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 80% in old age. Thus the environment and its interaction with
genes account for a high proportion of the variation in intelligence
seen in groups of young children, and for a small proportion of the
variation observed in groups of mature adults. 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.
Neurological theory
As
babies, our neuronal connections are completely undifferentiated.
Neurons make connections with neighboring neurons, and these become more
complex and more idiosyncratic as the child ages, up until the age of
16, when this process halts. This is also the time frame for development
of what is defined in psychometric studies as the general factor
of intelligence, or g, as measured by IQ tests. A person's IQ is
supposed to be relatively stable after they have reached maturity.
It is likely that the growth in neuronal connections is largely due to
an interaction with the environment, as there is not even enough genetic
material to code for all the possible neural connections. Even if there
was enough genetic material to code neural connections, it is unlikely
that they could produce such fine tuned connections. In contrast the
environment causes meaningful processing as the neurons adapt to stimuli
presented.
The capacity of the brain to adapt its connections to
environmental stimuli diminishes over time, and therefore it would
follow that there is a critical period for intellectual development as well. While the critical period for the visual cortex
ends in early childhood, other cortical areas and abilities have a
critical period that lasts up through maturity (age 16), the same time
frame for the development of fluid intelligence. In order for a person
to develop certain intellectual abilities, they need to be provided with
the appropriate environmental stimuli during childhood, before the
critical period for adapting their neuronal connections ends.
The existence of a critical period of language development is well
established. A case illustrating this critical period is that of E.M., a
young man who was born profoundly deaf and did not have any interaction
with the deaf community. At the age of 15 he was fitted with hearing
aids and taught Spanish; however, after 4 years he still had severe
difficulties in verbal comprehension and production.
Some researchers believe that the critical period effect is a
result of the manner by which intellectual abilities are acquired—that
changes in neuronal connections inhibit or prevent possible future
changes. However, the critical period is observed at approximately the
same age in all people, no matter what level of intellectual ability is
achieved.
Environmental influence
Sociocultural
Family
Having
access to resources of the home, and having a home life conducive to
learning is definitely associated with scores on intelligence tests.
However, it is difficult to disentangle possible genetic factors from a
parent's attitude or use of language, for example.
A child's ordinal position in their family has also been shown to
affect intelligence. A number of studies have indicated that as birth
order increases IQ decreases with first borns having especially superior
intelligence. Many explanations for this have been proposed but the
most widely accepted idea is that first borns receive more attention and
resources from parents and are expected to focus on task achievement,
whereas later borns are more focused on sociability.
The type and amount of praise received from family can also affect how intelligence develops. Research by Dweck
and colleagues suggests that feedback to a child on their academic
achievements can alter 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.
Children holding the entity theory of ability have been reported as
performing less well after a failure, perhaps because they believe that
failure on a task indicates that they are not intelligent, and that
therefore there is no point in trying to challenge themselves after
failure.
Dweck contrasts this with incremental theory beliefs – 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.
Peer group
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, but relatively few studies have examined the effect on tests of cognitive ability.
The peer group an individual identifies with can also influence intelligence through the stereotypes associated with that group. The stereotype threat, first introduced by Claude Steele,
is the idea that people belonging to a stereotyped group may perform
poorly in a situation where the stereotype is relevant. This has been
shown to be a factor in differences in intelligence test scores between
different ethnic groups, men and women, people of low and high social status
and young and old participants. For example, females who were told that
women are worse at chess than men, performed worse in a game of chess
than females who were not told this.
Education
IQ and educational attainment are strongly correlated (estimates range form .40 to over .60.) There is controversy, however, as to whether education affects intelligence – it may be both a dependent and independent variable with regard to IQ.
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.
Those who do better on childhood intelligence tests tend to have a
lower drop out rate, and complete more years of school and are
predictive of school success. For instance, one of the largest ever studies found a correlation of 0.81 between the general intelligence or g-factor and GCSE results. On the other hand, education has been shown to improve performance on intelligence tests.
Research controlling for childhood IQ and treating years of education
as a causal variable suggests that education causes an increase in total
IQ score, although general intelligence was not affected.
For instance a natural experiment in Norway where the school
leaving age was changed suggested that IQ was raised by additional year
of school. School may alter specific knowledge, rather than general ability or biological speed.
