October 12, 2004 | Volume 2 | Number 21
Socioeconomic Status and IQ
Socioeconomic Status and IQ
What is the relationship between socioeconomic status (SES) and genetic effects on intelligence as measured by IQ?
Ever since James Coleman and colleagues released the findings of the Equality of Educational Opportunity Study in 1966, the complex interaction between schools, socioeconomic status, and student achievement has been a point of contention among researchers. Some educators have argued that poverty's power in regards to student achievement is so strong that schools may find it difficult, if not impossible, to overcome its negative effects. Others have argued that schools have the ability to overcome the disadvantages imposed upon students by poverty and that their efforts in that direction can make a significant difference in student achievement. Research from the Tennessee Value-Added Assessment System, for example, strongly supports the claim that teachers can have a powerful effect on student achievement, even when taking into account the barriers presented by poverty.
Two other important issues play a role in the argument about school effects on student achievement. The first, the relevance of school resource levels to increases in student achievement, is frequently debated and has been the focus of past ResearchBriefs (Volume 2, Number 6). Although some researchers have found little correlation between funding and achievement, others have found a very strong relationship that spans many studies. The relationship between adequate school resources and the ability of high-poverty students to succeed educationally has also been the focus of two ASCD EDPolicy Updates (October 2003 and September 2004) examining adequacy lawsuits across the nation.
The second issue, and one that underlies all of the other issues in this debate, is the extent to which intelligence (as measured by IQ) is influenced by genetics and by environment. Previous research on intelligence has been mixed. Some studies, particularly those correlating the intelligence of twins and adoptees with their biological and adoptive parents, have suggested large genetic effects, whereas others comparing the mean IQ of children removed from poverty with that of their parents and impoverished siblings have found large differences due to environmental factors. As highlighted in the study examined here, however, the issue of intelligence is much more complex than the simplicities of an either/or comparison.
Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, and Irving Gottesman conducted the study highlighted in this issue of ResearchBrief (see below for full citation). The researchers used data from the National Collaborative Perinatal Project, which followed 48,197 mothers and their children (59,397) from birth to age 7. The project—which recruited participants from 12 urban hospitals—gathered a wealth of data, including the children's socioeconomic status at birth and at age 7, as well as their scores (at age 7) on three subtests of the Wechsler Intelligence Scale for Children—Verbal IQ, Performance IQ, and Full-Scale IQ. The Turkheimer and colleagues analysis was constructed using data from 319 twins in the study (out of 623 twin births, only 320 had complete data, and one of those was excluded as an outlier due to brain damage at birth). The study population included a high percentage of impoverished and minority families, allowing a different perspective than that taken by past studies, which generally included only a limited sample of high-poverty individuals. Twenty-five percent of household heads were not educated past 9th grade, and 25 percent of families had incomes below the poverty level.
The study examined variation in scores between fraternal and identical twins on the Wechsler Intelligence Scale for Children,1
and researchers used a complex set of statistical calculations to look at the interaction between socioeconomic status and the three components of intelligence traditionally associated with variance in IQ: variation in genotype, shared environment (the environment of siblings raised in the same family), and nonshared environment (the unique experiences of each individual). The researchers used a 95 percent confidence interval to minimize the risk of sampling error.2
In general, as socioeconomic status increased, the degree of environmental influence on measured IQ scores decreased. For the most impoverished families, almost 60 percent of the variability in scores was explained by environmental differences, whereas the percentage of variation in scores attributable to genetic difference was essentially zero. For the high-SES grouping, almost 90 percent of the variance in scores was explained by genetic differences.
The Bottom Line
The effect of environment on the IQ of young children can be significant, particularly for children living in poverty. As the influence of poverty decreases, the importance of environmental conditions as a limiting factor on intelligence also decreases. By addressing the environmental issues created by poverty, it may be possible to weaken the link between low socioeconomic status and poor student performance on IQ (and other) tests.
This study focused on 7-year-old children, particularly those living in poverty.
As noted by the authors, although the study focused on differences in socioeconomic status, both genetic and SES differences between high- and low-poverty groups are likely. The design of this study precludes the ability to determine the effect of any specific poverty-related environmental factors on intelligence (as identified by IQ assessments). The nature of IQ testing is controversial, and great debate exists about whether the complex range of abilities encompassed by “intelligence” can be quantified by a single measure. The findings were significant on Performance IQ and Full-Scale IQ measures, but were not significant on the Verbal IQ (although the Verbal IQ results fell in the predicted direction). Further, although studies of twins are often used to determine the likelihood that outcomes are the result of genetic or environmental differences, some researchers question the conclusiveness of such studies (see “A second look at twin studies” for further discussion).
Turkheimer, E., Haley, A., Waldron, M., D'Onofrio, B., & Gottesman, I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science 14(6). Retrieved September 20, 2004 from
http://www.blackwell-synergy.com/links/doi/10.1046/j.0956-7976.2003.psci_1475.x/abs. Free sample issue requires registration.
A second look at twin studies
American Psychological Association
Parsing the Achievement Gap
Educational Testing Service
What Research Says About Unequal Funding for Schools in America
EDPolicy Update, October 2003
EDPolicy Update, September 2004
Primer on 95% Confidence Intervals (PDF)
American College of Physicians
Researchers often study twins to determine the role of genetics and environment in human development. For example, if a measure under study is the result of a genetic difference, researchers would expect to find little or no variation between identical twins; if the outcome is due to environmental influences, however, then researchers would expect to find similar variation between identical and fraternal twins. Outcomes that are the result of both environmental and genetic differences would result in more variance between fraternal twins than between identical twins. This methodology relies on a number of assumptions that some researchers have questioned. See “A second look at twin studies” for further discussion of some of these issues.
A confidence interval is a statistical procedure used to determine the likelihood that, were the study repeated multiple times, the measured outcome would reflect the true measure 95 percent of the time.
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Dan Laitsch serves as ASCD's consultant editor for ResearchBrief. Laitsch is an assistant professor in the Faculty of Education at Simon Fraser University in British Columbia, Canada, and is coeditor of the International Journal for Education Policy and Leadership.
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