Environment and intelligence

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Environment and intelligence research investigates the impact of environment on intelligence. This is one of the most important factors in understanding human group differences in IQ test scores and other measures of cognitive ability. It is estimated that genes contribute about 20-40% of the variance in intelligence in childhood and about 60% in old age. Thus the environment and its interaction with genes account for the remaining approximate 50% of intelligence.[1] Historically, there has been great interest in the field of intelligence research to determine environmental influences on the development of cognitive functioning, in particular, fluid intelligence, as defined by its stabilization at 16 years of age. Despite the fact that intelligence stabilizes in early adulthood it is thought that genetic factors come to play more of a role in our intelligence during middle and old age and that the importance of the environment dissipates.[2]

Neurobiological theory[edit]




Environmental influences[edit]





The type and amount of praise received from family can also affect how intelligence develops. Carol Dweck and colleagues have conducted a lot of research into how the type of feedback the family gives a child on their academic achievements can shape their future intelligence scores. Telling a child that they are intelligent and praising them for this ‘intrinsic’ quality indicates that intelligence is fixed, known as entity theory. A child holding entity theory is likely to believe that there is no point in them trying to challenge themselves because if they fail that task it means that they are not intelligent. On the other hand is incremental theory: the idea that intelligence can be improved upon with effort. Children who hold this theory are more likely to develop a love for learning rather than for achievement. Parents who praise the child’s effort at a task rather than the result are more likely to instill this incremental theory of intelligence in their children and thus to improve their intelligence.[7]

Peer group[edit]

JR Harris suggested in The Nurture Assumption that an individual's peer group influences their intelligence greatly over time, and that different peer group characteristics may be responsible for the black-white IQ gap. Several longitudinal studies support the conjecture that peer groups significantly affect scholastic achievement,[8][9][10] but relatively few studies have examined the effect on tests of cognitive ability. There is some evidence that peer groups influence tests of cognitive ability, however.[11]



There is controversy as to whether education affects intelligence in that education has a complicated relationship with intelligence; it is both a dependent and independent variable.[5] Previously measured intelligence has been shown to predict educational achievement and attainment. The correlation between IQ and educational attainment is somewhere between .40 and .60.[13] Those who did better on intelligence tests in their childhood tend to have a lower drop out rate, and complete more years of school, therefore making intelligence a predictive factor of how well someone will succeed in schooling.[5] Intelligence also affects achievement in examinations with one study finding a correlation of 0.81 between general intelligence(g) and GCSE results.[14] However, on the other hand, education has been shown to improve a person’s performance on these intelligence tests, from a very young age.[5] A study by Ceci illustrates the numerous ways in which education can affect intelligence. It was found that; IQ decreases during summer breaks from schooling, children with delayed school entry have lower IQ's, those who drop out of education earlier have lower IQ's and children of the same age but of one years less schooling have lower IQ scores. Thus it is difficult to unravel the interconnected relationship of IQ and education where both seem to affect one another, however it appears that quantity of schooling may be important as it underpins the cognitive processes of performance on IQ tests.[15]

Training and interventions[edit]


In an experiment, groups of adults were first assessed using standard tests for fluid intelligence.[16] Then they trained groups for four different numbers of days, for half an hour each day, using an n-back exercise that worked on improving one’s working memory.[16] It supposedly does so through a few different components, involving having to ignore irrelevant items, manage tasks simultaneously, and monitor performance on exercise, while connecting related items.[16] After this training, the groups were tested again and those with training (compared against control groups who did not undergo training) showed significant increases in performance on the fluid intelligence tests.[16]

A study by Blackwell et al. found that they could improve a child’s mathematics achievement depending on which theory of intelligence they were taught; incremental or entity theory. Entity theory supposes that intelligence is fixed and cannot be altered by working harder. Incremental theory on the other hand assumes that intelligence is malleable and can be developed and improved with effort. Over the course of a year they found that student’s who had been taught the incremental theory of intelligence showed an upward trajectory in grades in mathematics throughout the year whereas those who had been taught entity theory showed no improvement. This indicates that teaching incremental theory may improve performance on academic tasks, though further research is needed to investigate whether the same results can be found for general intelligence.[17]

Other studies have looked at improving intelligence and preventing cognitive decline by using cognition enhancing substances known as nootropics. One such study gave participants a number of know nootropics in combination in the hopes of targeting numerous cellular mechanisms and increasing the effects on cognition that each would have if administered individually. They conducted a double blind test and administered the combination treatment or placebo to adults for 28 days. They administered Raven's Advanced Progressive Matrices as a measure of intelligence on the first day and after 28-days.The results indicated a significant improvement in performance for those who had taken the treatment compared to those taking the placebo. The effect was equivalent to an increase in IQ of around 6 points.[18]

Environmental enrichment[edit]


In measuring intellectual capacity, the rats who had both forms of enrichment performed best, the ones with social enrichment performed second best, and the ones with a toy in their cage performed still better than the rats with no toy or other rats.[19] When the volume of the rat's cortices was measured the amount of enrichment again correlated with larger volume, which is an indicator of more synaptic connections, and greater intelligence.[19] Attaining this sort of information in humans would be difficult as it requires histological research.