In terms of what matters about school, it appears that simple quantity
or years-in-school may be what underpins the linkage of education with
performance on IQ tests.
Training and interventions
Research
on the effectiveness of interventions, and the degree to which fluid
intelligence can be increased, especially after age 16, is somewhat
controversial. Fluid intelligence is typically thought of as something
more innate, and defined as immutable after maturity. One recent article
however, demonstrates that, at least for a period of time, fluid
intelligence can be increased through training in increasing an adult's working memory capacity. Working memory capacity is defined as the ability to remember something temporarily, like remembering a phone number just long enough to dial it.
In an experiment, groups of adults were first assessed using standard tests for fluid intelligence. 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.
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. 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.
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 students 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.
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 known 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.
Environmental enrichment
Environmental enrichment
affects cognition and intellectual development from a neurobiological
perspective. More stimulating environments can increase the number of
synapses in the brain which increases synaptic activity. In humans this
is most likely to occur during the development of the brain but can also
occur in adults. Most of the research on environmental enrichment has
been carried out on non human animals.
In an experiment, four different habitats were set up to test how
environmental enrichment or relative impoverishment affected rats'
performance on various measures of intelligent behavior. First, rats
were isolated, each to its own cage. In a second condition, the rats
were still in isolation, but this time they had some toy, or enriching
object in the cage with them. The third condition placed the rats in
cages with each other, so they were receiving social enrichment, without
any enriching object. The fourth and final condition exposed the rats
to both social interaction and some form of object enrichment.
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.
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. 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 fewer signs of cognitive aging and that
stimulating environments could be used in the treatment of cognitive aging dysfunctions such as dementia.
Biological influences
Nutrition
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. It has been shown that under-nutrition, particularly protein malnutrition, can lead to irregular brain maturation and learning disabilities.
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.
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.
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.
Post-natal malnutrition
can also have a significant influence on intellectual development. This
relationship has been harder to establish because the issue of
malnutrition is often conflated with socioeconomic
issues. However, it has been demonstrated in a few studies where
pre-schoolers in two Guatemalan villages (where undernourishment is
common) were given protein nutrition supplements for several years, and
even in the lowest socioeconomic class, those children showed an increase in performance on intelligence tests, relative to controls with no dietary supplement.
Malnutrition has been shown to affect organizational processes of the brain such as neurogenesis, synaptic pruning, cell migration and cellular differentiation. This thus results in abnormalities in the formation of neural circuits and the development of neurotransmitter systems. However, some of these effects of malnutrition have been shown to be improved upon with a good diet and environment. Early nutrition can also affect brain structures that are actually correlated to IQ levels. Specifically, the caudate nucleus is particularly affected by early environmental factors
and its volume correlates with IQ. In an experiment by Isaacs et al.,
infants born prematurely were either assigned a standard or
high-nutrient diet during the weeks directly after birth. When the
individuals were assessed later in adolescence, it was found that the
high-nutrient group had significantly larger caudate volumes and scored
significantly higher on verbal IQ tests. This study also found that the
extent to which the caudate volume size related selectively to verbal IQ
was much greater in male participants, and not very significant in
females. This may help explain the finding in other earlier research
that the effects of early diet on intelligence are more predominant in
males.
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. 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.
Breast feeding
has long been purported to supply important nutrients to infants and
has been correlated with increased cognitive gains later in childhood. The link between intelligence and breast feeding has even been shown to persist into adulthood.
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.
Other studies have indicated that breast feeding may be particularly important for children born Small for Gestational Age
(SGA). A study by Slykerman et al. found that there was no association
between breast feeding and higher intelligence in their full sample but
that when looking only at SGA babies there was a significant increase in
intelligence for those who had been breastfed over those who had not.
A 2007 study provides a possible resolution to the different
results found across studies investigating breastfeeding's effect on
intelligence. Caspi et al. found that whether breast feeding increased IQ was linked to whether the infant had a certain variant of the FADS2 gene.
Children with the C variant of the gene showed an IQ advantage of 7
points when breastfed, whereas those with the GG variant showed no IQ
advantages with breastfeeding. However, other studies have failed to replicate this result.
