The relationship between fertility and intelligence has been investigated in many
demographic
Demography () is the statistical study of populations, especially human beings.
Demographic analysis examines and measures the dimensions and dynamics of populations; it can cover whole societies or groups defined by criteria such as edu ...
studies. There is evidence that, on a population level, measures of intelligence such as
educational attainment Educational attainment is a term commonly used by statisticians to refer to the highest degree of education an individual has completed as defined by the US Census Bureau Glossary.
See also
*Academic achievement
*Academic degree
*Bachelor's degree ...
and
literacy
Literacy in its broadest sense describes "particular ways of thinking about and doing reading and writing" with the purpose of understanding or expressing thoughts or ideas in written form in some specific context of use. In other words, huma ...
are negatively correlated with
fertility
Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertili ...
rate in some contexts.
However, genetic studies have shown no evidence for
dysgenic
Dysgenics (also known as cacogenics) is the decrease in prevalence of traits deemed to be either socially desirable or well adapted to their environment due to selective pressure disfavoring the reproduction of those traits.
The adjective "dysgeni ...
effects in human populations. Theories about dysgenic and
eugenic
Eugenics ( ; ) is a fringe set of beliefs and practices that aim to improve the genetic quality of a human population. Historically, eugenicists have attempted to alter human gene pools by excluding people and groups judged to be inferior or ...
effects in human populations have historically been associated with
scientific racism
Scientific racism, sometimes termed biological racism, is the pseudoscience, pseudoscientific belief that empirical evidence exists to support or justify racism (racial discrimination), racial inferiority, or racial superiority.. "Few tragedies ...
.
Early views and research
The negative
correlation
In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...
between fertility and intelligence (as measured by
IQ) has been argued to have existed in many parts of the world. Early studies, however, were "superficial and illusory" and not clearly supported by the limited data they collected.
Some of the first studies into the subject were carried out on individuals living before the advent of IQ testing, in the late 19th century, by looking at the fertility of men listed in
Who's Who
''Who's Who'' (or ''Who is Who'') is the title of a number of reference publications, generally containing concise biography, biographical information on the prominent people of a country. The title has been adopted as an expression meaning a gr ...
, these individuals being presumably of high intelligence. These men, taken as a whole, had few children, implying a correlation.
More rigorous studies carried out on Americans alive after the
Second World War
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
returned different results suggesting a slight positive correlation with respect to intelligence. The findings from these investigations were consistent enough for Osborn and Bajema, writing as late as 1972, to conclude that fertility patterns were
eugenic
Eugenics ( ; ) is a fringe set of beliefs and practices that aim to improve the genetic quality of a human population. Historically, eugenicists have attempted to alter human gene pools by excluding people and groups judged to be inferior or ...
, and that "the reproductive trend toward an increase in the frequency of genes associated with higher IQ... will probably continue in the foreseeable future in the United States and will be found also in other industrial welfare-state democracies."
Several reviewers considered the findings premature, arguing that the samples were nationally unrepresentative, generally being confined to white people born between 1910 and 1940 in the Great Lakes States. Other researchers began to report a negative correlation in the 1960s after two decades of neutral or positive fertility.
In 1982,
Daniel R. Vining, Jr. sought to address these issues in a large study on the fertility of over 10,000 individuals throughout the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, who were then aged 25 to 34. The average fertility in his study was correlated at −0.86 with
IQ for white women and −0.96 for black women. Vining argued that this indicated a drop in the
genotypic
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
average IQ of 1.6 points per generation for the white population, and 2.4 points per generation for the black population.
In considering these results along with those from earlier researchers, Vining wrote that "in periods of rising birth rates, persons with higher intelligence tend to have fertility equal to, if not exceeding, that of the population as a whole," but, "The recent decline in fertility thus seems to have restored the dysgenic trend observed for a comparable period of falling fertility between 1850 and 1940." To address the concern that the fertility of this sample could not be considered complete, Vining carried out a follow-up study for the same sample 18 years later, reporting the same, though slightly decreased, negative correlation between IQ and fertility.
