Heritability of IQ

From Wikipedia, the free encyclopedia

Research on the heritability of IQ inquires into the degree of variation in IQ within a population that is due to genetic variation between individuals in that population. There has been significant controversy in the academic community about the heritability of IQ since research on the issue began in the late nineteenth century. Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene, and in the case of intelligence at least 500 genes. Further, explaining the similarity in IQ of closely related persons requires careful study because environmental factors may be correlated with genetic factors.

Early twin studies of adult individuals have found a heritability of IQ between 57% and 73%, with some recent studies showing heritability for IQ as high as 80%. IQ goes from being weakly correlated with genetics for children, to being strongly correlated with genetics for late teens and adults. The heritability of IQ increases with the child's age and reaches a plateau at 18–20 years old, continuing at that level well into adulthood. However, poor prenatal environment, malnutrition and disease are known to have lifelong deleterious effects.

Although IQ differences between individuals have been shown to have a large hereditary component, it does not follow that disparities in IQ between groups have a genetic basis. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.

Heritability and caveats

Main article: 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 heritability can be expressed in the form of the following question: "What is the proportion of the variation in a given trait within a population that is not explained by the environment or random chance?"

Estimates of heritability take values ranging from 0 to 1; a heritability estimate of 1 indicates that all variation in the trait in question is genetic in origin and a heritability estimate of 0 indicates that none of the variation is genetic. The determination of many traits can be considered primarily genetic under similar environmental backgrounds. For example, a 2006 study found that adult height has a heritability estimated at 0.80 when looking only at the height variation within families where the environment should be very similar. Other traits have lower heritability estimates, which indicate a relatively larger environmental influence. For example, a twin study on the heritability of depression in men estimated it as 0.29, while it was 0.42 for women in the same study.

Caveats

There are a number of points to consider when interpreting heritability:

• Heritability measures the proportion of variation in a trait that can be attributed to genes, and not the proportion of a trait caused by genes. Thus, if the environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability (because the variation or differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase. Thus, even in developed nations, a high heritability of a trait does not necessarily mean that average group differences are due to genes. Some have gone further, and used height as an example in order to argue that "even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability."
• A common error is to assume that a heritability figure is necessarily unchangeable. The value of heritability can change if the impact of environment (or of genes) in the population is substantially altered. If the environmental variation encountered by different individuals increases, then the heritability figure would decrease. On the other hand, if everyone had the same environment, then heritability would be 100%. The population in developing nations often has more diverse environments than in developed nations. This would mean that heritability figures would be lower in developing nations. Another example is phenylketonuria which previously caused mental retardation for everyone who had this genetic disorder and thus had a heritability of 100%. Today, this can be prevented by following a modified diet, resulting in a lowered heritability.
• A high heritability of a trait does not mean that environmental effects such as learning are not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high.
• Since heritability increases during childhood and adolescence, and even increases greatly between 16 and 20 years of age and adulthood, one should be cautious drawing conclusions regarding the role of genetics and environment from studies where the participants are not followed until they are adults. Furthermore, there may be differences regarding the effects on the g-factor and on non-g factors, with g possibly being harder to affect and environmental interventions disproportionately affecting non-g factors.
• Polygenic traits often appear less heritable at the extremes. A heritable trait is definitionally more likely to appear in the offspring of two parents high in that trait than in the offspring of two randomly selected parents. However, the more extreme the expression of the trait in the parents, the less likely the child is to display the same extreme as the parents. At the same time, the more extreme the expression of the trait in the parents, the more likely the child is to express the trait at all. For example, the child of two extremely tall parents is likely to be taller than the average person (displaying the trait), but unlikely to be taller than the two parents (displaying the trait at the same extreme). See also regression toward the mean.

Estimates

Various studies have estimated the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in the United States. It has been found that estimates of heritability increase as individuals age. Heritability estimates in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood. The brain undergoes morphological changes in development which suggests that age-related physical changes could contribute to this effect.

A 1994 article in Behavior Genetics based on a study of Swedish monozygotic and dizygotic twins found the heritability of the sample to be as high as 0.80 in general cognitive ability; however, it also varies by trait, with 0.60 for verbal tests, 0.50 for spatial and speed-of-processing tests, and 0.40 for memory tests. In contrast, studies of other populations estimate an average heritability of 0.50 for general cognitive ability.

In 2006, David Kirp, writing in The New York Times Magazine, summarized a century's worth of research as follows, "about three-quarters of I.Q. differences between individuals are attributable to heredity."

Shared family environment

See also: Environment and intelligence

There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, adoption studies show that by adulthood adoptive siblings aren't more similar in IQ than strangers, while adult full siblings show an IQ correlation of 0.24. However, some studies of twins reared apart (e.g. Bouchard, 1990) find a significant shared environmental influence, of at least 10% going into late adulthood. Judith Rich Harris suggests that this might be due to biasing assumptions in the methodology of the classical twin and adoption studies.

There are aspects of environments that family members have in common (for example, characteristics of the home). This shared family environment accounts for 0.25-0.35 of the variation in IQ in childhood. By late adolescence it is quite low (zero in some studies). There is a similar effect for several other psychological traits. These studies have not looked into the effects of extreme environments such as in abusive families.

The American Psychological Association's report "Intelligence: Knowns and Unknowns" (1996) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the shared family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests.

Non-shared family environment and environment outside the family

Although parents treat their children differently, such differential treatment explains only a small amount of non-shared environmental influence. One suggestion is that children react differently to the same environment due to different genes. More likely influences may be the impact of peers and other experiences outside the family. For example, siblings grown up in the same household may have different friends and teachers and even contract different illnesses. This factor may be one of the reasons why IQ score correlations between siblings decreases as they get older.

Malnutrition and diseases

Certain single-gene metabolic disorders can severely affect intelligence. Phenylketonuria is an example, with publications demonstrating the capacity of phenylketonuria to produce a reduction of 10 IQ points on average. Meta-analyses have found that environmental factors, such as iodine deficiency, can result in large reductions in average IQ; iodine deficiency has been shown to produce a reduction of 12.5 IQ points on average.

Heritability and socioeconomic status

The APA report "Intelligence: Knowns and Unknowns" (1996) also stated that:

"We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence."

A study (1999) by Capron and Duyme of French children adopted between the ages of four and six examined the influence of socioeconomic status (SES). The children's IQs initially averaged 77, putting them near retardation. Most were abused or neglected as infants, then shunted from one foster home or institution to the next. Nine years later after adoption, when they were on average 14 years old, they retook the IQ tests, and all of them did better. The amount they improved was directly related to the adopting family's socioeconomic status. "Children adopted by farmers and laborers had average IQ scores of 85.5; those placed with middle-class families had average scores of 92. The average IQ scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."

