, which had been rising for decades throughout the 20th century, are now on the decline, and researchers with the Ragnar Frisch Center for Economic Research in Norway set out to determine why.

The increasing trend, a phenomenon known as the Flynn effect, peaked in the 1970s and has been falling since. In fact, when the Norwegian researchers analyzed IQ scores from 730,000 men born between 1962 and 1975, IQ scores rose by about 3 percentage points a decade.1

Then, beginning with those born after 1975, they started to decline. IQ scores have often been tied to genetic factors, and it was long believed that people with higher IQs would have children with higher IQs and also that people with lower IQs would have more children (known as the dysgenic fertility theory), leading to an eventual lowering of IQ scores across the population.

But the Norwegian study challenges both of these assumptions, suggesting instead that declining IQ scores have little to do with genetics and everything to do with the environment.

Environmental Factors Are Leading to Lower IQ Scores

What made the study particularly interesting wasn't only the reveal that IQ scores are on the decline. Because the researchers analyzed IQ scores of brothers, they were able to spot differences in IQ scores among families. Surprisingly, brothers born to the same parents often had significantly different IQ scores, suggesting an environmental factor — not a genetic one — was causing the disparity.

Also revealing, parents with higher IQs tended to have more children than people with lower IQs, challenging the dysgenic fertility theory. Study author Ole Rogeberg said in a news release, "The causes in IQ increases over time and now the decline is due to environmental factors … It's not that dumb people are having more kids than smart people, to put it crudely. It's something to do with the environment, because we're seeing the same differences within families."2

As for what environmental factors are drawing down IQ scores, this remains to be seen, but access to education likely plays a role, with more education equating to higher IQ scores. Other environmental factors were also mentioned, including changes in nutrition and time spent on the internet or reading.

However, a number of environmental toxins that children are exposed to regularly have also been linked to cognitive effects, including lowered IQ. It could be that these toxic exposures are harming kids' intelligence — and have been doing so for decades.

Fluoride Exposure Linked to Lower IQ

The featured study didn't look in-depth into the environmental exposures that could be affecting IQ but other studies have. In the U.S., two-thirds of Americans' tap water contains fluoride, which is added under the guise of preventing cavities. Water fluoridation continues to occur in the majority of the U.S. even as research stacks up that fluoride is a neurotoxin that can harm brain function. Fluoride also leaches lead out of old pipes, which further magnifies its neurological risks.

A study of Mexican women and children, published in 2017, found that higher exposure to fluoride while in utero is associated with lower scores on tests of cognitive function in childhood, both at the age of 4 and 6 to 12 years.3

While the children's fluoride levels at ages 4 and 6 to 12 were not associated with their intelligence, the study found that exposure that occurs prenatally was linked to lower intelligence scores. In fact, women with higher levels of fluoride in their urine during pregnancy were more likely to have children with lower intelligence.

Specifically, each 0.5 milligram per liter increase in pregnant women's fluoride levels was associated with a reduction of 3.15 and 2.5 points on the children's General Cognitive Index (GCI) and Wechsler Abbreviated Scale of Intelligence (WASI) scores, respectively.

In 2012, Harvard researchers also revealed that children living in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas4 and suggested high fluoride exposure may have an adverse effect on children's neurodevelopment.

Then, in 2014, a review in Lancet Neurology classified fluoride as one of only 11 chemicals "known to cause developmental neurotoxicity in human beings,"5 alongside other known neurotoxins such as lead, methylmercury, arsenic and toluene. Among the proposed mechanisms of harm, studies have shown fluoride can:6

Interfere with basic functions of nerve cells in the brain

Reduce nicotinic acetylcholine receptors

Reduce lipid content in the brain

Damage the pineal gland through fluoride accumulation

Impair antioxidant defense systems

Damage the hippocampus

Damage purkinje cells

Increase uptake of aluminum, which has neurotoxic effects

Encourage formation of beta-amyloid plaques (the classic brain abnormality in Alzheimer's disease)

Exacerbate lesions induced by iodine deficiency

Increase manganese absorption, which has also been linked lower IQ in children

Impair thyroid function, which can also affect brain development

Flame Retardants Associated With Lower Intelligence in Children

Flame-retardant chemicals known as polybrominated diphenyl ethers (PBDEs) are found in everything from furniture and mattresses to electronics and baby toys. Along with being linked to cancer, infertility, hormone disruptions and birth defects, they're also contributors to neurodevelopmental delays and reduced IQ scores in children.