Critics note Vining's involvement with the
white supremacist
White supremacy or white supremacism is the belief that white people are superior to those of other Race (human classification), races and thus should dominate them. The belief favors the maintenance and defense of any Power (social and polit ...
journal ''
Mankind Quarterly
''Mankind Quarterly'' is a peer-reviewed journal that has been described as a "cornerstone of the scientific racism establishment", a "white supremacist journal", and "a pseudo-scholarly outlet for promoting racial inequality". It covers phys ...
'' and his acceptance of grants from the
Pioneer Fund
Pioneer Fund is an American non-profit foundation established in 1937 "to advance the scientific study of heredity and human differences". The organization has been described as racist and white supremacist in nature. One of its first projects w ...
.
Later research
In a 1988 study, Retherford and Sewell examined the association between the measured intelligence and fertility of over 9,000 high school graduates in Wisconsin in 1957, and confirmed the inverse relationship between IQ and fertility for both sexes, but much more so for females. If children had, on average, the same IQ as their parents, IQ would decline by .81 points per generation. Taking .71 for the additive heritability of IQ as given by Jinks and Fulker, they calculated a dysgenic decline of .57 IQ points per generation.
Another way of checking the negative relationship between IQ and fertility is to consider the relationship which educational attainment has to fertility, since
education
Education is a purposeful activity directed at achieving certain aims, such as transmitting knowledge or fostering skills and character traits. These aims may include the development of understanding, rationality, kindness, and honesty. Va ...
is known to be a reasonable proxy for IQ, correlating with IQ at .55; in a 1999 study examining the relationship between IQ and education in a large national sample, David Rowe and others found not only that achieved education had a high
heritability
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
(.68) and that half of the variance in education was explained by an underlying genetic component shared by IQ,
education
Education is a purposeful activity directed at achieving certain aims, such as transmitting knowledge or fostering skills and character traits. These aims may include the development of understanding, rationality, kindness, and honesty. Va ...
, and
SES
SES, S.E.S., Ses and similar variants can refere to:
Business and economics
* Socioeconomic status
* Scottish Economic Society, a learned society in Scotland
* SES, callsign of the TV station SES/RTS (Mount Gambier, South Australia)
* SES S.A., ...
. One study investigating fertility and education carried out in 1991 found that
high school
A secondary school describes an institution that provides secondary education and also usually includes the building where this takes place. Some secondary schools provide both '' lower secondary education'' (ages 11 to 14) and ''upper seconda ...
dropouts in the United States had the most children (2.5 on average), with high school graduates having fewer children, and college graduates having the fewest children (1.56 on average).
''
The Bell Curve
''The Bell Curve: Intelligence and Class Structure in American Life'' is a 1994 book by psychologist Richard J. Herrnstein and political scientist Charles Murray, in which the authors argue that human intelligence is substantially influenced by b ...
'' (1994) argued that the average
genotypic
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
IQ of the United States was declining due to both dysgenetic fertility and
large scale immigration of groups with low average IQ.
Controversial psychologist
Richard Lynn
Richard Lynn (born 20 February 1930) is a controversial English psychologist and author. He is a former professor emeritus of psychology at Ulster University, having had the title withdrawn by the university in 2018. He is former assistant edit ...
has been a strong advocate of dysgenic theories. In a 1999 study
Richard Lynn
Richard Lynn (born 20 February 1930) is a controversial English psychologist and author. He is a former professor emeritus of psychology at Ulster University, having had the title withdrawn by the university in 2018. He is former assistant edit ...
examined the relationship between the intelligence of adults aged 40 and above and their numbers of children and their siblings. Data was collected from a 1994
National Opinion Research Center
NORC at the University of Chicago is one of the largest independent social research organizations in the United States. Established in 1941 as the National Opinion Research Center, its corporate headquarters is located in downtown Chicago, with o ...
survey among a representative sample of 2992 English-speaking individuals aged 18 years. He found negative correlations between the intelligence of American adults and the number of children and siblings that they had, but only for females. He also reported that there was virtually no correlation between women's intelligence and the number of children they considered ideal.
