Is GPA correlated with socio-economic status

This article covers intelligence as a psychological term for the cognitive performance of people. For the different Intelligence models see intelligence theory; for those also referred to as "intelligence" Social group see the relevant section in the intellectual article.

intelligence (Latin intelligentia 'Insight', 'Understanding', 'Concept', 'Idea', from Latin casual 'Put together', 'catch the eye', 'choose') is a collective term in psychology for the cognitive performance of humans. Since individual cognitive abilities can vary in strength and there is no consensus on how to determine and differentiate them, there is no generally shared definition of intelligence. Rather, the various intelligence theories suggest different operationalizations of the everyday term. This article treats intelligence as Charles Spearmans unless otherwise notedg factor, one of the most famous models of intelligence.

General information on the general factor of intelligence according to Spearman

Spearman's General Factor of Intelligence (g) is one of the most widely used models of intelligence. g is one of the most reliable and most valid constructs in psychology. According to Robert Plomin, g predicts school success and prestige of the occupation achieved later better than any other measurable characteristic.[1] The predictive power of g on variables such as occupational prestige and income of an individual is reduced if one includes the socio-economic status of the parental home as a variable correlated with g. But even in this case, g makes an independent contribution to explaining the variance.[2]

It is undisputed that people are more similar to their relatives in terms of g than randomly selected people. However, it is unclear to what extent biological or social factors are the cause of this similarity. This question is one of the most widely discussed questions in psychology.[3]

While today there is broad consensus that both factors play a role under normal conditions[4][5] there is considerable disagreement about how strong the influence of which factor is.

The camp that assumes a strong heredity of g is called Hereditarians designated. Environmentalists however, represent the thesis that g is strongly conditioned by environmental influences. There is now a plethora of studies on the heredity of g,[6] which, however, are interpreted differently by the various camps. The interpretation is made more difficult by the fact that the heredity of g is not the same under all conditions (see below).

After all, the debate about the hereditary nature of intelligence has not remained free of scandals. Cyril Burt, for example (who, based on studies on identical twins raised separately, which may never have taken place, assumed a heredity of intelligence of 70 to 80 percent) are extremely controversial.[7] and Rick Heber (who, on the basis of an experiment whose existence had serious doubts, assumed that the IQ could be increased by around 35 points through appropriate programs).[8] Although these researchers are considered extremely controversial today, they are still (as of 2009) cited uncritically by other scientists from the relevant camps.[9]

General and differential psychology as well as neuropsychology deal with intelligence.

Intelligence in differential psychology and psychological diagnostics

Differential and personality psychology is the source of much of the research on the construct of intelligence. In this discipline, intelligence is seen as a sub-area of ​​personality in the broader sense. In doing so, efforts are made to avoid the fuzzy terminology that is used in everyday language (thinking, comprehension, rationality, logic, judgment) in order to characterize the mental abilities of humans, to make intelligence measurable and from other constructs of psychological research such as B. To delimit creativity (cf. discriminant validity, test quality criteria).

From the basic discipline of differential psychology, intelligence diagnostics or psychometry emerges as an area of ​​application. Here one tries to determine quantitative differences in intelligence between people. As a technical term in psychometry, “intelligence” was coined around 1900, with the content impulse coming from the French (Alfred Binet) and English-speaking areas (Louis Leon Thurstone, Charles Spearman). Some intelligence tests are adaptive and adapt to the level of difficulty of the subject (so-called adaptive testing).

Intelligence test

→ Main article: Intelligence test

An intelligence test is used to record a person's cognitive abilities. There is a large number of different tests for different target groups and use cases. The result of such a test is often what is known as the intelligence quotient (IQ).

Intelligence tests are based on the assumption that the population's intelligence quotient is normally distributed. The IQ thus describes the deviation from the mean value 100, a standard deviation is 15 IQ points.

Proof of validity of intelligence measurements is a prerequisite for using such results in psychodiagnostic practice.

Intelligence quotient

→ Main article: IQ

1904 established a society for child psychology, the Société Libre pour l’Etude Psychologique de l’Enfant, commissioned by the French government to create a test that could be used to identify mentally handicapped children who no longer benefit from normal schooling. Alfred Binet and Théodore Simon then developed the first IQ test.[10] The IQ was defined as the quotient of intelligence age and age. Later, other researchers (David Wechsler) introduced a new intelligence concept in which the performance of the individual is related to the mean value of the corresponding age group.


