Fertility and intelligence
Encyclopedia
Fertility and intelligence
research
investigates the relationship between fertility and intelligence. Demographic studies have indicated that in humans, fertility and intelligence tend to be inversely correlated, that is to say, the more intelligent, as measured by IQ
tests, exhibit a lower total fertility rate
than the less intelligent. Survival rates are also correlated with IQ
, so the net effect on population intelligence is unclear. It is theorized that if the inverse correlation of IQ with fertility is stronger than the correlation of survival rate, and if this continued over a significant number of generations, it could lead to a decrease in population IQ scores, a form of dysgenics
. Other correlates of IQ include income
and educational attainment, which are also inversely correlated with fertility.
between fertility and intelligence (as measured by IQ, see the article on IQ for a discussion of the concept) has been argued to have existed in many parts of the world at various times.
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
, these individuals being presumably of high intelligence. These men, taken as a whole, had few children, implying a correlation.
William Shockley
controversially argued from the mid-1960s through the 1980s that "the future of the population was threatened because people with low IQs had more children than those with high IQs."
Robert Klark Graham
argued that genocide
and class warfare
, in particular discussing the examples of the French Revolution
and the Russian Revolution, have had a dysgenic effect through the killing of the more intelligent by the less intelligent, and "might well incline humanity toward a more primitive, more brutish level of evolutionary achievement."
In 1963, Weyl and Possony asserted that comparatively small differences in average intelligence can become very large differences in the very high I.Q. ranges. A decline in average psychometric intelligence of only a few points will mean a much smaller population of gifted individuals.
More rigorous studies carried out on Americans alive after the Second World War 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, 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 whites 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 Vining sought to address these issues in a large study on the fertility of over 10,000 individuals throughout the United States
, 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
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.
employed, later research
has generally supported that of Vining.
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 & 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 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
(.68) and that half of the variance in education was explained by an underlying genetic component shared by IQ, education
, and SES
. One study investigating fertility and education carried out in 1991 found that high school
dropouts in America 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
(1994) argued that the average genotypic IQ of the United States was declining due to both dysgenetic fertility and large scale immigration of groups with low average IQ.
In his book Dysgenics: Genetic Deterioration in Modern Populations (1996), Richard Lynn
estimated that genotypic IQ have declined by 5 IQ points in Britain between 1890 and 1980. For the United States, genotypic IQ was estimated to have declined by 2.5 IQ points for whites and 6.2 IQ points for blacks between 1890 and 1960–1964.
In a 1999 study Lynn examined the relationship between the intelligence of adults aged 40 and above and their numbers of children and their siblings. Data were collected from the 1994 National Opinion Research Center
survey among a representative sample of 2992 English-speaking individuals aged 18 years. Findings revealed that weak negative correlations of −0.05 and −0.09, respectively were found. Further analysis showed that the negative correlation was present only in females. The correlation for females between intelligence and ideal number of children was effectively zero.
In 2004 Richard 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.
Shatza in a 2008 study found that SAT-derived state IQ scores negatively correlated with state fertility rates measured by three different indicators. The relationship remained relatively unchanged after controlling for state demographic and educational characteristics.
Meisenberg (2010) found that intelligence in the US was negatively related to the number of children, with age-controlled correlations of −.156, −.069, −.235 and −.028 for White females, White males, Black females and Black males. This effect was related mainly to the general intelligence factor
and was caused in part by education and income, and to a lesser extent by the more "liberal" gender attitudes of those with higher intelligence. Without migration the average IQ of the US population will decline by about 0.8 points per generation.
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 IQ is projected for the years 2000–2050." In the first period this effect had been compensated for by the Flynn effect
causing a rise in phenotypic 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.
As nations with higher IQ scores have access to more resources, and thus, prophylactics and fertility education, than nations with lower IQ scores, the birth rate would be expected to be lower. Meisenberg (2009) found that both log of per capita GDP and intelligence were negatively associated with fertility. Liberal democracy had only a weak and inconsistent relationship.
while income and IQ are positively correlated, fertility is inversely correlated with income, that is, the higher incomes, the lower the fertility rates and vice versa. This well-studied inverse correlation is known as the demographic-economic paradox
, which shows an inverse correlation between wealth and fertility within and between nations. The higher the level of education
and GDP
per capita of a human population
, subpopulation or social stratum, the fewer children are born. In a 1974 UN population conference in Bucharest
, Karan Singh
, a former minister of population in India
, illustrated this trend by stating "Development is the best contraceptive".