Stress
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.
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.
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
effects as experiencing childhood maltreatment or trauma.
Maternal age
Maternal
age has been shown to be related to intelligence with younger mothers
tending have children of lower intelligence than older mothers.
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.
Exposure to toxic chemicals and other substances
Lead exposure has been proven to have significant effects on the intellectual development of a child. 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. 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.
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.
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.
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.
However, more recent studies have found that low to moderate alcohol
consumption is not associated with children's intelligence scores.
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.
In another study, prenatal drug exposure was shown to have
significantly negative effects on cognitive functioning, as measured at
the age of five, compared again controls matched for socioeconomic
status and inner-city environment. The researchers concluded that
prenatally drug-exposed children are at greater risk for learning difficulties
and attention problems in school, and therefore should be the subject
of interventions to support educational success. It could be
hypothesized that the effect of these drugs on the development of the
brain prenatally, and axon guidance could be the root of the negative consequences on later deficits in intellectual development.
Specifically, prenatal exposure to marijuana affects development of intelligence later in childhood, in a nonlinear fashion, with the degree of exposure. 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.
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.
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.
Exposure to tobacco smoking has been associated with diminished intelligence and attentional problems.
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.
However, other studies have found no direct link between IQ and
tobacco smoking with maternal intelligence accounting entirely for the
relationship.
Perinatal factors
There
is also evidence that birth complications and other factors around the
time of birth (perinatal) can have serious implications on intellectual
development. For example, a prolonged period of time without access to oxygen during the delivery can lead to brain damage and mental retardation. Also, low birth weights have been linked to lower intelligence scores later in lives of the children. 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.
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.
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.
Development of genius
It
has been hypothesized that the development of genius in an area results
from early environmental exposure to the topic in which the "genius"
has prodigious knowledge or skill. This is utilizing the definition of
genius that is not just a significantly higher than average IQ score,
but also having some type of exceptional understanding or ability in a
specific field. Einstein is often used as an example of genius;
he did not demonstrate generalized exceptional intelligence as a child;
however, there is evidence that he started exploring the ideas of physics and the universe at a young age.
This fits with the model of development of fluid intelligence
before age of maturity because the neuronal connections are still being
made in childhood. The idea is if you expose a child to concepts of, for
example theoretical physics,
before their brain stops responding to the environment in a plastic
way, then you get exceptional understanding of that field in adulthood,
because there was a framework developed for it in early childhood.
However, Garlick proposes that early environmental experience with
their field of genius, is necessary but not sufficient to the
development of genius.
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.
There are many environmental influences on intelligence, typically divided into biological
and non-biological factors, often involving social or cultural factors.
The commonality between these two divisions is the exposure in early
childhood. It seems as though exposure to these various positive or
negative influences on intelligence levels needs to happen early on in
the development of the brain, before the neuronal connections have
ceased forming.
Parents of gifted children also tend to have above average
educational achievement and at least one tends to work in an
intellectual profession. There is also evidence that the probability of a
gifted child becoming a genius may be increased if the child has had to
face adversity or trauma and that a traditional upbringing may
encourage conformity and discourage the necessary divergent thinking.
Training
Training at an early age reduces synaptic pruning, which helps save neurons.
Musical
Early musical training in children is said to improve IQ.
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. However, a study claimed that musical training improves verbal, but not visual memory.
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
because musicians from a young age translate visually perceived musical
notes into motor commands whilst listening to the auditory output.
Studies have shown that listening to Mozart before taking an IQ test will improve scores. This is called the Mozart Effect. The Mozart Effect improves spatial-temporal reasoning.
For example, one study found that college students scores on a spatial
abilities test increased by 8-9 points after they had listened to Mozart
whereas there was no increase when they listened to relaxation
instructions or silence.
Chess
Studies have shown that chess requires auditory-verbal-sequential skills, not visuospatial skills. 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.
Similarly, a study looking at young Belgian chess experts found that
they have an average IQ of 121, a verbal IQ of 109 and a performance IQ
of 129.
However, a recent study looking at an elite group of young chess
players found that intelligence was not a significant factor in chess
skill.
One study found that students who were taking a chess class improved mathematical and comprehension performance.
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.