Stoolmiller (1999) argued that the range of environments in previous adoption studies was restricted. Adopting families tend to be more similar on, for example, socio-economic status than the general population, which suggests a possible underestimation of the role of the shared family environment in previous studies. Corrections for range restriction to adoption studies indicated that socio-economic status could account for as much as 50% of the variance in IQ.

On the other hand, the effect of this was examined by Matt McGue and colleagues (2007), who wrote that "restriction in range in parent disinhibitory psychopathology and family socio-economic status had no effect on adoptive-sibling correlations [in] IQ"

Turkheimer and colleagues (2003) argued that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in a study on seven-year-old twins, in impoverished families, 60% of the variance in early childhood IQ was accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.

In contrast to Turkheimer (2003), a study by Nagoshi and Johnson (2005) concluded that the heritability of IQ did not vary as a function of parental socioeconomic status in the 949 families of Caucasian and 400 families of Japanese ancestry who took part in the Hawaii Family Study of Cognition.

Asbury and colleagues (2005) studied the effect of environmental risk factors on verbal and non-verbal ability in a nationally representative sample of 4-year-old British twins. There was not any statistically significant interaction for non-verbal ability, but the heritability of verbal ability was found to be higher in low-SES and high-risk environments.

Harden, Turkheimer, and Loehlin (2007) investigated adolescents, most 17 years old, and found that, among higher income families, genetic influences accounted for approximately 55% of the variance in cognitive aptitude and shared environmental influences about 35%. Among lower income families, the proportions were in the reverse direction, 39% genetic and 45% shared environment."

In the course of a substantial review, Rushton and Jensen (2010) criticized the study of Capron and Duyme, arguing their choice of IQ test and selection of child and adolescent subjects were a poor choice because this gives a relatively less hereditable measure. The argument here rests on a strong form of Spearman's hypothesis, that the hereditability of different kinds of IQ test can vary according to how closely they correlate to the general intelligence factor (g); both the empirical data and statistical methodology bearing on this question are matters of active controversy.

A 2011 study by Tucker-Drob and colleagues reported that at age 2, genes accounted for approximately 50% of the variation in mental ability for children being raised in high socioeconomic status families, but genes accounted for negligible variation in mental ability for children being raised in low socioeconomic status families. This gene–environment interaction was not apparent at age 10 months, suggesting that the effect emerges over the course of early development.

A 2012 study based on a representative sample of twins from the United Kingdom, with longitudinal data on IQ from age two to age fourteen, did not find evidence for lower heritability in low-SES families. However, the study indicated that the effects of shared family environment on IQ were generally greater in low-SES families than in high-SES families, resulting in greater variance in IQ in low-SES families. The authors noted that previous research had produced inconsistent results on whether or not SES moderates the heritability of IQ. They suggested three explanations for the inconsistency. First, some studies may have lacked statistical power to detect interactions. Second, the age range investigated has varied between studies. Third, the effect of SES may vary in different demographics and different countries.

A 2017 King's College London study suggests that genes account for nearly 50 per cent of the differences between whether children are socially mobile or not.

Maternal (fetal) environment

A meta-analysis by Devlin and colleagues (1997) of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (fetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%. They argue that the shared maternal environment may explain the striking correlation between the IQs of twins, especially those of adult twins that were reared apart. IQ heritability increases during early childhood, but whether it stabilizes thereafter remains unclear. These results have two implications: a new model may be required regarding the influence of genes and environment on cognitive function; and interventions aimed at improving the prenatal environment could lead to a significant boost in the population's IQ.

Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different from previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports. They write that:

Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al. (1997a) conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950), in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion (Price, 1978). Research subsequent to the 1978 review largely reinforces Price's hypothesis (Bryan, 1993; Macdonald et al., 1993; Hall and Lopez-Rangel, 1996; see also Martin et al., 1997, box 2; Machin, 1996).

Dickens and Flynn model

Dickens and Flynn (2001) argued that the "heritability" figure includes both a direct effect of the genotype on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues. This model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects.

The Flynn effect is the increase in average intelligence test scores by about 0.3% annually, resulting in the average person today scoring 15 points higher in IQ compared to the generation 50 years ago. This effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program. Most of the improvements have allowed for better abstract reasoning, spatial relations, and comprehension. Some scientists have suggested that such enhancements are due to better nutrition, better parenting and schooling, as well as exclusion of the least intelligent people from reproduction. However, Flynn and a group of other scientists share the viewpoint that modern life implies solving many abstract problems which leads to a rise in their IQ scores.

Influence of genes on IQ stability

Recent research has illuminated genetic factors underlying IQ stability and change. Genome-wide association studies have demonstrated that the genes involved in intelligence remain fairly stable over time. Specifically, in terms of IQ stability, "genetic factors mediated phenotypic stability throughout this entire period [age 0 to 16], whereas most age-to-age instability appeared to be due to non-shared environmental influences". These findings have been replicated extensively and observed in the United Kingdom, the United States, and the Netherlands. Additionally, researchers have shown that naturalistic changes in IQ occur in individuals at variable times.

Influence of parents genes that are not inherited

Kong reports that, "Nurture has a genetic component, i.e. alleles in the parents affect the parents' phenotypes and through that influence the outcomes of the child." These results were obtained through a meta-analysis of educational attainment and polygenic scores of non-transmitted alleles. Although the study deals with educational attainment and not IQ, these two are strongly linked.

Spatial ability component of IQ

Spatial ability has been shown to be unifactorial (a single score accounts well for all spatial abilities), and is 69% heritable in a sample of 1,367 pairs of twins from the ages 19 through 21. Further only 8% of spatial ability can be accounted for by shared environmental factors like school and family. Of the genetically determined portion of spatial ability, 24% is shared with verbal ability (general intelligence) and 43% was specific to spatial ability alone.

Molecular genetic investigations

A 2009 review article identified over 50 genetic polymorphisms that have been reported to be associated with cognitive ability in various studies, but noted that the discovery of small effect sizes and lack of replication have characterized this research so far. Another study attempted to replicate 12 reported associations between specific genetic variants and general cognitive ability in three large datasets, but found that only one of the genotypes was significantly associated with general intelligence in one of the samples, a result expected by chance alone. The authors concluded that most reported genetic associations with general intelligence are probably false positives brought about by inadequate sample sizes. Arguing that common genetic variants explain much of the variation in general intelligence, they suggested that the effects of individual variants are so small that very large samples are required to reliably detect them. Genetic diversity within individuals is heavily correlated with IQ.

A novel molecular genetic method for estimating heritability calculates the overall genetic similarity (as indexed by the cumulative effects of all genotyped single nucleotide polymorphisms) between all pairs of individuals in a sample of unrelated individuals and then correlates this genetic similarity with phenotypic similarity across all the pairs. A study using this method estimated that the lower bounds for the narrow-sense heritability of crystallized and fluid intelligence are 40% and 51%, respectively. A replication study in an independent sample confirmed these results, reporting a heritability estimate of 47%. These findings are compatible with the view that a large number of genes, each with only a small effect, contribute to differences in intelligence.