→ Main article: Intelligence theory

The author of the first intelligence test, Alfred Binet, saw intelligence as a bundle of numerous individual abilities, even if his test seemed to show that intelligence was something uniform, whole. However, he did not create a more precise structural model.

In the course of time, various explanatory models have emerged, which mainly rely on factor analysis.

Intelligence as a personality trait

Intelligence correlates with a number of other variables. For example, intelligent people are more successful at school than less intelligent people[11] and occupy higher professional positions on average (this applies in particular to men, whose IQ value correlates around 0.7 with the professional prestige, for women the correlation is lower due to child-rearing, among other things).[12] Among students and trainees, the more intelligent perform better than the less intelligent.[13] Above-average talented people generally live healthier lives and have a longer life expectancy.[14]

However, the influence of intelligence on income is limited. It could be shown that, at least in the USA, social origin has a much stronger influence on earnings than intelligence.[13]

However, intelligence also correlates with disease. For example, intelligent people are more likely to be nearsighted.[15][16][17][18] There is also a connection with certain hereditary diseases.

For mental disorders such as schizophrenia, it has been shown that both particularly intelligent and particularly less intelligent people suffer more from it - but that are on average intelligent far less often.[19][20][21]

Biological correlates

It can also be observed that intelligence and brain volume are positively related.[22][23]

Analyzes after death show that educated and intelligent people with more synapses live longer than less educated and less intelligent people. It is not known whether people with more synapses are looking for higher education, education stimulates synapses to grow, or both.[24]

Critique of the intelligence concept of differential psychology

Main Products:Criticism of the concept of intelligence

Intelligence is often criticized as a statistical construct. There is a strong connection between IQ and social class. People from the lower social classes and their children achieve a lower IQ on standardized intelligence tests than people from the upper social classes and their children.[25] It is debated whether this is because traditional intelligence tests are unfair to workers and their children,[26] IQ tests have therefore been criticized as classic.[27] With regular practice, the tasks of intelligence tests can be trained and thus achieve a better result than unprepared people would achieve. This is known as Test intelligence (English: test-wiseness).

Intelligence from the perspective of other disciplines

General Psychology

Research into intelligence in the field of general psychology is now often referred to as cognitive psychology. This in turn draws on methods and findings from brain research, developmental psychology and, increasingly, artificial intelligence.


Neuropsychology also deals with the neural basis of intelligence and the processing of signals and information in humans. The processes in the cerebrum (cf. also cortex) are particularly relevant for intelligence, whereas the cerebellum (Latin: cerebellum) and phylogenetically older areas (e.g. the brain stem) receive less attention in research on the neural basis of intelligence. However, this does not mean that intelligence can be localized in certain areas; despite everything, a certain decentralized organization of information processing cannot be dismissed out of hand.

Computer science

In computer science, the topic is dealt with in the context of research on artificial intelligence (AI). It describes the replication of human intelligence within computer science. AI is increasingly being used in engineering or medical technology. Possible application scenarios are: optimization problems (travel planning, rail traffic), dealing with natural language (automatic language understanding, automatic translation, search engines on the Internet), dealing with natural signals (image understanding and pattern recognition). But AI is also often used in computer games for computer-controlled opponents. But it still does not come close to human intelligence.

The concept of "replicating human intelligence" must not be interpreted too narrowly here: AI is only implemented as a rule-based system (e.g. expert system) (as a set of conditions and derivation rules, i.e. ultimately as a program in which the so-called inference machine itself is looking for the correct order in which the rules are processed). On the other hand, human intelligence can also be viewed as a rule-based system, albeit of a much higher complexity. A clear distinction between artificial and human intelligence is therefore difficult. What the AI ​​certainly lacks is self-confidence (see also Turing test).

The regression to the middle

Francis Galton was the first to determine that the intelligence scores of children against their parents show a "regression to the mean", which means that they are approaching the average value. Children of the gifted and highly gifted are on average not quite as intelligent as their parents, but their intelligence is somewhat lower (although still above average). In contrast, the children of very poorly intelligent people are, on average, not quite as poorly intelligent (although still below average).

