Relationship between Socioeconomic Status and Intelligence

Socioeconomic status (SES) can be defined as the social and economic standing of an individual or group, relative to others. Components that typically determine SES are occupation, education and income. The Equality Trust (2017) presented figures that showed the top one-fifth of households in the United Kingdom have 40% of national income. In contrast, the bottom fifth have just 8%. Moreover, the Office for National Statistics (ONS, 2014) reported figures for the period of 2012 to 2014 where the wealthiest 10% of households owned 45% of household wealth, whereas the least wealthy 50% owned less than 9%. There is strong reason to suggest that SES, and the material circumstances in which people live, can profoundly influence various aspects of an individual’s life, including physical and psychological health. For instance, Stringhini et al. (2017) conducted a multi-cohort study and meta-analysis on 1.7 million men and women, revealing that participants with low SES had greater mortality compared with those with high SES. Additionally, between the ages of 40 to 85 years, low SES was also related with a 2.1-year decrease in life expectancy. Therefore, it is unsurprising that SES is often associated with another important aspect of life - intelligence.

McCall (1981) illustrated the association between SES and cognitive performance beginning as early as infancy, and an abundance of research shows children from disadvantaged backgrounds, on average, produce lower scores than their high SES counterparts on intelligence tests (Bradley & Corwyn, 2002; Strenze, 2007). Adoption studies show that adopted children (who have been moved from low SES homes to high SES homes) typically score 12 points or higher than comparison children, further strengthening the association between SES and intelligence. There is also evidence indicating that these differences may not diminish over the years – in fact, they can widen. Stumm and Plomin (2015) found that in their study, children of the lowest and highest SES backgrounds were divided by 6 IQ points at the age of 2 years. At the age of 16 years, this gap had almost tripled. Not only do low SES children perform worse in intelligence tests, this disparity can intensify over time. There are a number of possible reasons for the discrepancies mentioned above, such as those that index SES (education, occupation and income), as well as how these may affect access to resources, socialization and parental attitudes.

The measures used in the Health Examination Survey (family income, maternal education, paternal education) were found to predict intellectual attainment, with education standing out as the best predictor (Mercy & Steelman, 1982). On the other hand, White (1982) noticed that family income was the highest single correlate of academic achievement amongst traditional measures of SES. Parcel and Menaghan (1990) observed that mothers in occupations, where a variety of tasks and problem-solving opportunities were offered, provided greater warmth and support, as well as more stimulating materials. As a result, their children were able to develop more advanced verbal competence. These contrasting findings illustrate the existing debate surrounding which aspects of SES are connected to cognitive development the most strongly. However, DeGarmo et al. (1999) demonstrated that each SES indicator correlated with better parenting, ultimately affecting school achievement through activities that build useful skills and establishing appropriate school behavior through effective discipline. Other studies have suggested that the input of family income and parental education are dependent on the number of siblings present in the household (Anastasi, 1956; Walberg & Marjoribanks, 1976). Thus, the relationship between SES and intelligence is quite complex and different aspects of SES play different roles in the development of cognitive skills.

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Other ways in which SES may be related to intelligence go beyond the specific indices of SES and are concerned with how SES translates to the immediate environment. Bradley and Caldwell (1980) discovered a substantial relationship between the home environment during an infant’s first year of life and their IQ at age 3. Data from the National Longitudinal Survey of Youth and the National Household Education Survey (Bradley, Corwyn, Burchinal, McAdoo & García Coll, 2001) revealed that children from poorer families have less access to a wide variety of different recreational and learning materials. They are also less likely to have access to cultural resources, e.g., going on school trips, visiting a museum or attending theatrical performances. Inevitably, this limited access will have an impact on the cognitive growth of children in low-SES households and there has been evidence for this. Whitehurst and Lonigan (1998) note that economically disadvantaged children are slower at acquiring language skills and display delayed letter recognition and phonological sensitivity, putting them at greater risk of reading difficulties. Moreover, an association between the frequency of pre-school shared reading and individuals has been found in the reading, spelling and IQ scores of 550 13-year-olds (Stevenson & Fredman, 1990). These findings may explain why low-SES children frequently perform worse than their higher-SES counterparts in tests designed to measure intelligence, e.g., standardized tests. If the children cannot fully understand the questions or what is being asked of them, it cannot be expected that they will perform better than those who do. In terms of explanations that have been proposed to account for the strong relationship between SES and intelligence, limited access to cognitively stimulating materials and experiences due to economic inequalities is one of the most convincing.