Correlations between IQ and degree of genetic relatedness

The relative influence of genetics and environment for a trait can be calculated by measuring how strongly traits covary in people of a given genetic (unrelated, siblings, fraternal twins, or identical twins) and environmental (reared in the same family or not) relationship. One method is to consider identical twins reared apart, with any similarities that exist between such twin pairs attributed to genotype. In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share on average half alleles and the correlation of their scores would be 0.50 if IQ were affected by genes alone (or greater if there is a positive correlation between the IQs of spouses in the parental generation). Practically, however, the upper bound of these correlations are given by the reliability of the test, which is 0.90 to 0.95 for typical IQ tests.

If there is biological inheritance of IQ, then the relatives of a person with a high IQ should exhibit a comparably high IQ with a much higher probability than the general population. In 1982, Bouchard and McGue reviewed such correlations reported in 111 original studies in the United States. The mean correlation of IQ scores between monozygotic twins was 0.86, between siblings 0.47, between half-siblings 0.31, and between cousins 0.15.

The 2006 edition of Assessing adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger reports correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings. These numbers are not necessarily static. When comparing pre-1963 to late 1970s data, researches DeFries and Plomin found that the IQ correlation between parent and child living together fell significantly, from 0.50 to 0.35. The opposite occurred for fraternal twins.

Every one of these studies presented next contains estimates of only two of the three factors which are relevant. The three factors are G, E, and GxE. Since there is no possibility of studying equal environments in a manner comparable to using identical twins for equal genetics, the GxE factor can not be isolated. Thus the estimates are actually of G+GxE and E. Although this may seem like nonsense, it is justified by the unstated assumption that GxE=0. It is also the case that the values shown below are r correlations and not r(squared), proportions of variance. Numbers less than one are smaller when squared. The next to last number in the list below refers to less than 5% shared variance between a parent and child living apart.

Another summary:

• Same person (tested twice over time) .85 or above
• Identical twins—Reared together .86
• Identical twins—Reared apart .76
• Fraternal twins—Reared together .55
• Fraternal twins—Reared apart .35
• Biological siblings—Reared together .47
• Biological siblings—Reared apart .24
• Biological siblings—Reared together—Adults .24
• Unrelated children—Reared together—Children .28
• Unrelated children—Reared together—Adults .04
• Cousins .15
• Parent-child—Living together .42
• Parent-child—Living apart .22
• Adoptive parent–child—Living together .19

Between-group heritability

Main article: Race and intelligence

In the US, individuals identifying themselves as Asian generally tend to score higher on IQ tests than Caucasians, who tend to score higher than Hispanics, who tend to score higher than African Americans –– despite the fact that greater variation in IQ scores exists within each ethnic group than between them. Yet, although IQ differences between individuals have been shown to have a large hereditary component, it does not follow that between-group differences in average IQ have a genetic basis. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups. Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.

Arguments in support of a genetic explanation of racial differences in average IQ are sometimes fallacious. For instance, some hereditarians have cited as evidence the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races. Jensen and Rushton, in their formulation of Spearman's Hypothesis, argued that cognitive tasks that have the highest g-load are the tasks in which the gap between black and white test takers is greatest, and that this supports their view that racial IQ gaps are in large part genetic. However, in separate reviews, Mackintosh, Nisbett et al. and Flynn have all concluded that the slight correlation between g-loading and the test score gap offers no clue to the cause of the gap. Further reviews of both adoption studies and racial admixture studies have also found no evidence for a genetic component behind group-level IQ differences. Hereditarian arguments for racial differences in IQ have been criticized from a theoretical point of view as well. For example, the geneticist and neuroscientist Kevin Mitchell has argued that "systematic genetic differences in intelligence between large, ancient populations" are "inherently and deeply implausible" because the "constant churn of genetic variation works against any long-term rise or fall in intelligence." As he argues, "To end up with systematic genetic differences in intelligence between large, ancient populations, the selective forces driving those differences would need to have been enormous. What's more, those forces would have to have acted across entire continents, with wildly different environments, and have been persistent over tens of thousands of years of tremendous cultural change."

In favor of an environmental explanation, on the other hand, numerous studies and reviews have shown promising results. Among these, some focus on the gradual closing of the black–white IQ gap over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers. For instance, Vincent reported in 1991 that the black–white IQ gap was decreasing among children, but that it was remaining constant among adults. Similarly, a 2006 study by Dickens and Flynn estimated that the difference between mean scores of black people and white people closed by about 5 or 6 IQ points between 1972 and 2002, a reduction of about one-third. In the same period, the educational achievement disparity also diminished. Reviews by Flynn and Dickens, Mackintosh, and Nisbett et al. all accept the gradual closing of the gap as a fact. Other recent studies have focused on disparities in nutrition and prenatal care, as well as other health-related environmental disparities, and have found that these disparities may account for significant IQ gaps between population groups. Still other studies have focused on educational disparities, and have found that intensive early childhood education and test preparation can diminish or eliminate the black–white IQ test gap. In light of these and similar findings, a consensus has formed that genetics does not explain differences in average IQ test performance between racial groups.

Socioeconomic status

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Socioeconomic_status 

 

An 1880 painting by Jean-Eugène Buland showing a stark contrast in socioeconomic status

Socioeconomic status (SES) is an economic and sociological combined total measure of a person's work experience and of an individual's or family's economic access to resources and social position in relation to others. When analyzing a family's SES, the household income, earners' education, and occupation are examined, as well as combined income, whereas for an individual's SES only their own attributes are assessed. Recently, research has revealed a lesser recognized attribute of SES as perceived financial stress, as it defines the "balance between income and necessary expenses". Perceived financial stress can be tested by deciphering whether a person at the end of each month has more than enough, just enough, or not enough money or resources. However, SES is more commonly used to depict an economic difference in society as a whole.

Socioeconomic status is typically broken into three levels (high, middle, and low) to describe the three places a family or an individual may fall into. When placing a family or individual into one of these categories, any or all of the three variables (income, education, and occupation) can be assessed.

Education in higher socioeconomic families is typically stressed as much more important, both within the household as well as the local community. In poorer areas, where food, shelter and safety are a priority, education is typically regarded as less important. Youth audiences are particularly at risk for many health and social problems in the United States, such as unwanted pregnancies, drug abuse, and obesity.

Additionally, low income and education have been shown to be strong predictors of a range of physical and mental health problems, including respiratory viruses, arthritis, coronary disease, and schizophrenia. These problems may be due to environmental conditions in their workplace, or, in the case of disabilities or mental illnesses, may be the entire cause of that person's social predicament to begin with.