One systematic review and meta-analysis looked into exposures to PBDEs that occurred near conception or during in utero, perinatal or childhood time periods. Greater exposures to flame retardants during pregnancy were associated with lower intelligence in the child. Specifically, for every tenfold increase in prenatal exposure to PBDEs, there was a 3.7-point decline in IQ test scores in children.7

Past research has also demonstrated that children born to mothers with higher levels of flame-retardant chemicals in their body had a 4.5-point average decrease in IQ,8 while exposure in childhood is strongly associated with poor attention span, reduced fine motor coordination and a decrease in cognitive ability.9

While a few-point reduction in IQ may seem small, the widespread exposure to flame retardants makes the decrease especially serious. Study coauthor Tracey Woodruff told Medicine Net, "Even the loss of a few IQ points on a populationwide level means more children who need early interventions, and families who may face personal and economic burdens for the rest of their lives."10

One way that exposure to flame retardants during pregnancy may harm children's IQ could be via their influence on thyroid hormones. PBDEs are known to disrupt estrogenic activity as well as thyroid levels and, in one study, women with the highest concentrations of PBDEs in their blood had an increased risk of thyroid disease compared to those with lower concentrations.11

Meanwhile, research published in the New England Journal of Medicine (NEJM) revealed that children aged 7 to 9 who had mothers with untreated hypothyroidism in pregnancy had IQ scores about 7 points lower than youngsters of women without such a deficiency.12

Pesticide Exposure May Lower IQ

Exposure to organophosphate pesticides is another environmental factor linked to lower IQ. In a series of studies published in the journal Environmental Health Perspectives, women who had higher exposure to pesticides during pregnancy had children with lower IQ scores when they reached school age.13

In one of the studies, children with the highest pesticide exposures had a seven-point lower score on intelligence tests compared to children with the lowest exposures and, as The New York Times reported, "every tenfold increase in organophosphate exposure detected during pregnancy corresponded to a 5.5 point drop in overall I.Q. scores."14

Further, according to estimates by David Bellinger, a professor in the department of environmental health at Harvard Medical School, Americans have lost a total of 16.9 million IQ points due to exposure to organophosphate pesticides. He also cites exposures to lead and methylmercury, via seafood consumption, as other contributing factors to loss of IQ.

Children Are Especially Vulnerable to Chemical Exposures

Part of what makes environmental chemical exposures so damaging is their widespread prevalence, and the fact that children are exposed to multiple chemicals both in utero and after birth. Bellinger wrote:15

"Based on the estimated number of FSIQ [full-scale IQ] points lost, the population burdens associated with environmental chemical exposures of children are surprisingly large — in some cases larger than those estimated for major medical conditions and events [i.e., preterm birth, traumatic brain injury, brain tumors, congenital heart disease].

This is attributable not so much to the magnitude of the effect sizes associated with chemicals, but to the prevalence of exposures associated with adverse impacts."

Children are especially vulnerable, as they experience greater exposure to chemicals pound-for-pound than adults,16 and though the blood-brain barrier is fully formed at birth, its function may be immature, which may allow greater chemical exposures to reach their developing brain.

Children also have lower levels of some chemical-binding proteins, according to the Environmental Working Group (EWG), which allows more of a chemical to reach their organs, while systems that detoxify and excrete chemicals in adults are not fully developed.

These factors, coupled with the fact that a child will be around for 80 years or more, allowing more than enough time for chemicals to do their damage, signals a major challenge for kids born today. Exposure that occurs in utero may be even more problematic, as EWG reported:

"The pace and complexity of growth and development in the womb are unmatched later in life. Three weeks after conception, an embryo, still only 1/100th the size of a water droplet, has nevertheless grown at such an explosive rate that were it not to slow down, it would be born literally the size of a million Earths.

… At no other time in life does a person create so much from so little in so short a time. Industrial chemicals that interrupt this intricate process can, at high levels, wreak havoc in the form of severe birth defects, or at lower levels cause subtle but important changes in development that surface later in childhood as learning or behavioral problems, or in adulthood in the form of certain cancers or perhaps neurodegenerative disease."