Causes of differences in intelligence

Heritage or environment?

There is a broad consensus in psychology these days that both heredity and environmental factors play a role in intelligence development.[28][29]

According to a 2010 review, estimates of the genetic portion of the variance in intelligence range from 30 to 80%. The heritability of intelligence increases with age, from around 30% in early childhood to 70 to 80% in adulthood.[30] This increase in the heritability of intelligence with age could be due to the fact that adults have a greater tendency than children to choose and shape their environment according to their genotype, so that genetic differences are amplified.[31][32]

The intelligence test values ​​in the population roughly follow a normal distribution, with the exception of a slight overhang at the lower end, triggered by severe cognitive disorders. The male intelligence distribution has a slightly higher variance, i.e. a higher proportion of men with extremely high and extremely low intelligence.[30]

Influence of genes

Numerous researchers are of the opinion that intelligence always has a hereditary component, since “the meta-analysis by Bouchard and McGue (1981), which is now to be regarded as classic, the empirical search for the answer to the question of whether general intelligence is hereditary, with an unambiguous 'Yes' ”.[33]

Other authors also point out that the relevant environmental influences can usually not be identified in more detail and that these are environmental aspects that are not shared within the family, i.e. those that affect siblings growing up together in different ways, for example.[34]

“The various approaches to investigating the hereditary nature of intelligence do not produce completely consistent results. The highest heredity estimates of around h (2) = 70% resulted from studies on identical twins raised separately, while both the comparison of identical twins with dizygotic twins and adoption studies produce heredity estimates that are closer to h (2) = 50%, sometimes even below that . There is no doubt, however, that the findings converge to the effect that individual differences in IQ are, to a large extent, genetically determined. Furthermore, the findings converge to the effect that the different quality of the family environment contributes to individual differences in intelligence. The estimates for c (2) fluctuate between 20% and 40%. "

- Peter Borkenau: Plant and environment[35]

Importance of the social environment

Richard Lewontin argues that the opinion that intelligence is largely hereditary and that environmental influences are not to be found within the family came about through incorrect interpretation of adoption studies. If the studies are correctly interpreted, it is obvious that the family environment shared by siblings plays a major role.[36]

Lewontin explains this thesis with a parable:

Imagine having a sack full of wheat grains. Divide this sack in half at random. One half would be sown on fertile soil that is well watered and fertilized. The other half is thrown into a barren field.
If you now look at the first field, you will notice that the ears of wheat are of different sizes. One can trace this back to the genes, because the environment was the same for all ears.
If you look at the second field, you will be able to attribute the variation within the field to the genes.
But it will also be noticeable that there are big differences between the first field and the second field. In the first field the differences are 100% genetic, in the second field the differences are 100% genetic, but that does not mean that the differences between field 1 and field 2 are also genetic.

Lewontin also sees it with the social environment: the IQ differences within a stratum can be genetic to a certain extent, but this would not mean that the differences between two strata also have to be genetic. He cites adoption studies as evidence, for example those by Skodak and Skeels or the Minnesota Transracial Adoption Study.

As an analogy, he also cites body size, which is known to be largely genetic. However, this cause can only be regarded as sufficient within one class, but there is still a difference between different social classes, which is now given as three to four centimeters. With increasing economic prosperity, the size of entire nations also increases.[36]


Borkenau criticizes Lewontin's assessment as inaccurate and generalized:

“[It is] not appropriate to pick the study that implies the lowest or highest heredity estimate, at will, and rate that study as the most conclusive. This is the way [t] [...] Lewontin [...] proceeds. "

- Peter Borkenau: Plant and Environment[35]

Furthermore, Riemann and Spinath point out that the share of heredity in children and adults is different from what Lewontin states:

“Apparently, the environmental conditions shared by family members only have an effect on intelligence until the people leave the family. […] While effects of the shared environment explain a quarter of the variation in intelligence in childhood, these can no longer be demonstrated in adulthood.However, the influences of the specific environment are increasing. "

- Riemann & Spinath: Genetics and Personality[37]

Influence of risk factors in childhood

Influence of risk factors
Risk factors
Average IQ
of the children *
no 119
1 116
2 113
4 93
8 85
* These are IQ values ​​after a
US scale

So-called risk factors such as the parents' drug consumption, poverty or poor mental health of the parenting person can have a significant negative influence on the development of intelligence. One study found that child development is only severely impaired when several risk factors occur at the same time.