Important factors

Income

Income refers to wages, salaries, profits, rents, and any flow of earnings received. Income can also come in the form of unemployment or worker's compensation, social security, pensions, interests or dividends, royalties, trusts, alimony, or other governmental, public, or family financial assistance. It can also come from monetary winnings, as from lotteries and other games or contests where money is awarded as a prize.

Income can be looked at in two terms, relative and absolute. Absolute income, as theorized by economist John Maynard Keynes, is the relationship in which as income increases, so will consumption, but not at the same rate. Relative income dictates a person's or family's savings and consumption based on the family's income in relation to others. Income is a commonly used measure of SES because it is relatively easy to figure for most individuals.

Income inequality is most commonly measured around the world by the Gini coefficient, where 0 corresponds to perfect equality and 1 means perfect inequality. Low-income families focus on meeting immediate needs and do not accumulate wealth that could be passed on to future generations, thus increasing inequality. Families with higher and expendable income can accumulate wealth and focus on meeting immediate needs while being able to consume and enjoy luxuries and weather crises.

Education

Education also plays a role in determining income. Median earnings increase with each level of education. As conveyed in the chart, the highest degrees, professional and doctoral degrees, make the highest weekly earnings while those without a high school diploma earn less. Higher levels of education are associated with better economic and psychological outcomes (i.e.: more income, more control, and greater social support and networking).

Education plays a pivotal role in skillsets for acquiring jobs, as well as specific qualities that stratify people with higher SES from lower SES. Annette Lareau speaks on the idea of concerted cultivation, where middle class parents take an active role in their children's education and development by using controlled organized activities and fostering a sense of entitlement through encouraging discussion. Laureau argues that families with lower income do not participate in this movement, causing their children to have a sense of constraint. An interesting observation that studies have noted is that parents from lower SES households are more likely to give orders to their children in their interactions while parents with a higher SES are more likely to interact and play with their children. A division in education attainment is thus born out of these two differences in child-rearing. Research has shown how children who are born in lower SES households have weaker language skills compared to children raised in higher SES households. These language skills affect their abilities to learn and thus exacerbate the problem of education disparity between low and high SES neighbourhoods. Lower-income families can have children who do not succeed to the levels of the middle-income children, who can have a greater sense of entitlement, be more argumentative, or be better prepared for adult life.

Research shows that lower SES students have lower and slower academic achievement as compared with students of higher SES. When teachers make judgments about students based on their class and SES, they are taking the first step in preventing students from having an equal opportunity for academic achievement. Educators need to help overcome the stigma of poverty. A student of low SES and low self-esteem should not be reinforced by educators. Teachers need to view students as individuals and not as a member of an SES group. Teachers looking at students in this manner will help them to not be prejudiced towards students of certain SES groups. Raising the level of instruction can help to create equality in student achievement. Teachers relating the content taught to students' prior knowledge and relating it to real-world experiences can improve achievement. Educators also need to be open and discuss class and SES differences. It is important that all are educated, understand, and be able to speak openly about SES.

Occupation

Occupational prestige, as one component of SES, encompasses both income and educational attainment. The occupational status reflects the educational attainment required to obtain the job and income levels that vary with different jobs and within ranks of occupations. Additionally, it shows achievement in skills required for the job. Occupational status measures social position by describing job characteristics, decision-making ability and control, and psychological demands on the job.

Occupations are ranked by the Census (among other organizations) and opinion polls from the general population are surveyed. Some of the most prestigious occupations are physicians and surgeons, lawyers, chemical and biomedical engineers, university professors, and communications analysts. These jobs, considered to be grouped in the high SES classification, provide more challenging work and greater control over working conditions but require more ability. The jobs with lower rankings include food preparation workers, counter attendants, bartenders and helpers, dishwashers, janitors, maids and housekeepers, vehicle cleaners, and parking lot attendants. The jobs that are less valued also offer significantly lower wages, and often are more laborious, very hazardous, and provide less autonomy.

Occupation is the most difficult factor to measure because so many exist, and there are so many competing scales. Many scales rank occupations based on the level of skill involved, from unskilled to skilled manual labour to professional, or use a combined measure using the education level needed and income involved.

In sum, the majority of researchers agree that income, education and occupation together best represent SES, while some others feel that changes in family structure should also be considered. SES affects students' cognitive abilities and academic success. Several researchers have found that SES affects students' abilities.

Other variables

Wealth

Wealth distribution in the United States by net worth (2007). The net wealth of many people in the lowest 20% is negative because of debt. By 2014 the wealth gap deepened.

  Top 1% (34.6%)

  Next 4% (27.3%)

  Next 5% (11.2%)

  Next 10% (12%)

  Upper Middle 20% (10.9%)

  Middle 20% (4%)

  Bottom 40% (0.2%)

Wealth, a set of economic reserves or assets, presents a source of security providing a measure of a household's ability to meet emergencies, absorb economic shocks, or provide the means to live comfortably. Wealth reflects intergenerational transitions as well as accumulation of income and savings.

Income, age, marital status, family size, religion, occupation, and education are all predictors of wealth attainment.

The wealth gap, like income inequality, is very large in the United States. There exists a racial wealth gap due in part to income disparities and differences in achievement resulting from institutional discrimination. According to Thomas Shapiro, differences in savings (due to different rates of incomes), inheritance factors, and discrimination in the housing market lead to the racial wealth gap. Shapiro claims that savings increase with increasing income, but African Americans cannot participate in this, because they make significantly less than Americans of European descent (whites). Additionally, rates of inheritance dramatically differ between African Americans and Americans of European descent. The amount a person inherits, either during a lifetime or after death, can create different starting points between two different individuals or families. These different starting points also factor into housing, education, and employment discrimination. A third reason Shapiro offers for the racial wealth gap are the various discriminations African Americans must face, like redlining and higher interest rates in the housing market. These types of discrimination feed into the other reasons why African Americans end up having different starting points and therefore fewer assets.

Effects

Health

Recently, there has been increasing interest from epidemiologists on the subject of economic inequality and its relation to the health of populations. Socioeconomic status has long been related to health, those higher in the social hierarchy typically enjoy better health than those below. Socioeconomic status is an important source of health inequity, as there is a very robust positive correlation between socioeconomic status and health. This correlation suggests that it is not only the poor who tend to be sick when everyone else is healthy, but that there is a continual gradient, from the top to the bottom of the socio-economic ladder, relating status to health. Parents with a low socioeconomic status cannot afford many of the health care resources which is the reason that their children may have a more advanced illness because of the lack of treatment. This phenomenon is often called the "SES Gradient" or according to the World Health Organisation the "Social Gradient". Lower socioeconomic status has been linked to chronic stress, heart disease, ulcers, type 2 diabetes, rheumatoid arthritis, certain types of cancer, and premature aging.

There is debate regarding the cause of the SES Gradient. Researchers see a definite link between economic status and mortality due to the greater economic resources of the wealthy, but they find little correlation due to social status differences.