In 2004 Lynn and Marian Van Court attempted a straightforward replication of Vining's work. Their study returned similar results, with the genotypic decline measuring at 0.9 IQ points per generation for the total sample and 0.75 IQ points for whites only.
However, Lynn's research has been widely criticized as
pseudo-scientific
Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claim ...
and having poor research practices, as well as for promoting
scientific racism
Scientific racism, sometimes termed biological racism, is the pseudoscience, pseudoscientific belief that empirical evidence exists to support or justify racism (racial discrimination), racial inferiority, or racial superiority.. "Few tragedies ...
and
white supremacy
White supremacy or white supremacism is the belief that white people are superior to those of other races and thus should dominate them. The belief favors the maintenance and defense of any power and privilege held by white people. White su ...
, particularly with regards to Lynn's status as
editor-in-chief
An editor-in-chief (EIC), also known as lead editor or chief editor, is a publication's editorial leader who has final responsibility for its operations and policies.
The highest-ranking editor of a publication may also be titled editor, managing ...
of the journal
Mankind Quarterly
''Mankind Quarterly'' is a peer-reviewed journal that has been described as a "cornerstone of the scientific racism establishment", a "white supremacist journal", and "a pseudo-scholarly outlet for promoting racial inequality". It covers phys ...
culminating in the revocation of his title as professor
emeritus
''Emeritus'' (; female: ''emerita'') is an adjective used to designate a retired chair, professor, pastor, bishop, pope, director, president, prime minister, rabbi, emperor, or other person who has been "permitted to retain as an honorary title ...
of Psychology at
Ulster University
sco, Ulstèr Universitie
, image = Ulster University coat of arms.png
, caption =
, motto_lang =
, mottoeng =
, latin_name = Universitas Ulidiae
, established = 1865 – Magee College 1953 - Magee Un ...
in 2018.
Boutwell et al. (2013) reported a strong negative association between county-level IQ and county-level fertility rates in the United States.
A 2014 study by
Satoshi Kanazawa
Satoshi Kanazawa (born 16 November 1962) is an American-born British evolutionary psychologist and writer. He is currently Reader in Management at the London School of Economics. Kanazawa's comments and research on race and intelligence, healt ...
using data from the
National Child Development Study
The National Child Development Study (NCDS) is a continuing, multi-disciplinary longitudinal study which follows the lives of 17,415 people born in England, Scotland and Wales from 17,205 women during the week of 3–9 March 1958. The results from ...
found that more intelligent women and men were more likely to want to be childless, but that only more intelligent women – not men – were more likely to actually be childless.
International research
![Total Fertility Rate Map by Country](https://upload.wikimedia.org/wikipedia/commons/d/d7/Total_Fertility_Rate_Map_by_Country.svg)
Although much of the research into intelligence and fertility has been restricted to individuals within a single nation (usually the United States), Steven Shatz (2008) extended the research internationally; he finds that "There is a strong tendency for countries with lower national IQ scores to have higher fertility rates and for countries with higher
national IQ scores to have lower fertility rates."
Lynn and Harvey (2008) found a correlation of −0.73 between national IQ and fertility. They estimated that the effect had been "a decline in the world's genotypic IQ of 0.86 IQ points for the years 1950–2000. A further decline of 1.28 IQ points in the world's
genotypic
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
IQ is projected for the years 2000–2050." In the first period this effect had been compensated for by the
Flynn effect
The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially standa ...
causing a rise in
phenotypic
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
IQ but recent studies in four developed nations had found it has now ceased or gone into reverse. They thought it probable that both genotypic and phenotypic IQ will gradually start to decline for the whole world.