The quantitative results of this study are shown in the table on the right.

Another longitudinal study came to the same conclusion. One or two risk factors had very little effect on cognitive development, but when others were added, the effects were strong. Children who were affected by eight to nine risk factors even had an IQ that was on average 30 points lower than children with no stress.[38]

Adoption Studies

Studies on adopted children make it possible to examine the influence of the social environment on a child's intelligence development. Since the adopted children are not related to their parents and siblings, their IQ would have to be independent of that of the adopting family with a purely inherited intelligence. However, if only the socio-economic environment influences the development of intelligence, there should be no significant difference in IQ between the adopted child and his adoptive parents or siblings.

The 1975 started Minnesota Transracial Adoption Study was a methodologically complex, large-scale and detailed documented study on the adoption of children from families of the lower and working class who were adopted by families of the upper middle class. At the beginning of the study, both the adoptive parents and their biological children were tested when the adopted children were 7 years old. The IQ of the birth parents was not recorded, only their education, on the basis of which the average IQ was estimated at around 85 to 90. 10 years later, all candidates who could still be localized were tested again with another test (the results are slightly lower due to the test).

Overall, it becomes clear that the adopted children are very similar in the IQ to their adoptive parents and siblings:

Results of the Minnesota Transracial Adoption Study[39]
Age of children: 7 17
IQ of the adoptive parents: 120 115
Background of children number IQ IQ GPA * Class percentile School achievement percentile
biological children 104 117 109 3,0 64 69
adopted, two white parents 16 112 106 2,8 54 59
adopted one white / black parent 55 109 99 2,2 40 53
adopted, two black parents 21 97 89 2,1 36 42
adopted, asian / indigenous 12 101 96
* The GPA corresponds to the grade point average, whereby 4.0 corresponds to the German 1.0.

Based on the data that were obtained when the candidates were 17 years old, a correlation with the education of the biological mother (but not that of the biological father) could be shown (factor: 0.23). Furthermore, a significant correlation with the age at which the child was given up for adoption was found: the younger it was, the more intelligent it was later (−0.30). While no connection to the adoptive parents' income or education was found, their intelligence quotients were decisive (father: 0.20, mother: 0.18). The “quality of the foster family before adoption” also correlated (0.30).[39] This shows how much a child's intelligence depends on their social environment.

The authors Scarr and Weinberg explain the poorer performance of black children with racial discrimination and the fact that many were only given up for adoption at an older age. Eyferth's study also suggests the great influence of environmental factors on the results. With regard to bi-ethnic children, there is the comparative study carried out in Germany by the psychologist Klaus Eyferth from 1959, who tested the average intelligence of 264 "occupation children", i.e. children of a German mother and white or African American soldiers stationed in Germany determined:

group Boys girl average
know white 101 93 97,2
White black 97 96 96,5
difference 4 −3

The research shows that there was no significant difference between children with two white parents and children with one black and one white parent.[40] While it is often criticized that the black soldiers were not representatives of the Afro-American population because they were possibly intelligent above average due to the selection criteria of the US Army (based on their ethnicity), the test result protocols of the army showed that black soldiers were on average achieved lower IQ values ​​than white soldiers, which also corresponds to the overall picture of the US-American intelligence distribution.[41]In 1980, Flynn drew the conclusion, after extensive analysis, that selection criteria (from the US Army) could not explain this result either, and noted that there must have been substantial differences in intelligence between the white and African-American fathers involved, which, contrary to expectations, had no influence on the intelligence of the two groups of children.[42]

Adoption studies by Clark and Hanisee (1982) and Winick, Meyer and Harris (1975) also suggest the connection between the child's intelligence and his or her social environment: It was shown that Vietnamese and Korean orphans raised by American middle-class families later had an above-average IQ. In the studies, it was on average between 112 and 120 for adequately nourished children. Malnourished children had an average IQ of around 102 to 106. Orphans who were raised by relatives in Vietnam or raised in homes, on the other hand, had a below-average IQ.[43][44]