Other researchers such as Richard G. Wilkinson, J. Lynch, and G.A. Kaplan have found that socioeconomic status strongly affects health even when controlling for economic resources and access to health care. Most famous for linking social status with health are the Whitehall studies—a series of studies conducted on civil servants in London. The studies found that although all civil servants in England have the same access to health care, there was a strong correlation between social status and health. The studies found that this relationship remained strong even when controlling for health-affecting habits such as exercise, smoking and drinking. Furthermore, it has been noted that no amount of medical attention will help decrease the likelihood of someone getting type 2 diabetes or rheumatoid arthritis—yet both are more common among populations with lower socioeconomic status.

Political participation

Political scientists have established a consistent relationship between SES and political participation. For example, in 2004, the American Political Science Task Force on Inequality and American Democracy has found that those with higher socioeconomic status participate at higher rates than those with lower status.

Psychological

Language development

Home environment

The environment of low SES children is characterized by less dialogue from parents, minimal amounts of book reading, and few instances of joint attention, the shared focus of the child and adult on the same object or event, when compared to the environment of high SES children. In contrast, infants from high SES families experience more child-directed speech. At 10 months, children of high SES hear on average 400 more words than their low SES peers.

Language ability differs sharply as a function of SES, for example, the average vocabulary size of 3-year-old children from professional families was more than twice as large as for those on welfare.

Children from lower income households had greater media access in their bedrooms but lower access to portable play equipment compared to higher income children. This eventually leads children from lower socioeconomic backgrounds to be at a disadvantage when comparing them with their counterparts in terms of access to physical activities.

Parental interactions

In addition to the amount of language input from parents, SES heavily influences the type of parenting style a family chooses to practice. These different parenting styles shape the tone and purpose of verbal interactions between parent and child. For example, parents of high SES tend toward more authoritative or permissive parenting styles. These parents pose more open-ended questions to their children to encourage the latter's speech growth. In contrast, parents of low SES tend toward more authoritarian styles of address. Their conversations with their children contain more imperatives and yes/no questions that inhibits child responses and speech development.

Parental differences in addressing children may be traced to the position of their respective groups within society. Working class individuals often hold low-power, subordinate positions in the occupational world. This standing in the social hierarchy requires a personality and interaction style that is relational and capable of adjusting to circumstances. An authoritarian style of address prepares children for these types of roles, which require a more accommodating and compliant personality. Therefore, low-SES parents see the family as more hierarchical, with the parents at the top of the power structure, which shapes verbal interaction. This power differential emulates the circumstances of the working class world, where individuals are ranked and discouraged from questioning authority.

Conversely, high-SES individuals occupy high-power positions that call for greater expressivity. High-SES parents encourage their children to question the world around them. In addition to asking their children more questions, these parents push their children to create questions of their own. In contrast with low-SES parents, these individuals often view the power disparity between parent and child as detrimental to the family. Opting instead to treat children as equals, high-SES conversations are characterized by a give and take between parent and child. These interactions help prepare these children for occupations that require greater expressivity.

Disparities in language acquisition

The linguistic environment of low and high SES children differs substantially, which affects many aspects of language and literacy development such as semantics, syntax, morphology, and phonology.

Semantics

Semantics is the study of the meaning of words and phrases. Semantics covers vocabulary, which is affected by SES. Children of high SES have larger expressive vocabularies by the age of 24 months due to more efficient processing of familiar words. By age 3, there are significant differences in the amount of dialogue and vocabulary growth between children of low and high SES. The effects of SES on vocabulary extend from childhood to adolescence and even into early adulthood according to a large socioeconomically diverse study. A lack of joint attention in children contributes to poor vocabulary growth when compared to their high SES peers. Joint attention and book reading are important factors that affect children's vocabulary growth. With joint attention, a child and adult can focus on the same object, allowing the child to map out words. For example, a child sees an animal running outside and the mom points to it and says, "Look, a dog." The child will focus its attention to where its mother is pointing and map the word dog to the pointed animal. Joint attention thus facilitates word learning for children.

Syntax

Syntax refers to the arrangement of words and phrases to form sentences. SES affects the production of sentence structures. Although 22- to 44-month-old children's production of simple sentence structures does not vary by SES, low SES does contribute to difficulty with complex sentence structures. Complex sentences include sentences that have more than one verb phrase. An example of a complex sentence is, "I want you to sit there". The emergence of simple sentence structures is seen as a structure that is obligatory in everyday speech. Complex sentence structures are optional and can only be mastered if the environment fosters its development.

This lag in the sentence formation abilities of low SES children may be caused by less frequent exposure to complex syntax through parental speech. Low SES parents ask fewer response-coaxing questions of their children which limits the opportunities of these children to practice more complex speech patterns. Instead, these parents give their children more direct orders, which has been found to negatively influence the acquisition of more difficult noun and verb phrases. In contrast, high SES households ask their children broad questions to cultivate speech development. Exposure to more questions positively contributes to children's vocabulary growth and complex noun phrase constructions.

Morphology

Children's grasp of morphology, the study of how words are formed, is affected by SES. Children of high SES have advantages in applying grammatical rules, such as the pluralization of nouns and adjectives compared to children of low SES. Pluralizing nouns consists of understanding that some nouns are regular and -s denotes more than one, but also understanding how to apply different rules to irregular nouns. Learning and understanding how to use plural rules is an important tool in conversation and writing. In order to communicate successfully that there is more than one dog running down the street, an -s must be added to dog. Research also finds that the gap in ability to pluralize nouns and adjectives does not diminish by age or schooling because low SES children's reaction times to pluralize nouns and adjectives do not decrease.

Phonology

Phonological awareness, the ability to recognize that words are made up of different sound units, is also affected by SES. Children of low SES between the second and sixth grades are found to have low phonological awareness. The gap in phonological awareness increases by grade level. This gap is even more problematic if children of low SES are already born with low levels of phonological awareness and their environment does not foster its growth. Children who have high phonological awareness from an early age are not affected by SES.

Positive outcomes of low SES

Given the large amount of research on the setbacks children of low SES face, there is a push by child developmental researchers to steer research to a more positive direction regarding low SES. The goal is to highlight the strengths and assets low income families possess in raising children. For example, African American preschoolers of low SES exhibit strengths in oral narrative, or storytelling, that may promote later success in reading. These children have better narrative comprehension when compared to peers of higher SES. Since 2012, there has also been some research on the Shift-and-persist model, which attempts to account for the counterintuitive positive health outcomes that can occur in individuals who grow up in low SES families.