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 birth control is directly correlated with proper usage, an alternate interpretation of the data would indicate lower IQ women were more prone to misuse of birth control. Another study found that after an unwanted pregnancy 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 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."
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
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
, has been argued to be evidence against dysgenic arguments. Geneticist
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
IQ may decrease even while phenotypic
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.
Studies find that while low IQ families have relatively more children, large families do not produce low IQ children.
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.
and brain gain).
.
Dynamics:
Intelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
research
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
investigates the relationship between fertility and intelligence. Demographic studies have indicated that in humans, fertility and intelligence tend to be inversely correlated, that is to say, the more intelligent, as measured by IQ
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...
tests, exhibit a lower total fertility rate
Total Fertility Rate
The total fertility rate of a population is the average number of children that would be born to a woman over her lifetime if she were to experience the exact current age-specific fertility rates through her lifetime, and she...
than the less intelligent. Survival rates are also correlated with IQ
Cognitive epidemiology
Cognitive epidemiology is a field of research that examines the associations between intelligence test scores and health, more specifically morbidity and mortality. Typically, test scores are obtained at an early age, and compared to later morbidity and mortality...
, so the net effect on population intelligence is unclear. It is theorized that if the inverse correlation of IQ with fertility is stronger than the correlation of survival rate, and if this continued over a significant number of generations, it could lead to a decrease in population IQ scores, a form of dysgenics
Dysgenics
Dysgenics is the study of factors producing the accumulation and perpetuation of defective or disadvantageous genes and traits in offspring of a particular population or species. Dysgenic mutations have been studied in animals such as the mouse and the fruit fly...
. Other correlates of IQ include income
Income
Income is the consumption and savings opportunity gained by an entity within a specified time frame, which is generally expressed in monetary terms. However, for households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings...
and educational attainment, which are also inversely correlated with fertility.
Early views and research
A negative correlationCorrelation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
between fertility and intelligence (as measured by IQ, see the article on IQ for a discussion of the concept) has been argued to have existed in many parts of the world at various times.
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
Who's Who is the title of a number of reference publications, generally containing concise biographical information on a particular group of people...
, these individuals being presumably of high intelligence. These men, taken as a whole, had few children, implying a correlation.
William Shockley
William Shockley
William Bradford Shockley Jr. was an American physicist and inventor. Along with John Bardeen and Walter Houser Brattain, Shockley co-invented the transistor, for which all three were awarded the 1956 Nobel Prize in Physics.Shockley's attempts to commercialize a new transistor design in the 1950s...
controversially argued from the mid-1960s through the 1980s that "the future of the population was threatened because people with low IQs had more children than those with high IQs."
Robert Klark Graham
Robert Klark Graham
Robert Klark Graham was born in Harbor Springs, Michigan, USA. He was a eugenicist and businessman who made millions by developing shatter-proof plastic eyeglass lenses, and who later founded the Repository for Germinal Choice, a sperm bank for geniuses in the hope of implementing a eugenics...
argued that genocide
Genocide
Genocide is defined as "the deliberate and systematic destruction, in whole or in part, of an ethnic, racial, religious, or national group", though what constitutes enough of a "part" to qualify as genocide has been subject to much debate by legal scholars...
and class warfare
Class conflict
Class conflict is the tension or antagonism which exists in society due to competing socioeconomic interests between people of different classes....
, in particular discussing the examples of the French Revolution
French Revolution
The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...
and the Russian Revolution, have had a dysgenic effect through the killing of the more intelligent by the less intelligent, and "might well incline humanity toward a more primitive, more brutish level of evolutionary achievement."
In 1963, Weyl and Possony asserted that comparatively small differences in average intelligence can become very large differences in the very high I.Q. ranges. A decline in average psychometric intelligence of only a few points will mean a much smaller population of gifted individuals.
More rigorous studies carried out on Americans alive after the Second World War 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, 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 whites 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 Vining sought to address these issues in a large study on the fertility of over 10,000 individuals throughout the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, who were then aged 25 to 34. The average fertility in his study was correlated at −0.86 with IQ
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...
for white women and −0.96 for black women. Vining argued that this indicated a drop in the genotypic
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
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.
Later research
Regardless of the methodologyMethodology
Methodology is generally a guideline for solving a problem, with specificcomponents such as phases, tasks, methods, techniques and tools . It can be defined also as follows:...
employed, later research
Research
Research can be defined as the scientific search for knowledge, or as any systematic investigation, to establish novel facts, solve new or existing problems, prove new ideas, or develop new theories, usually using a scientific method...
has generally supported that of Vining.