In a French adoption study it was shown that even through comparatively late adoption, combined with an improvement in the social environment, the IQ of a neglected / abused child who was below average intelligent before adoption can be increased. It was also shown that children who were adopted by families with a high socio-economic status developed a higher level of intelligence (IQ average: 98) than children who were adopted by families with a low socio-economic status (IQ average: 85).[45]

The adoption study by Harold M. Skeels and Skodak is also much cited. These originally examined 181 adoptive children for their IQ. They followed their mental development up to adolescence. At this point in time there were still 100 people in the sample. They came to the following results:

  • Adopted children develop very favorably compared to their birth mothers.
  • Children adopted by middle-class couples have an IQ equal to that of the biological children of this class.
  • The IQ of the birth mother correlates clearly with the IQ of her child. Children born to birth mothers with an IQ of less than 70 had an average IQ of 104 in adolescence. Children born to birth mothers with an IQ of 110 or more had an average IQ of 129 in adolescence. However, it is not certain whether this is due to biological factors, since it was American adoption practice at the time to place the children of the most intelligent mothers in the hands of the richest adoption seekers.
  • Emotional and personal factors play an important role in intellectual development in the adoptive family.[46]

An adoption study carried out by Capron and Duyme came to the conclusion that both genes and the environment play a role. In the vast majority of cases, the biological parents of adopted children come from the poor. Little is known about the adopted children whose birth parents are wealthy. The adoptive parents, on the other hand, mostly come from the upper classes, if only because poor people are rarely allowed to adopt a child. In order to close this knowledge gap, Capron and Duyme were now specifically looking for children with wealthy birth parents and also children with poor adoptive parents. It could be shown that three groups of children achieved an IQ above 100:

  • Children who had both wealthy birth and wealthy adoptive parents,
  • Children who had poor birth parents and wealthy adoptive parents,
  • Children who had wealthy birth parents and poor adoptive parents.

In contrast, achieved an IQ of less than 100:

  • Children who had both poor birth and poor adoptive parents.

The following table shows the results in detail:

poor adoptive parents wealthy adoptive parents
poor birth parentsAverage IQ: 92.4 Average IQ: 103.6
wealthy birth parentsAverage IQ: 107.5 Average IQ: 119.6

It has been criticized, however, that the heredity of the IQ is overestimated, as a majority of adoptive parents are “white, of legal age, wealthy, educated and living in stable marriages”. This means that the adopting families do not have the full spectrum of environmental influences. The environment in these families is usually particularly conducive to IQ development, so that one can only conclude from the small influence of the environment in these studies that it does not matter whether a child is in the educated, wealthy couple A or in the educated , wealthy couple B growing up.[48]

Differences in the influence of the socio-economic environment

In all studies on the subject, class-specific differences in the intelligence of young people were found.[49] However, these are not equally pronounced everywhere: the differences in rural areas are far smaller than those in cities. The exact reasons for this are unknown. It is assumed that lower-class milieus are developing in cities more than in rural areas and that social problems, such as unemployment and drug use, contribute to the inadequate support of the children.[50]

There are no direct studies for Germany. However, the PISA study examined “problem-solving skills”, which is very similar to intelligence. Here, too, it was found that the differences between the classes were smaller in the countryside than in the city. There were strong differences between East and West Germany. In eastern Germany, young people from different social classes are much more similar in terms of their problem-solving skills than in the west. The reasons for this are unclear.[51]

Genetic predisposition

A connection with the development of intelligence has been demonstrated for some genes. However, the influence is relatively small, so that genetic predisposition is by no means sufficient as the sole explanation for differences in intelligence. It is also controversial whether the findings are reproducible.[52]

Intelligence is not determined by a single master gene or just a small group of genes, but is a multi-genetic predisposition. With the help of the SNP microarray technique, a total of 47 gene segments were identified that correlated with the development of intelligence. However, none of these gene variants contribute more than 0.4 percent to intelligence, the six most influential gene variants taken together only contribute a little more than one percent to the development of the intelligence of an individual. Since a person's intelligence is closely linked to the brain and at least half of the genome contributes to its individual structure, the researchers suspect a large number of other genes.[53] Other scientists found slightly more influential gene variants. This could explain up to 3% of the IQ.[54] A master gene could not be found.