Possible causes
Income
A theory to explain the fertility-intelligence relationship is that while income and IQ are positively correlated,
income is also in itself a
fertility factor that correlates inversely with fertility, that is, the higher the incomes, the lower the fertility rates and
vice versa
References
Additional references
*
*
{{Latin phrases
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ca:Locució llatina#V
da:Latinske ord og vendinger#V
fr:Liste de locutions latines#V
id:Daftar frasa Latin#V
it:Locuzioni latine#V
nl:Lijst van Latijnse spreekwoorden en ui ...
. There is thus an inverse correlation between
income and fertility
Income and fertility is the association between monetary gain on one hand, and the tendency to produce offspring on the other. There is generally an inverse correlation between income and the total fertility rate within and between nations. The hi ...
within and between nations. The higher the level of
education
Education is a purposeful activity directed at achieving certain aims, such as transmitting knowledge or fostering skills and character traits. These aims may include the development of understanding, rationality, kindness, and honesty. Va ...
and
GDP
Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced and sold (not resold) in a specific time period by countries. Due to its complex and subjective nature this measure is often ...
per capita of a human
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
, sub-population or
social stratum
Social stratification refers to a society's categorization of its people into groups based on socioeconomic factors like wealth, income, race, education, ethnicity, gender, occupation, social status, or derived power (social and political). As su ...
, the fewer children are born. In a 1974 UN population conference in
Bucharest
Bucharest ( , ; ro, București ) is the capital and largest city of Romania, as well as its cultural, industrial, and financial centre. It is located in the southeast of the country, on the banks of the Dâmbovița River, less than north of ...
,
Karan Singh
Karan Singh (born 9 March 1931) is an Indian politician and philosopher. He is the son of the last ruling Maharaja of the princely state of Jammu and Kashmir, Sir Hari Singh. He was the prince regent of Jammu and Kashmir until 1952. From 19 ...
, a former minister of population in
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, encapsulated this relationship by stating "Development is the best contraceptive".
Education
In most countries, education is inversely correlated to childbearing. People often delay childbearing in order to spend more time getting education, and thus have fewer children. Conversely, early childbearing can interfere with education, so people with early or frequent childbearing are likely to be less educated. While education and childbearing place competing demands on a person's resources, education is positively correlated with IQ.
While there is less research into men's fertility and education, in developed countries evidence suggests that highly-educated men display higher levels of childbearing compared to less-educated men.
As a country becomes more developed, education rates increase and fertility rates decrease for both men and women. Fertility has fallen faster for both less-educated men and women than it has for highly-educated men and women. In the Nordic countries of Denmark, Norway, and Sweden, fertility for less-educated women has now fallen enough that childlessness is now highest among the least educated women just as it is for men.
Birth control and intelligence
Among a sample of women using
birth control
Birth control, also known as contraception, anticonception, and fertility control, is the use of methods or devices to prevent unwanted pregnancy. Birth control has been used since ancient times, but effective and safe methods of birth contr ...
methods of comparable theoretical effectiveness, success rates were related to IQ, with the percentages of high, medium and low IQ women having unwanted births during a three-year interval being 3%, 8% and 11%, respectively. Since the effectiveness of many methods of birth control is directly correlated with proper usage, an alternative interpretation of the data would indicate lower IQ women were less likely to use birth control consistently and correctly. Another study found that after an
unwanted pregnancy
Unintended pregnancies are pregnancies that are mistimed, unplanned or unwanted at the time of conception.
Sexual activity without the use of effective contraception through choice or coercion is the predominant cause of unintended pregnancy. W ...
has occurred, higher IQ couples are more likely to obtain abortions; and unmarried teenage girls who become pregnant are found to be more likely to carry their babies to term if they are doing poorly in school.