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 & 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 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
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
(.68) and that half of the variance in education was explained by an underlying genetic component shared by IQ, education
Education
Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...
, and SES
Social class
Social classes are economic or cultural arrangements of groups in society. Class is an essential object of analysis for sociologists, political scientists, economists, anthropologists and social historians. In the social sciences, social class is often discussed in terms of 'social stratification'...
. One study investigating fertility and education carried out in 1991 found that high school
High school
High school is a term used in parts of the English speaking world to describe institutions which provide all or part of secondary education. The term is often incorporated into the name of such institutions....
dropouts in America 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
The Bell Curve is a best-selling and controversial 1994 book by the Harvard psychologist Richard J. Herrnstein and political scientist Charles Murray...
(1994) argued that the average genotypic IQ of the United States was declining due to both dysgenetic fertility and large scale immigration of groups with low average IQ.
In his book Dysgenics: Genetic Deterioration in Modern Populations (1996), Richard Lynn
Richard Lynn
Richard Lynn is a British Professor Emeritus of Psychology at the University of Ulster who is known for his views on racial and ethnic differences. Lynn argues that there are hereditary differences in intelligence based on race and sex....
estimated that genotypic IQ have declined by 5 IQ points in Britain between 1890 and 1980. For the United States, genotypic IQ was estimated to have declined by 2.5 IQ points for whites and 6.2 IQ points for blacks between 1890 and 1960–1964.
In a 1999 study Lynn examined the relationship between the intelligence of adults aged 40 and above and their numbers of children and their siblings. Data were collected from the 1994 National Opinion Research Center
National Opinion Research Center
NORC at the University of Chicago, established in 1941 as the National Opinion Research Center, is one of the largest and most highly respected social research organizations in the United States. Its corporate headquarters are located on the University of Chicago campus...
survey among a representative sample of 2992 English-speaking individuals aged 18 years. Findings revealed that weak negative correlations of −0.05 and −0.09, respectively were found. Further analysis showed that the negative correlation was present only in females. The correlation for females between intelligence and ideal number of children was effectively zero.
In 2004 Richard 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.
Shatza in a 2008 study found that SAT-derived state IQ scores negatively correlated with state fertility rates measured by three different indicators. The relationship remained relatively unchanged after controlling for state demographic and educational characteristics.
Meisenberg (2010) found that intelligence in the US was negatively related to the number of children, with age-controlled correlations of −.156, −.069, −.235 and −.028 for White females, White males, Black females and Black males. This effect was related mainly to the general intelligence factor
General intelligence factor
The g factor, where g stands for general intelligence, is a statistic used in psychometrics to model the mental ability underlying results of various tests of cognitive ability...
and was caused in part by education and income, and to a lesser extent by the more "liberal" gender attitudes of those with higher intelligence. Without migration the average IQ of the US population will decline by about 0.8 points per generation.
International research
Race and intelligence
The connection between race and intelligence has been a subject of debate in both popular science and academic research since the inception of intelligence testing in the early 20th century...
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 IQ is projected for the years 2000–2050." In the first period this effect had been compensated for by the Flynn effect
Flynn effect
The Flynn effect is the name given to a substantial and long-sustained increase in intelligence test scores measured in many parts of the world. When intelligence quotient tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100...
causing a rise in phenotypic 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.
As nations with higher IQ scores have access to more resources, and thus, prophylactics and fertility education, than nations with lower IQ scores, the birth rate would be expected to be lower. Meisenberg (2009) found that both log of per capita GDP and intelligence were negatively associated with fertility. Liberal democracy had only a weak and inconsistent relationship.
General factors causing reduced fertility
According to the UN Department of Economic and Social Affairs Population Division, the single universal factor affecting fertility rate decline is mortality decline, regardless of race, religion or political context. Extrapolated from this sole factor of mortality, there are other universal factors effecting fertility rates, regardless of race, religion or political context.This well-proven and exhaustively studied group of factors has affected the population policy of all UN member nations- Female age at marriage. The younger the female at first marriage, the higher the rate of fertility and vice versa.
- Female literacy and education. The higher the female education, the lower the fertility rate.
- Female mortality rates in childbirth and infant/child mortality. The higher the rates of death in childbirth, and crude infant or child deaths, the higher the crude fertility rate.
- Female economic participation. The greater the female participation in any form of economic activity or capacity, the lower their fertility.
- Access to contraception.