A correlation could be demonstrated for the following genes, among others:

  • One of six variants of the gene DTNBP1, responsible for the formation of the proteinDysbindin-1seems to lower the intelligence. At the same time, there is also likely to be a link with schizophrenia. The mean IQ of people with this gene variant is 3 points below the mean for the entire population.[55][56][57]
  • A certain variant of the receptor gene for the insulin-like growth hormone IGF-2 seems to occur in gifted children with 50% about twice as often as in normally gifted children (25%). However, this only explains a difference in IQ of around 4 points. Compared to others, this variant is associated with more frequent myopia and the occurrence of allergies as well as tall stature and a slim body shape.[58][59]
  • For some variations of the COMT gene, a connection with slightly increased intelligence could be demonstrated. At the same time, it is also linked to schizophrenia.[60][61]
  • Polymorphisms in the interleukin-1β gene may also have an influence on intelligence: According to a study, test subjects with the CC genotype are more intelligent than the rest of the population.[62]
  • The CHRM2 gene is currently the favored gene when it comes to the influence of genes on intelligence and school performance. It was found that some variations in the CHRM2 gene lead to increased intelligence. It was also found that these variations of the gene have a positive influence on the level of education achieved. However, the influence of each individual variation is very small.[63] However, the cumulative impact of all the variations in this gene taken together could be far greater.[64]
  • We are currently investigating the two cases where a person has all or all of the intelligence-enhancing or all intelligence-impairing variations in the CHRM2 gene. It is believed that in this rare case there would be significant IQ differences. Presumably, a person with all intelligence-enhancing variations would outperform a person with all intelligence-impairing variations by 15 to 20 IQ points. However, people who only have intelligence-enhancing versions or only intelligence-impairing versions of the gene are extremely rare, and the statements about this small group of people have so far been pure speculation.[64]

Certain forms of intellectual disability are genetic. These include Down syndrome, fragile X syndrome and phenylketonuria.

Socio-economic environment

Social class

The social class also has an influence on intelligence. This shows that intelligence tests that make high linguistic requirements determine a greater connection with intelligence than intelligence tests that only have low verbal requirements. Such tests are known as culture fair tests.[65]

Turkheimer has pointed out that social class plays a major role in the inheritance of intelligence. While intelligence in the middle class is largely hereditary, it is not so in the lower class. Twin studies have so far been carried out mainly in the middle and upper classes and have thus led to a result that does not take into account the greater importance of the environment in the lower classes. The poor environmental conditions in the lower class meant that the children could not develop their genetically given potential. On a scale from 0.00 to 1.00, the IQ in the middle class is 0.72 determined by genes, but only 0.10 in the lower class, according to Turkheimer.[66]


Hertzig, Birch, Richardson and Tizard found in 1972 that malnutrition in early childhood has serious consequences for the development of intelligence and social behavior in children. They examined children who were hospitalized for malnutrition and then returned to their families. Her average IQ was 58.[67] Clark and Hanisee studied the lives of children adopted from developing countries who were malnourished and had traumatic childhood experiences. The children were adopted by upper-middle-class American families. Contrary to the assumption that these children would suffer from severe impairments, they turned out to be above average intelligent and above average socially competent. They achieved an IQ of 120 in the Peabody Picture Vocabulary Test, and an average of 137 points on the Vineland Social Maturity Scale. 100 points are considered to be average, 137 as extremely good. Clark and Hanisee came to the conclusion that malnourished and traumatized children prove to be surprisingly resilient when they are adopted into stable family relationships.[43] Winick, Meyer, and Harris studied Korean adopted children who were adopted by American couples under the age of three. They divided the children into three groups: one severely malnourished, borderline and one adequately nourished. The severely malnourished group achieved an IQ of 102, the borderline cases an IQ of 106 and 112 for the children who were not malnourished. Winick Meyer and Harris concluded that malnutrition in early childhood has a detrimental effect on IQ development, but in no way condemns to a life with intellectual disabilities. If they are adopted by the age of three at the latest, even severely malnourished children will achieve a normal IQ. The above-average IQ values ​​of adequately nourished children can probably be explained by the adoptive families.Families that are allowed to adopt a child usually have a high socio-economic status and can offer the children particularly good living conditions.[44]