Literacy development

A gap in reading growth exists between low SES and high SES children, which widens as children move on to higher grades. Reading assessments that test reading growth include measures on basic reading skills (i.e., print familiarity, letter recognition, beginning and ending sounds, rhyming sounds, word recognition), vocabulary (receptive vocabulary), and reading comprehension skills (i.e., listening comprehension, words in context). The reading growth gap is apparent between the spring of kindergarten and the spring of first-grade, the time when children rely more on the school for reading growth and less on their parents. Initially, high SES children begin as better readers than their low SES counterparts. As children get older, high SES children progress more rapidly in reading growth rates than low SES children. These early reading outcomes affect later academic success. The further children fall behind, the more difficult it is to catch up and the more likely they will continue to fall behind. By the time students enter high school in the United States, low SES children are considerably behind their high SES peers in reading growth.

Home environment

Home environment is one of the leading factors of a child's well-being. Children living in a poor home with inadequate living conditions are more likely to be susceptible to illness and injuries. The disparities in experiences in the home environment between children of high and low SES affect reading outcomes. The home environment is considered the main contributor to SES reading outcomes. Children of low SES status are read to less often and have fewer books in the home than their high SES peers, which suggests an answer to why children of low SES status have lower initial reading scores than their high SES counterparts upon entering kindergarten. Low SES parents are also less involved in their children's schooling. The fact that many students go to school outside of their home to learn does not mean that it is the only determinant of their literacy growth. Parenting at home plays a role in shaping emotional, physical and mental health, all things that are extremely important to educational success in the classroom. This is a crucial factor that must be acknowledged by educators because boundaries such as constant parenting stress and approach to learning, for example, have a major impact on the students' literacy development.

The home environment makes the largest contribution to the prediction of initial kindergarten reading disparities. Characteristics of the home environment include home literacy environment and parental involvement in school. Home literacy environment is characterized by the frequency with which parents engage in joint book reading with the child, the frequency with which children read books outside of school, and the frequency with which household members visited the library with the child. Parental involvement in school is characterized by attending a parent–teacher conference, attending a parent–teacher association (PTA) meeting, attending an open house, volunteering, participating in fundraising, and attending a school event. Resources, experiences, and relationships associated with the family are most closely associated with reading gaps when students' reading levels are first assessed in kindergarten. The influence of family factors on initial reading level may be due to children experiencing little schooling before kindergarten—they mainly have their families to rely on for their reading growth.

Socioeconomic status plays a role in the involvement of certain parents over others. It affects parenting practices and as a result proves to be a strong predictor of child achievement when comparing households. A parent’s involvement in their child's reading literacy performance progress is often overcome by demographic factors such as poverty, racial and ethnic identity, family and parenting stress, and the parent's educational level. Studies show that when parents become involved in reading-related activities with their children outside of school, reading performance, literacy, love for reading and language skills are more likely to improve. Parent involvement in students’ education is a large factor in their literacy achievement, but the way they parent has a large impact on the overall development of the child. These kinds of involvements are often determined by privilege and the level of stress that a parent must endure, especially when of low socioeconomic status. The reading literacy gap has been further exposed by the enhancement of these already existing inequalities. Studies have found a direct link between Family Processes (including parenting stress and discipline practices), Social-Emotional Readiness (including approaches to learning and self control), and Reading Literacy. Although seeming unrelated, the way that a parent interacts with their child and their child's learning at home sets the stage for how well they will be able to improve their reading literacy in school.

The disadvantages of the achievement gap have exposed itself further for students and children as students have been forced to practice remote learning of the 2020 pandemic. Limited access to the correct school resources affects a child's literacy level dramatically, even more so during the switch to online learning, given the combination of decreased parent involvement and access to outdoor play. Low to lower-middle class households had the highest rate of employment change during the pandemic, which includes loss of employment, reduced hours and/or reduced pay. Large historical events like this one have only extenuated and exposed already existing inequities and in turn have negatively affected students of these demographics. The US Department of Labor revealed that layoffs that occurred during the COVID 19 pandemic had the biggest impact on historically minorities groups, which include Black, Latino, low income workers, and women. This means that children of these same working adults experienced disparities as well. In a 2013 report by the US Department of Commerce, it was found that only 55% of African American and 58% of rural households had any internet access in their home. This can be compared to the 74% of white and 81% of Asian American homes that had reliable internet. Comparing this 2013 report to the occurrences existing in 2020 are not very different given that the demographic students still experience this "digital gap" and disproportionate lack in access to the internet and/or technological equipment necessary. Without access to the correct materials at home, including books and digital tools, students cannot perform as well in reading literacy as their more privileged classmates.

Family SES is also associated with reading achievement growth during the summer. Students from high SES families continue to grow in their ability to read after kindergarten and students from low SES families fall behind in their reading growth at a comparable amount. Additionally, the summer setback disproportionately affects African American and Hispanic students because they are more likely than White students to come from low SES families. Also, low SES families typically lack the appropriate resources to continue reading growth when school is not in session. After the long summer break, it is found that the reading literacy gap between middle and lower class students is about 3 months long. This is a substantial amount of skills lost over a period of break from classes that, if not addressed, can grow extremely worse over time. It is especially important to address this issue and create solutions for young students of low SES in order to address the cycle of disadvantages faced by these communities. Studies show that by providing books to disadvantaged students over the summer, the reading achievement dramatically improves for elementary school students. Specifically, providing access to self-selected books consistently over the months of summers is successful in limiting reading setbacks. Many of these students continue to feel discouraged, have less motivation and therefore fall more behind. By providing encouragement through opportunity, there is more chance of future success in literacy development.

Neighborhood influence

The neighborhood setting in which children grow up contributes to reading disparities between low and high SES children. These neighborhood qualities include but are not limited to garbage or litter in the street, individuals selling or using drugs in the street, burglary or robbery in the area, violent crime in the area, vacant homes in the area, and how safe it is to play in the neighborhood. Low SES children are more likely to grow up in such neighborhood conditions than their high SES peers. Community support for the school and poor physical conditions surrounding the school are also associated with children's reading. Neighborhood factors help explain the variation in reading scores in school entry, and especially as children move on to higher grades. As low SES children in poor neighborhood environments get older, they fall further behind their high SES peers in reading growth and thus have a more difficult time developing reading skills at grade level.

In a study by M. Keels, it was determined that when low-income families are moved from poor neighborhoods to suburban neighborhoods, there are reductions in delinquency in children. When comparing different social statuses of families, the environment of a neighborhood turns out to be a major factor in contributing to the growth of a child.

School influence

School characteristics, including characteristics of peers and teachers, contribute to reading disparities between low and high SES children. For instance, peers play a role in influencing early reading proficiency. In low SES schools, there are higher concentrations of less skilled, lower SES, and minority peers who have lower gains in reading. The number of children reading below grade and the presence of low-income peers were consistently associated with initial achievement and growth rates. Low SES peers tend to have limited skills and fewer economic resources than high SES children, which makes it difficult for children to grow in their reading ability. The most rapid growth of reading ability happens between the spring of kindergarten and the spring of first grade. Teacher experience (number of years teaching at a particular school and the number of years teaching a particular grade level), teacher preparation to teach (based on the number of courses taken on early education, elementary education, and child development), the highest degree earned, and the number of courses taken on teaching reading all determine whether or not a reading teacher is qualified. Low SES students are more likely to have less qualified teachers, which is associated with their reading growth rates being significantly lower than the growth rates of their high SES counterparts.