Socioeconomic circumstances of certain households also impact the extent to which physiological needs are met, which may subsequently influence intelligence. It has been established that prolonged malnutrition during childhood can have long-term effects on intellect. Pollitt and colleagues (1993) conducted an intervention study with a sample of preschool children in two Guatemalan villages where undernourishment is common. The sample was given ad-libbed access to a protein dietary supplement for several years. When tested 10 years later, compared to controls, many of these children (who were from the poorest socioeconomic levels) scored significantly higher on school-related achievement tests. However, it is possible that the effects detected are indirectly linked – malnourished children are less motivated to learn, less active in exploration and less responsive to adults than more nourished peers. Despite the fact that the extent of malnourishment observed in this study rarely occurs in Western societies, nutritional influences on intelligence are still possible. Studies on ‘micro-nutrients’ have illustrated that experimental groups of children, who were given vitamin/mineral supplements, showed test score gains significantly surpassing those of the controls, who were given placebos (Schoenthaler, Amos, Eysenck, Peritz & Yudkin, 1991). Qian et al. (2005) also found deficiencies in iodine predicted lower IQ scores. In a meta-analysis, they discovered that children in iodine-deficient areas who were given a supplement gained approximately 8.7 IQ points. The results of these studies confirm that not only does inadequate dietary intake result in poor health, but it can also have long-term effects on intelligence. This provides further explanation as to why individuals from high-SES backgrounds perform better on intelligence tests - they are able to purchase goods and services that are essential for health, whereas those from low-SES backgrounds are not.

Parental attitudes, expectations and styles of interacting with children are also a part of an observed connection between SES and intelligence. High-SES parents tend to engage their children in more conversations, read to them more and provide more teaching experiences (Shonkoff & Phillips, 2000). The conversations typically involve more responsiveness and include greater efforts in eliciting child speech (Hoff-Ginsberg & Tardif, 1995). Conversely, low-SES parents are less likely to purchase reading and learning materials and less likely to take their children to educational/cultural events (Bradley et al., 2001). As a result of the dissimilarities in parenting practices, there is a strong difference between SES and children’s intellectual and academic performance. Nonetheless, the relationship between parental attitudes and intelligence is not as clear-cut as it seems; both the amount of crowding and the number of siblings in the home can result in differences in allocation of time for each child (Bradley et al., 1994; Mercy & Steelman, 1982; Walberg & Marjoribanks, 1976). Both of these factors can lead to high amounts of distress and distraction, which can result in fewer, less-rich parent-child exchanges.

All of the aforementioned reasons for the strong association between SES and intelligence are all primarily environmental influences. However, there is always a genetic aspect to the effects of the environment. The heritability of cognitive ability in childhood is well-documented (McGue, Bouchard, Iacono, & Lykken, 1993; Plomin, 1999) and research has also shown that the degree to which intelligence is heritable differs between socioeconomic groups. A study conducted by Fischbein (1980) used a Swedish sample of 87 MZ and 126 DZ pairs of 12-year-old schoolchildren. For both verbal ability and inductive reasoning, heritability was highest in the high-SES group and lowest in the low-SES group. Additionally, Turkheimer, Haley, Waldron, D’Orofrio and Gottesman (2003) demonstrated that SES modified heritability of IQ in young children. It is suggested that, in impoverished families, 60% of the variance in IQ was accounted for by the environment, whereas the contribution of genes was close to zero. In affluent families, the result is almost exactly the reverse. It would be naïve to interpret SES as solely an environmental variable of intelligence, particularly when all environmental effects involve the genes or structures to which the genes have contributed. Thus, the effect of SES on the genetic influences of IQ makes it the most conclusive and strong explanation for the relationship between SES and intelligence.

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