It has long been known that iodine deficiency can lead to intellectual deficiency in pregnancy or early childhood. A meta-analysis from 10 different clinical studies showed that chronic iodine deficiency led to a mean IQ reduction of 13.5 points.[68] Studies from all parts of the world have shown that chronic iodine deficiency leads to a reduction in intelligence in children.[69][70][71] Iodine deficiency is considered to be the world's greatest single cause of preventable brain damage and mental retardation (the world's largest single cause of preventable brain damage and intellectual disability).[72]


Breastfeeding has a positive influence on intelligence development. It increases IQ by seven points, but apparently only if the child has a certain version of the FADS2 gene.[73] Other studies also found a positive influence of breastfeeding on intelligence.[74][75]


Initially, this does not rule out that upbringing is one of these environmental influences, since research on parenting styles shows that the same parents raise their individual children differently. Proponents of the heritage standpoint, however, interpret this in such a way that parents of several children react differently to different genetically determined temperaments of their different children (cf. reciprocal interactionism).

The complexity of the interaction between genetic material and the environment was evident early on from experiments on the inheritance of learning achievements, which are now classified as classic. Rats were initially exposed to so-called disruptive selection pressure with the aim of changing their learning performance when crossing a maze.[76] Over seven generations - while maintaining the same keeping conditions - on the one hand only the offspring of those mothers who learned to cross the labyrinth particularly quickly in the breeding line of the "clever" rats were bred. At the same time, in a second breeding line, starting from the same initial population, the offspring of those mothers who learned to cross the labyrinth particularly slowly were bred. Finally, it was possible to demonstrate statistically significant differences between the test animals of the two breeding lines: As a result of the removal of the unsuitable test animals from the breeding program, changes in the learning ability had resulted which could only be explained by a change in the gene pool of the two breeding lines; the rats' ability to learn has a genetic basis. Robert Rosenthal, however, considered a different explanation: He argued that it may have been a so-called experimenter effect. In an experiment, he analyzed the behavior of researchers who allegedly had to test “clever” and “stupid” rats. The result was that the test animals selected purely at random showed strong differences in the extent of their behavior expected by the investigator. Rosenthal attributed this to an unconscious stronger affection for the supposedly smarter rats.[77]

A few years later, a further study on animals of such “intelligent” or “unintelligent” breeding lines revealed that the genes only determine the rats' learning behavior under certain environmental conditions.[78] Test animals from a slowly learning breed line were raised and kept in cages that were particularly varied with tunnels, slides and toys; Conversely, test animals from the fast-learning breed line were housed in a particularly low-irritation environment: under these changed environmental conditions, no difference between the two breeding lines was detectable. With an unchanged gene pool in each of the two breeding lines, this is evidence that the environment has a decisive influence on learning performance in the labyrinth. The authors of the study therefore argued that only the interaction of heritage and environment produces visible behavior and a separation into innate and acquired ultimately neither useful nor possible.

Parenting style

Longitudinal studies show that there are clear differences in intelligence between children whose parents value intellectual performance and children of parents who do not. The first group of children were more intelligent. Another study shows that the children of parents who displayed a warm and democratic upbringing behavior were more intelligent than children of parents who behaved in an authoritarian and punitive manner.[79]

Language environment

The language environment also plays an important role and is closely correlated with the social status of the parents. One study found that parents from the middle and upper classes spoke to their children much more frequently and significantly more than those from the lower classes, and that they formed more complex sentences. According to the authors, this has an enormous influence on the development of intelligence; the IQ of the disadvantaged children averaged 79, while the socially well-off children who were talked to a lot averaged 117.[80]


According to René A. Spitz, neglect in early childhood can lead to hospitalism. This is characterized, among other things, by mental retardation and a low IQ. However, hospitalism is curable if the child is lovingly cared for later.[81] Interesting in this context are Harry Harlow's experiments with young rhesus monkeys and Harold M. Skeels' research on the development of intelligence in people who grew up in homes.