Influences on nonverbal behavior

Children of parents with a high SES tended to express more disengagement behaviors than their peers of low SES. This study by Michael Kraus and Dacher Keltner was published in the December 2008 issue of Psychological Science. In this context, disengagement behaviors included self-grooming, fidgeting with nearby objects, and doodling while being addressed. In contrast, engagement behaviors included head nods, eyebrow raises, laughter and gazes at one's partner. These cues indicated an interest in one's partner and the desire to deepen and enhance the relationship. Participants of low SES tended to express more engagement behaviors toward their conversational partners, while their high SES counterparts displayed more disengagement behaviors. Authors hypothesized that, as SES rises, the capacity to fulfill one's needs also increases. This may lead to greater feelings of independence, making individuals of high SES less inclined to gain rapport with conversational partners because they are less likely to need their assistance in the future.

Economic mobility

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Economic_mobility 

 

Illustration from a 1916 advertisement for a vocational school in the back of a US magazine. Education has been seen as a key to economic mobility, and this advertisement appealed to Americans' belief in the possibility of self-betterment, as well as threatening the consequences of downward mobility in the great income inequality existing during the Industrial Revolution.

Economic mobility is the ability of an individual, family or some other group to improve (or lower) their economic status—usually measured in income. Economic mobility is often measured by movement between income quintiles. Economic mobility may be considered a type of social mobility, which is often measured in change in income.

Types of mobility

There are many different ideas in the literature as to what constitutes a good mathematical measure of mobility, each with their own advantages and drawbacks.

Mobility may be between generations ("inter-generational") or within a person's or group's lifetime ("intra-generational"). It may be "absolute" or "relative".

Inter-generational mobility compares a person's (or group's) income to that of her/his/their parents. Intra-generational mobility, in contrast, refers to movement up or down over the course of a working career.^{[citation needed]} Absolute mobility involves widespread economic growth and answers the question “To what extent do families improve their incomes over a generation?” Relative mobility is specific to individuals or groups and occurs without relation to the economy as a whole. It answers the question, "how closely are the economic fortunes of children tied to that of their parents?" Relative mobility is a zero-sum game, absolute is not.

• Exchange mobility is the mobility that results from a "reshuffling" of incomes among the economic agents, with no change in the income amounts. For example, in the case of two agents, a change in income distribution might be {1,2}->{2,1}. This is a case of pure exchange mobility, since they have simply exchanged incomes. More generally, for a set of incomes x_i, any permutation of the x_i will yield a pure exchange mobility. Measures of inequality (e.g. the Gini coefficient) will not change under pure exchange mobility.
• Structural mobility is mobility that results from a change in the income distribution function without regard to the identity of the agents. For two agents, a change in income distribution might be {1,3}->{2,2}. This case may involve some exchange mobility, depending on one's definition, but there is certainly some structural mobility since it does not involve a simple reshuffling of incomes.
• Growth mobility is mobility that results from an increase in total income. For two agents, a change in income distribution might be {1,2}->{2,4} or perhaps {1,2}->{3,5}. Growth mobility is certainly positive in these cases, since the sum of the incomes increases.

United States

Main article: Socio-economic mobility in the United States

Intergenerational mobility

According to the 2007 "American Dream Report" study, "by some measurements"—relative mobility between generations—"we are actually a less mobile society than many other nations, including Canada, France, Germany and most Scandinavian countries. This challenges the notion of America as the land of opportunity." Other research places the U.S. among the least economically mobile countries.

Another 2007 study ("Economic Mobility Project: Across Generations") found significant upward "absolute" mobility from the late 1960s to 2007, with two-thirds of those who were children in 1968 reporting more household income than their parents (although most of this growth in total family income can be attributed to the increasing number of women who work since male earnings have stayed relatively stable throughout this time).

However, in terms of relative mobility it stated: "contrary to American beliefs about equality of opportunity, a child's economic position is heavily influenced by that of his or her parents." 42% of children born to parents in the bottom fifth of the income distribution ("quintile") remain in the bottom, while 39% born to parents in the top fifth remain at the top. Only half of the generation studied exceeded their parents economic standing by moving up one or more quintiles. Moving between quintiles is more frequent in the middle quintiles (2–4) than in the lowest and highest quintiles. Of those in one of the quintiles 2–4 in 1996, approximately 35% stayed in the same quintile; and approximately 22% went up one quintile or down one quintile (moves of more than one quintile are rarer). 39% of those who were born into the top quintile as children in 1968 are likely to stay there, and 23% end up in the fourth quintile. Children previously from lower-income families had only a 1% chance of having an income that ranks in the top 5%. On the other hand, the children of wealthy families have a 22% chance of reaching the top 5%.

Intragenerational mobility

According to a 2007 study by the US Treasury Department, Americans concerned over the recent growth in inequality (after-tax income of the top 1% earners has grown by 176% percent from 1979 to 2007 while it grew only 9% for the lowest 20%) can be reassured by the healthy income mobility in America: "There was considerable income mobility of individuals [within a single generation] in the U.S. economy during the 1996 through 2005 period as over half of taxpayers moved to a different income quintile over this period".

Other studies were less impressed with the rate of individual mobility in the United States. A 2007 inequality and mobility study by Wojciech Kopczuk and Emmanuel Saez and 2011 CBO study on "Trends in the Distribution of Household Income, found the pattern of annual and long-term earnings inequality "very close", or "only modestly" different. Another source described it as the mobility of "the guy who works in the college bookstore and has a real job by his early thirties," rather than poor people rising to middle class or middle income rising to wealth.

Relative vs. absolute

There are two different ways to measure economic mobility: absolute and relative. Absolute mobility measures how likely a person is to exceed their parents' family income at the same age. Research by the Pew Economic Mobility Project shows that the majority of Americans, 84 percent, exceed their parents' income. However, the size of absolute income gains is not always enough to move them to the next rung of the economic ladder.

A focus on how Americans' rank on the income ladder compares to their parents, their peers, or even themselves over time is a measure of relative mobility. The Pew Economic Mobility Project's research shows that forty percent of children in the lowest income quintile remain there as adults, and 70 percent remain below the middle quintile, meaning 30% moved up two quintiles or more in one generation.

Worldwide

In recent years several large studies have found that vertical inter-generational mobility is lower in the United States than in most developed countries. A 1996 paper by Daniel P. McMurrer, Isabel V. Sawhill found "mobility rates seem to be quite similar across countries." However a more recent paper (2007) found a person's parents is a great deal more predictive of their own income in the United States than other countries. The United States had about 1/3 the ratio of mobility of Denmark and less than half that of Canada, Finland and Norway. France, Germany, Sweden, also had higher mobility, with only the United Kingdom being less mobile.