Conversely, while desired family size in the United States is apparently the same for women of all IQ levels,
highly educated women are found to be more likely to say that they desire more children than they have, indicating a "deficit fertility" in the highly intelligent. In her review of reproductive trends in the United States, Van Court argues that "each factor – from initially employing some form of contraception, to successful implementation of the method, to termination of an accidental pregnancy when it occurs – involves selection against intelligence."
Criticisms
Preston and Campbell (1993) argued that it is a mathematical fallacy that such differences in fertility would result in a progressive change of IQ, and applies only when looking at closed subpopulations. In their mathematical model, with constant differences in fertility, since children's IQ can be more or less than that of their parents, a steady-state equilibrium is argued to be established between different subpopulations with different IQ. The
mean
There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value (magnitude and sign) of a given data set.
For a data set, the ''arithme ...
IQ will not change in the absence of a change of the fertility differences. The steady-state IQ distribution will be lower for negative differential fertility than for positive, but these differences are small. For the extreme and unrealistic assumption of
endogamous
Endogamy is the practice of marrying within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close personal relationships.
Endogamy is common in many cultu ...
mating in IQ subgroups, a differential fertility change of 2.5/1.5 to 1.5/2.5 (high IQ/low IQ) causes a maximum shift of four IQ points. For random mating, the shift is less than one IQ point.
James S. Coleman, however, argues that Preston and Campbell's model depends on assumptions which are unlikely to be true.
The general increase in IQ test scores, the
Flynn effect
The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially standa ...
, has been argued to be evidence against dysgenic arguments.
Geneticist
A geneticist is a biologist or physician who studies genetics, the science of genes, heredity, and variation of organisms. A geneticist can be employed as a scientist or a lecturer. Geneticists may perform general research on genetic processe ...
Steve Connor wrote that Lynn, writing in ''
Dysgenics: Genetic Deterioration in Modern Populations,'' "misunderstood modern ideas of genetics." "A flaw in his argument of genetic deterioration in intelligence was the widely accepted fact that intelligence as measured by IQ tests has actually increased over the past 50 years."
If the genes causing IQ have been adversely affected, IQ scores should reasonably be expected to change in the same direction, yet the reverse has occurred. However, it has been argued that
genotypic
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
IQ may decrease even while
phenotypic
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
IQ rises throughout the population due to environmental effects such as better nutrition and education.
The Flynn effect may now have ended or reversed in some developed nations.
Some of the studies looking at relation between IQ and fertility cover the fertility of individuals who have attained a particular age, thereby ignoring positive correlation between
IQ and survival. To make conclusions about effects on IQ of future populations, such effects would have to be taken into account.
Recent research has shown that education and socioeconomic status are better indicators of fertility and suggests that the relationship between intelligence and number of children may be spurious. When controlling for education and socioeconomic status, the relationship between intelligence and number of children, intelligence and number of siblings, and intelligence and ideal number of children reduces to statistical insignificance. Among women, a post-hoc analysis revealed that the lowest and highest intelligence scores did not differ significantly by number of children.
Most research involves studying female fertility, while male fertility is ignored. When male fertility rates are compared to education attainment men with more education father more children.
Other research suggest that siblings born further apart achieve higher educational outcomes. Therefore, sibling density, not number of siblings, may explain the negative association between IQ and number of siblings.
See also
*
Dysgenics
Dysgenics (also known as cacogenics) is the decrease in prevalence of traits deemed to be either socially desirable or well adapted to their environment due to selective pressure disfavoring the reproduction of those traits.
The adjective "dysgeni ...
*
Fertility-development controversy
*
Fertility factor (demography)
Fertility factors are determinants of the number of children that an individual is likely to have. Fertility factors are mostly positive or negative correlations without certain causations.
Factors generally associated with increased fertility i ...
*
List of countries and territories by fertility rate
A ''list'' is any set of items in a row. List or lists may also refer to:
People
* List (surname)
Organizations
* List College, an undergraduate division of the Jewish Theological Seminary of America
* SC Germania List, German rugby union ...
References
{{Population
Factors related to intelligence
Fertility
Eugenics