This article or section needs to be revised. Details are given on the disc. Help improve it, and then remove Template: SSend this mark.
Children from poor backgrounds are often less intelligent, have poorer language skills and develop poorer social skills. In Germany, for example, 13.8 percent of all poor children are impaired in their intellectual development, but only 0.8% among the rich.[82] Overall, the IQ of poor children is 6 to 13 points below the average IQ.[83]

80–90% of the children in schools for people with learning disabilities come from a poor environment. According to Schlack, the insecure professional and financial situation of the parents, poor living conditions, life in socially disadvantaged areas, incomplete families, restricted and one-sided suggestions and social isolation contribute to the fact that the needs of children cannot be satisfied in this living environment. This leads to the fact that they cannot reach their intellectual potential.[84]

Malnutrition and malnutrition may play a role in this. Providing healthy nutrition to young children can reduce the impact of poverty on IQ.[85]

But there are exceptions to the rule that poverty leads to low intelligence. For example, the Oakland Growth and Berkeley Guidance Studies did not show any significant effects of poverty among working-class and middle-class boys. Poor middle-class boys had an average IQ of 115.9 and poor upper-class boys had an average IQ of 113.1.[86] The results of these studies, which deal with individuals who were born in California between 1920 and 1929, cannot be fully applied to the present day. These are also boys who, despite being poor, grew up in a relatively good social environment.[87]

Growth hormones

The more growth hormone IGF-1 (not to be confused with IGF-2) a person has in their blood, the higher their intelligence tends to be. One possible explanation could be that the hormone makes the brain grow. A number of things affect IGF-1 levels, according to David Gunnell of the University of Bristol. For example, premature babies usually have lower levels of the hormone. This would also explain why prematurely born children often have intelligence deficits. The older the child's mother is at the time of pregnancy, the higher the level of the hormone in question, and in fact, it has been shown that older mothers tend to have more intelligent children. It may be possible to boost the intelligence of children with low IGF-1 levels through targeted administration of the hormone. But you also have to consider the risks: The hormone can potentially lead to cancer. High IGF-1 levels have also been linked to schizophrenia.[88]

type of school

In Germany, due to the multi-tier school system, studies are available on the question of whether, given the same entry requirements, the intelligence performance increases more by attending grammar school than by attending secondary or secondary school. The Max Planck Institute for Human Development carried out a study on this (the BIJU). Strong effects could be proven: When checking the output power in the intelligence test in class 7[89] the pupils who attended the grammar school were able to increase their intelligence performance by 11.39 points more than the pupils who attended the secondary school.[90]

In America, a study of 1,450 schools showed a connection between the qualifications of teachers and the IQ of the children they teach. Even after controlling the effects of other factors (such as poverty), being taught by an unskilled teacher was correlated with lower IQ scores.[91]

Threat from stereotypes

Main Products:Threat from stereotypes

Baby with facial features typical of fetal alcohol syndrome (small eyes, smooth philtrum, narrow upper lip), which is often associated with intellectual disability

A stereotype threat is when a person believes they belong to an underperforming group and therefore fails an IQ test. The phenomenon could be proven for women, for example: Steele let male and female students take part in a test of math skills. Shortly before the test, half of the sample were told that there were usually large gender differences in this test. In fact, women now fared significantly worse than men. The other half of the sample did not receive this information. No significant gender differences could be shown here.[92] Other groups can also be threatened by stereotypes - such as members of ethnic minorities or members of the lower social classes. Sometimes a stereotype threat can also be demonstrated for entire nations. Harold Stevenson was able to prove that Americans were more threatened by stereotypes than members of Asian nations and therefore performed poorly on tests of mathematical skills.[93]


Alcohol consumption by the mother during pregnancy can cause fetal alcohol syndrome (also called FAS or alcohol embryopathy). This is often associated with a low IQ. Fetal alcohol syndrome is the most common type of intellectual disability that is not genetic. And: It is 100% avoidable.

Scientists believe that pollution, especially lead, has negative consequences for the development of intelligence.[94][95]

IQ differences between generations

In industrialized countries, increases in average IQ test scores were observed until the 1990s. This phenomenon is known as the Flynn effect. The IQ has stagnated or decreased in some industrialized countries since the 1990s.

IQ differences between ethnic groups and castes

IQ differences in the US

The intelligence debate in the US is characterized by two contradicting research findings:

  • Ethnic groups differ in terms of their average IQ
  • Groups that score high on intelligence tests are on average more successful in school and have a higher average income[96]