However studies where environmental deprivation has occurred provide insight and indicate that a lack of stimulation can lead to cognitive impairment. Further research using educational attainment as an indicator of cognitive stimulation have found that those with higher levels of education show less signs of cognitive aging and that stimulating environments could be used in the treatment of cognitive aging dysfunctions such as dementia.

Biological influences[edit]


Nutrition has been shown to affect intelligence prenatally and postnatally. The idea that prenatal nutrition may affect intelligence comes from Barker's hypothesis of fetal programming, which states that during critical stages of development the intrauterine environment affects or ‘programmes’ how the child will develop. Barker cited nutrition as being one of the most important intrauterine influences affecting development and that under-nutrition could permanently change the physiology and development of the child.[20] It has been shown that under-nutrition, particularly protein malnutrition, can lead to irregular brain maturation and learning disabilities.[21]

As prenatal nutrition is difficult to measure, birth weight has been used as a surrogate marker of nutrition in many studies. Birth weight needs to be corrected for gestational length to ensure that the effects are due to nutrition and not prematurity.[22] The first longitudinal study looking at the effects of under-nutrition, as measured by birth weight, and intelligence focused on males who were born during the Dutch famine.[5] The results indicated that there were no effects of under-nutrition on intellectual development. However, many studies since have found a significant relationship and a meta-analysis by Shenkin and colleagues indicates that birth weight is associated with scores on intelligence tests in childhood.[22]



Another study done by Lucas et al. confirms the conclusions about the importance of nutrition in the cognitive development of individuals born prematurely. It also found that the cognitive function of males was significantly more impaired by poorer postnatal nutrition.[24] A unique finding however, was that there was a higher incidence of cerebral palsy in the individuals who were fed the non-nutrient enhanced formula.[25]

Breast feeding has long been purported to supply important nutrients to infants and has been correlated with increased cognitive gains later in childhood.[26] The link between intelligence and breast feeding has even been shown to persist into adulthood.[27] However this view has been challenged in recent times by studies which have found no such link between breast feeding and cognitive abilities. A meta-analysis by Der, batty and Deary concluded that there was no link between IQ and breastfeeding when maternal intelligence had been accounted for and that mothers' intelligence is likely to be the link between breastfeeding and intelligence.[28] a

Contrary to this, when controlling for other environmental influences and maternal IQ, a study by Johnson et al. still found that the initiation of breast feeding predicted the scores of three-year-olds on intelligence tests. On average, breast feeding resulted in a 4.6 higher score on intelligence tests.[29] Thus even when maternal IQ is considered, the relationship between breastfeeding and later cognitive abilities persisted.




Maternal stress levels may affect the developing child’s intelligence. The timing and duration of stress can greatly alter the fetus’ brain development which can have long term effects on intelligence. Maternal reactions to stress such as increased heart rate are dampened during pregnancy in order to protect the fetus. The impact of stress can be seen across many different species and can be an indicator of the outside environment which can help the fetus to adapt for surviving in the outside world. However not all maternal stress has been perceived as bad as some has been seen to induce advantageous adaptions.[33]

Stress during early childhood may also affect the child’s development and have negative consequences on neural systems underlying fluid intelligence. A 2006 study found that IQ scores were related to the number of traumas and symptoms of Post Traumatic Stress Disorder (PTSD) in children and adults.[34] Similarly, another study found that exposure to violence in the community and the subsequent distress, were related to a significant decrease in intelligence scores and reading abilities in children aged 6–7 years. Exposure to violence in the community had similar cognitive affects as experiencing childhood maltreatment or trauma.[35]

Maternal age[edit]

Maternal age has been shown to be related to intelligence with younger mothers tended have children of lower intelligence than older mothers.[36] However this relationship may be non-linear with older mothers being at increased risk of giving birth to children with down syndrome which greatly affects cognitive abilities.[37]

Exposure to toxic chemicals and other substances[edit]

Lead exposure has been proven to have significant effects on the intellectual development of a child.[5] In a long-term study done by Baghurst et al. 1992, children who grew up next to a lead-smelting plant had significantly lower intelligence test scores, negatively correlated with their blood-lead level exposure.[5] Even though lead levels have been reduced in our environment, some areas in the United States, particularly inner cities, are still at risk for exposing their children.[5]

Furthermore, prenatal exposure to alcohol can greatly affect a child’s performance on intelligence tests, and their intellectual growth. At high doses, fetal alcohol syndrome can develop, which causes mental retardation, as well as other physical symptoms, such as head and face deformities, heart defects and slow growth.[5][29] It is estimated that 1 in 1,000 babies born in the general population are born with fetal alcohol syndrome, as a result of heavy use of alcohol during pregnancy.[24]