Top Tips for Protecting Intelligence

There are many factors that contribute to a person's intelligence, but from the standpoint of environmental ones, limiting exposure to the chemicals listed above makes sense. You can do this by:

Filtering Your Water to Remove Fluoride — Fluoride is a very small molecule, making it tremendously difficult to filter out once added to your water supply. Any simple countertop carbon filter, like Brita, will not remove it. Among the more effective filtering systems for fluoride removal are:

Reverse osmosis (RO). The drawback is that it will remove many valuable minerals and trace elements as well. RO systems also need frequent cleaning to avoid bacterial growth. So, use a tankless RO system with a compressor

Water distillation, which, like RO, gets everything out, including beneficial minerals. You then need to restructure the water

Bone char filters and biochar

Clearly, the simplest, most effective, most cost-effective strategy is to not put fluoride in the water to begin with. To learn more about fluoride and how you can help end this harmful practice, I highly recommend getting a copy of "The Case Against Fluoride." You can also download my free report on water fluoridation, a book by Paul Connett, Ph.D., toxicologist, environmental chemist and former director of the Fluoride Action Network (FAN).

Avoiding Flame-Retardant Chemicals — These chemicals are widespread in consumer products, but there are steps you can take to reduce your exposure, including these tips from the Green Science Policy Institute:17

Avoid upholstered furniture with a TB117 label. If the label states, "This article meets the flammability requirements of California Bureau of Home Furnishings Technical Bulletin 117 …" it most likely contains flame retardants. However, even upholstered furniture that's unlabeled may contain flame retardants.

Furniture products filled with cotton, wool or polyester tend to be safer than chemical-treated foam; some products also state that they are "flame retardant free." Organic wool (100 percent) is naturally flame-resistant.

Avoid baby products with foam. Nursing pillows, high chairs, strollers and other products containing polyurethane foam most likely contain flame retardants.

Avoid foam carpet padding. If possible, minimize the use of foam carpet padding, which often contains flame retardants. If removing carpeting, take precautions to avoid exposures. You'll want to isolate your work area from the rest of your house to avoid spreading it around, and use a HEPA filter vacuum to clean up.

PBDEs are often found in household dust, so clean up with a HEPA-filter vacuum and/or a wet mop often. Washing your hands regularly can also help.

Minimizing Exposure to Pesticides — Research shows that eating organic leads to lower levels of pesticides in your body. In one study, when a family of five switched to an all-organic diet for two weeks, their body levels of pesticides dropped significantly, decreasing by a factor of 6.7.18 If you must choose between which products to purchase organic, I recommend prioritizing organic animal foods first and then using EWG's Dirty Dozen list for produce.

For the nonorganic produce you consume, washing with a solution of baking soda may help to remove some of the pesticides on the surface of the fruit or vegetable,19 although it won't remove chemical residues that have penetrated beyond the peel. Peeling is another option to reduce pesticide residue, but this also means you're removing the healthy compounds contained in the peel (and there can still be residues that have penetrated into the produce flesh).

For these reasons, the best way to avoid pesticide residues in your food is to choose those that haven't been exposed to them to begin with, i.e., go organic. In addition, avoid spraying pesticides around your home; opt for natural solutions to pest control instead.

Optimizing Your Diet Is Crucial for a Healthy Brain

Nutrition also plays a role in intelligence, and this includes during pregnancy. Omega-3 fats are incredibly important and have been linked to cognitive benefits for children both during pregnancy and in early life.20 When boys were given an omega-3 supplement, there were significant increases in the activation of the dorsolateral prefrontal cortex part of the brain.21

This is an area of your brain associated with working memory. They also noticed changes in other parts of the brain, including the occipital cortex (the visual processing center) and the cerebellar cortex (which plays a role in motor control).

Small cold-water fish that are rich in animal-based omega-3 fats but have a low risk of contamination are among your best choices for healthy omega-3s. This includes anchovies, sardines, mackerel, herring and wild-caught Alaskan salmon. Krill oil is a supplement alternative.

Other important nutrients include choline, found in cruciferous veggies, leafy greens and pastured organic egg yolks. Choline intake during pregnancy "super-charged" the brain activity of animals in utero, indicating that it may boost cognitive function, improve learning and memory, and even diminish age-related memory decline and the brain's vulnerability to toxins during childhood, as well as conferring protection later in life.22

In addition, people with higher choline intakes were shown to have better cognitive performance, doing better on tests of verbal and visual memory, than those with low intake.23 Ultimately, protecting the intelligence of future generations will depend on removing known neurotoxins from the environment, but you can also make a difference individually by avoiding chemical exposures, eating right and leading a healthy lifestyle overall.


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