Income
A theory to explain the fertility-intelligence relationship is thatwhile income and IQ are positively correlated, fertility is inversely correlated with income, that is, the higher incomes, the lower the fertility rates and vice versa. This well-studied inverse correlation is known as the demographic-economic paradox
Demographic-economic paradox
The demographic-economic paradox is the inverse correlation found between wealth and fertility within and between nations. The higher the degree of education and GDP per capita of a human population, subpopulation or social stratum, the fewer children are born in any industrialized country...
, which shows an inverse correlation between wealth and fertility within and between nations. The higher the level of education
Education
Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...
and GDP
Gross domestic product
Gross domestic product refers to the market value of all final goods and services produced within a country in a given period. GDP per capita is often considered an indicator of a country's standard of living....
per capita of a human population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
, subpopulation or social stratum, the fewer children are born. In a 1974 UN population conference in Bucharest
Bucharest
Bucharest is the capital municipality, cultural, industrial, and financial centre of Romania. It is the largest city in Romania, located in the southeast of the country, at , and lies on the banks of the Dâmbovița River....
, Karan Singh
Karan Singh
Karan Singh is an MP in the Rajya Sabha, a senior member of the ruling Indian National Congress Party serving as Chairman of the Foreign Affairs Department, President of the Indian Council for Cultural Relations , India's Ambassador to UNESCO, Chairman of the Auroville Foundation and of the Temple...
, a former minister of population in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, illustrated this trend by stating "Development is the best contraceptive".
Birth control and intelligence
Among a sample of women using birth controlBirth control
Birth control is an umbrella term for several techniques and methods used to prevent fertilization or to interrupt pregnancy at various stages. Birth control techniques and methods include contraception , contragestion and abortion...
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 birth control is directly correlated with proper usage, an alternate interpretation of the data would indicate lower IQ women were more prone to misuse of birth control. Another study found that after an unwanted pregnancy 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 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
While it may seem obvious that such differences in fertility would result in a progressive change of IQ, Preston and Campbell (1993) argued that this is a mathematical fallacy that 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 meanMean
In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
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
Endogamy is the practice of marrying within a specific ethnic group, class, or social group, rejecting others on such basis as being unsuitable for marriage or other close personal relationships. A Greek Orthodox Christian endogamist, for example, would require that a marriage be only with another...
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
Flynn effect
The Flynn effect is the name given to a substantial and long-sustained increase in intelligence test scores measured in many parts of the world. When intelligence quotient tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100...
, has been argued to be evidence against dysgenic arguments. Geneticist
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
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
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
IQ may decrease even while phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
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.
Studies find that while low IQ families have relatively more children, large families do not produce low IQ children.
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
Cognitive epidemiology
Cognitive epidemiology is a field of research that examines the associations between intelligence test scores and health, more specifically morbidity and mortality. Typically, test scores are obtained at an early age, and compared to later morbidity and mortality...
. To make conclusions about effects on IQ of future populations, such effects would have to be taken into account.
Brain drain and gain
Weiss in a 2009 study argues that for many nations the effect of fertility on IQ is currently overshadowed by the effects of selective immigration and emigration of groups with varying average IQs (brain drainBrain drain
Human capital flight, more commonly referred to as "brain drain", is the large-scale emigration of a large group of individuals with technical skills or knowledge. The reasons usually include two aspects which respectively come from countries and individuals...
and brain gain).
Other traits
For hypothesized dysgenic trends for traits other than intelligence, see the article on dysgenicsDysgenics
Dysgenics is the study of factors producing the accumulation and perpetuation of defective or disadvantageous genes and traits in offspring of a particular population or species. Dysgenic mutations have been studied in animals such as the mouse and the fruit fly...
.
See also
- Total fertility rateTotal Fertility RateThe total fertility rate of a population is the average number of children that would be born to a woman over her lifetime if she were to experience the exact current age-specific fertility rates through her lifetime, and she...
- List of countries and territories by fertility rate
- Fertility-development controversyFertility-development controversyThe relationship between the total fertility rate and socio-economic development, which is measured by the human development index , is the subject of debate in social sciences....
- Cognitive epidemiologyCognitive epidemiologyCognitive epidemiology is a field of research that examines the associations between intelligence test scores and health, more specifically morbidity and mortality. Typically, test scores are obtained at an early age, and compared to later morbidity and mortality...
Dynamics:
- Heritability of IQ
- Demographic-economic paradoxDemographic-economic paradoxThe demographic-economic paradox is the inverse correlation found between wealth and fertility within and between nations. The higher the degree of education and GDP per capita of a human population, subpopulation or social stratum, the fewer children are born in any industrialized country...
- HypergamyHypergamyHypergamy is the act or practice of seeking a spouse of higher socioeconomic status, or caste status than oneself....