Economic mobility in developing nations (such as those in Africa) is thought to be limited by both historical and global economic factors. Economic mobility is everywhere correlated with income and wealth inequality.

Men and women

Women in their 30s have substantially higher incomes today than their counterparts did in their parents' generation. Between 1974 and 2004, average income for women in their 30s has increased almost fourfold. This is a stark contrast to the growth in income of their male counterparts. The average income of men in their 30s has increased from 31,000 in 1964 to 35,000 in 2004, an increase of only 4,000.

However, much of this can be attributed to employment rates. The employment rate of women in their 30s has increased from 39% in 1964 to 70% in 2004; whereas, the rate of employment for men in this same age group has decreased from 91% in 1964 to 86% in 2004. This sharp increase in income for working women, in addition to stable male salaries, is the reason upward economic mobility is attributed to women.

See: De-industrialization crisis

Black and white families

Data shows substantial social segregation correlating with economic income groups. However, social connectedness to people of higher income levels is a strong predictor of upward income mobility.

Average income for both White and Black families has increased since the 1970s. However, average income for White families in their 30s has increased from $50,000 to $60,000 from 1975 to 2005, compared to an increase from $32,000 to $35,000 for Black families of the same age over the same period. So in addition to receiving a lower average income, its growth is also less for Black families (10% growth) than their White counterparts (19% growth). One way this can be explained is that even though marriage rates have declined for both races, Blacks are 25% less likely to be in a married couple. However, Blacks also have less economic mobility and are less likely to surpass their parents' income or economic standing than Whites. Two of three White children born into families in the middle quintile have achieved a higher family income than their parents. Conversely, only one of three Black children born into families in the middle quintile has achieved a higher family income than their parents. On average, Black children whose parents were in the bottom or second quintile do exceed their parents' income, but those whose parents were in the middle or fourth quintile actually have a lower income than their parents. This is a very large difference compared to Whites, who experience intergenerational income growth in every quintile except the highest. This shows that in addition to lower wages with less growth over time, it is less likely for Black families to experience upward economic mobility than it is for Whites.

Education

It is a widespread belief that there is a strong correlation between obtaining an education and increasing one's economic mobility. In the United States, the education system has always been considered the most effective and equal process for all individuals to improve one's economic standing. Despite the increasing availability to education for all, family background continues to play a huge role in determining economic success. To individuals who do not have or cannot obtain an education, the greater overall levels of education can act as a barrier, increasing their chance of being left behind at the bottom of the economic or income ladder. In this regard, education policy that allocates high ability students from lower social economic background to quality schools can have a large impact on economic mobility.

Studies have shown that education and family background has a great effect on economic mobility across generations. Family background or one's socioeconomic status affects the likelihood that students will graduate from high school or college, what type of college or institution they will attend, and how likely they are to graduate and complete a degree. According to studies, when split into income quintiles including the bottom, second, middle, fourth and top, adult children without a college degree and with parents in the bottom quintile remained in the bottom quintile. But if the adult children did have a college degree, there was only a 16% chance that they would remain at the bottom of the quintile. Therefore, it was proven that education provided an increase in economic status and mobility for poorer families. Not only does obtaining a college degree make it much more likely for individuals to make it to the top two quintiles, education helps those who were born in the top quintiles to remain in the top quintiles. Therefore, hard work and increasing education from those who are born into the lower quintiles can boost economic status and help them move ahead, but children born into wealthier families do seem to have the advantage. Even when the likelihood of attending college is ignored, studies have shown that out of all the students that enroll in college, socioeconomic status or family background still has an effect on graduation rates with 53% of those from the top quintile receiving bachelor's degrees along with 39% from the middle and 22% from the bottom quintile. According to the 2002 US Census, students can expect to earn on average about $2.1 million with a bachelor's degree over the course of their working career. That is almost $1 million more than what individual's without a college degree can expect to earn.

Considering that inflation rates have not kept up with increasing tuition rates, disadvantaged families have a much harder time affording college. Especially considering the increased competition for college admittances at public schools, students from lower economic quintiles are at an even greater disadvantage. Tuition rates over the past ten years has risen 47% at public universities and 42% at private universities. While having to take out more loans and work jobs while taking classes, students from lower income quintiles are considering college to be “a test of their endurance rather than their intelligence”.

By obtaining an education, individuals with low economic status can increase the income potential and therefore earn more than their parents and possibly surpass those in the upper income quintiles. Overall, each additional level of education an individual achieves whether it be a high school, college, graduate, or professional degree can add greatly to income levels. On the other hand, there are reports that disagree with the idea that individuals can work hard, obtain an education and succeed because there is the notion that America is actually getting poorer and actually more likely to stay poor as compared to any other western country. Some claim that the idea of the “American Dream” is starting to fade since the middle-class family income has remained constant since 1973. But upward mobility clearly still exists. One study claims that economic mobility is 3 times stronger in Denmark, 2.5 times higher in Canada, and 1.5 times higher in Germany as compared to the United States.

Immigration

According to the U.S. Census Bureau, the number of legal immigrants has been rising steadily since the 1960s. The number has increased from about 320,000 to almost a million per year. About 500,000 illegal immigrants also remain in the United States each year. People immigrate to the United States in hopes of greater economic opportunities and most first generation immigrants experience a boost in their income from the American economy. But since most do not have an education, their wages quickly begin to fall relative to non-immigrants. According to studies, there is a great upward jump in economic mobility from the first to the second immigrant generation because of education. These second generation immigrants exceed the income levels of the first generation immigrants as well as some non-immigrants.

Through intergenerational mobility research, the mobility of immigrants and their children from different nations can be measured. Considering relative wages from male workers from certain nations in 1970 to second generation male workers in 2000, conclusions can be drawn about economic mobility. In 1970, if immigrants had come from an industrialized nation, then their average wages tended to be more than the average wages of non-immigrant workers during that time. In 2000, the second generation workers had experienced a downfall in relative mobility because their average wages were much closer to the average wages of a non-immigrant worker. In 1970, for the immigrant workers migrating from less industrialized countries, their average wages were less than the average wages of non-immigrant workers. In 2000, the second generation workers from less industrialized nations have experienced an increase in relative mobility because their average wages have moved closer to those of non-immigrants.

By computing the intergenerational correlation between relative wages of first and second generation workers from the same country a conclusion was made regarding whether or not first generation immigrants influence the wages of the second generation immigrants. This computation was also reported for native-born first and second generation American families. The study found that both immigrants and natives pass along almost exactly the same level of economic advantages or disadvantages to their offspring. These conclusions predict diminishing correlations in wages from the first and second generations if change in the level of education for each immigrant is considered. Since the majority of immigrants have low levels of education, it may be increasingly difficult for future second generation immigrants to ever surpass the average wages of non-immigrants.