However, studies have shown that even at slightly less severe doses, prenatal exposure to alcohol can still affect the intelligence of the child in development, without having the full syndrome. Through a study done by Streissguth, Barr, Sampson, Darby, and Martin in 1989, it was shown that moderate prenatal doses of alcohol, defined as the mother ingesting 1.5 oz. daily, lowered children’s test scores by 4 point below control levels, by the age of four. They also showed that prenatal exposure to aspirin and antibiotics is correlated with lower performance on intelligence tests as well.[5] However more recent studies have found that low to moderate alcohol consumption is not associated with children’s intelligence scores.[38] This contradictory evidence could perhaps be explained by findings that the effects of alcohol may depend on the genetic makeup of the fetus. In a recent study Lewis et al., looked at alcohol dehydrogenase genes and their mutations, which humans can have between 0 and 10. These mutations slow the breakdown of alcohol so the more mutations the fetus has the slower they will breakdown alcohol. They found that in children whose mothers had drank moderately, those children with four or more mutations performed worse on an intelligence test than those with two or less mutations.[39]


Specifically, prenatal exposure to marijuana affects development of intelligence later in childhood, in a nonlinear fashion, with the degree of exposure.[40] Heavy use by the mother within the first trimester is associated with lower verbal reasoning scores on the Stanford-Binet Intelligence Scale; heavy use during the second trimester is associated with deficits in composite, short-term memory as well as lower quantitative scores on the test; high exposure in third trimester associated with lower quantitative scores as well.[41] A study by Fried and Smith indicated that marijuana exposure did not lead to a decrease in global intelligence but that it did lead to problems with executive functions in childhood.[42] However another study found that when influences such as maternal age, mother’s personality and home environment, there was no longer a difference between children exposed to marijuana and those not exposed with relation to executive functions.[43]

Exposure to tobacco smoking has been associated with diminished intelligence and attentional problems.[44] One study indicated that children whose mothers had smoked 10 or more cigarettes a day were between 3 and 5 months behind schoolmates in reading, maths and general ability.[45] However other studies have found no direct link between IQ and tobacco smoking with maternal intelligence accounting entirely for the relationship.[46]

Perinatal factors[edit]

There is also evidence that birth complications and other factors around the time of birth (perinatal) can have serious implications on intellectual development.[5] For example, a prolonged period of time without access to oxygen during the delivery can lead to brain damage and mental retardation.[29] Also, low birth weights have been linked to lower intelligence scores later in lives of the children.[5][29] There are two reasons for low birth weight, either premature delivery or the infant’s size is just lower than average for its gestational age; both contribute to intellectual deficits later in life.[5] A meta analysis of low birth weight babies found that there is a significant relationship between low birth weight and impaired cognitive abilities, however the relationship is small and they concluded that although it may not be relevant at an individual level it may instead be relevant at a population level.[47] Other studies have also found that the correlations are relatively small unless the weight is extremely low (less than 1,500 g) - in which case the effects on intellectual development are more severe and often result in mental retardation.[5]

Development of genius[edit]



Intelligence alone is not enough for the development of genius but the pathways and neural connections for divergent thinking are also necessary. Thus the home must encourage creativity. The parents of gifted children tend to supply enriching environments with intellectually and culturally stimulating materials thus increasing the child’s likelihood to engage in creative activities.[48]




Training at an early age reduces synaptic pruning, which helps save neurons.[49]


Early musical training in children is said to improve IQ.[50][51] Schellenberg conducted a study in which children either received music lessons, drama lessons or no lessons and measured their intelligence scores. He found that children in the music group showed a greater overall increase in IQ scores than the children in the other groups.[50] However, a study claimed that musical training improves verbal, but not visual memory.[52] Significant differences in brain structure between musicians and non-musicians have been found. It was shown that there were significant differences in gray matter volume in motor, auditory and visual-spatial regions of the brain. The authors suggest that this could in part be due to the fact that musicians from a young age translate visually perceived musical notes into motor commands whilst listening to the auditory output.[53]

Studies have shown that listening to Mozart before taking an IQ test will improve scores. This is called the Mozart Effect.[54] The Mozart Effect improves spatial-temporal reasoning.[55] For example one study found that college students scores on a spatial abilities test increased by 8-9 points after they had listened to Motzart whereas there was no increase when they listened to relaxation instructions or silence.[56]


Studies have shown that chess requires auditory-verbal-sequential skills, not visuospatial skills.[57] A German study found that Garry Kasparov, a Soviet / Russian former World Chess Champion, regarded by many as the greatest chess player of all time, has an IQ of 135 and an extremely good memory.[57] Similarly a study looking at young Belgian chess experts fond that they have an average IQ of 121, a verbal IQ of 109 and a performance IQ of 129.[58] However a recent study looking at an elite group of young chess players found that intelligence was not a significant factor in chess skill.[59]

One study found that students who were taking a chess class improved mathematical and comprehension performance.[57] Despite this a recent study found that chess did not improve students’ academic or cognitive abilities. At risk students were put into 2 groups, one group was given a chess class once a week for 90 minutes, the other group was not. The results indicated no differences between the groups in changes in math, reading, writing or general intelligence.[60]

See also[edit]


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