As well as the level of socioeconomic inequality, in this empirical chapter I was also concerned about whether the level of socioeconomic inequality has changed over time. Although measuring the degree of socioeconomic inequality in cognitive ability has been a relatively prominent research area for a number of years, very few published studies have explicitly attempted to measure the changes in the socioeconomic inequalities in cognitive ability across time, with the few that have generating mixed results. To my knowledge, only five empirical studies have either directly or indirectly examined whether socioeconomic inequalities in cognitive ability have significantly changed over time in the UK4.
Blanden and Machin (2007) considered the indirect relationship between parental income and a range of child outcomes (including cognitive test scores, non-cognitive ability and degree attainment) in a variety of British datasets (NCDS, BCS, MCS, British Household Panel Survey) in the context of changing social mobility. Using both OLS and 2SLS models, the authors found little evidence that the relationships between these intermediate variables had significantly changed from the older studies (for example the NCDS and BCS) to the more recent MCS and British Household Panel Survey. However, the results from the 2SLS models should be treated with caution, due to the fact that the variables used to instrument income were measure of parental education, employment status and housing tenure at age 16. Although it is almost certain that these variables will be highly correlated with household income, it is extremely unlikely that these variables will be exogenous to the main equation, as one may expect a large vector of unobservable factors to be related to both education level and income, such as underlying ability.
Schoon (2010) investigated the relationship between family socioeconomic background (measured by parental occupation), general cognitive ability and academic attainment, using the 1946 National Survey of Health and Development, the NCDS and the BCS. General measures of cognitive ability were calculated through principal components analysis (PCA) and SEM methods, with the author finding that the association between social background and cognitive ability marginally increased between 1946 and 1970 cohorts, despite the
4 Reardon (2011), Duncan and Murnane (2011) and Michelmore and Dynarski (2017) have examined the
widening achievement gap between the rich and poor in the USA in the past fifty years, whereas Maika et al., (2013) have investigated changes in the inequality of cognitive ability in an Indonesian sample from 2000-2007. However, due to the different institutional contexts of these studies and the interests of space, these studies are not discussed in greater detail.
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introduction of the 1944 Education Act aimed at increasing educational opportunities irrespective of socioeconomic background.
Blanden and Machin (2010) compared the inequality in cognitive ability between the second and third waves of the MCS with the children of respondents of the original NCDS and BCS birth cohorts. In all cohorts, the authors found a significant association between parental income levels and child cognitive ability, with these income related cognitive ability gaps once more relatively stable over time. Although this cross-cohort comparison allowed the authors to compare children using the exact same cognitive test, the time range examined was relatively short (1991-2005) and the children of the NCDS and BCS samples were relatively small in comparison to the MCS sample.
The most prominent study in this small literature is that of Gregg and Macmillan (2010), who analysed the relationship between standardised family income and cognitive ability across groups of cohorts from both the late 1950s (NCDS) and the 1990s (the ALSPAC study and a sample from the BHPS). In contrast to Blanden and Machin (2007), using OLS methods the authors found a small, yet consistent narrowing of the social gradient in the relationship between family background and cognitive ability between the older cohorts (such as the NCDS and the BCS) and the newer youth cohorts (such as the ALSPAC and BHPS). The authors attributed this change of relationship to changes in the UK education system over time, such as increased spending on education as a share of GDP.
The most recent study to investigate the changing relationship of socioeconomic disparities in child cognitive ability over time was Connelly (2013), who used the NCDS, BCS and MCS to examine changes over time using SEM methods. Using a latent measure of SES calculated using information on parental occupational classification and parental education, the author found no significant change in the degree of socioeconomic inequality between the three cohort studies. The latent measure of cognitive ability used in the study was a combined measure of cognitive ability created using PCA. Although PCA methods allowed the authors to combine various cognitive test into a single measure, and have also been used in several high profile publications utilising the British cohort studies (for instance Feinstein 2003 and Galindo-Rueda and Vignoles 2005), this combined measure does not take into account that different cognitive tests may have radically different socioeconomic distributions, and therefore may underestimate or overestimate the level of socioeconomic inequality, depending on the measure of cognitive ability in question.
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Although they did not explicitly examine the socioeconomic distribution of child cognitive ability over time, at this point it is also worth mentioning the recent studies of Goisis et
al,.(2017a) and Goisis et al,.(2017b), as both studies utilised the NCDS, BCS and MCS to
compare child cognitive test scores over time. In the first of their studies (Goisis et al,. 2017a), the authors examined the changing relationship between birth weight and child cognitive ability (as measured by verbal ability at age 10/11) in the three separate cohort studies. Using pooled linear regression models, results showed a marginal narrowing of the relationship between birth weight and child cognitive ability over time from the NCDS and BCS to the MCS.
In the second of their studies (Goisis et al., 2017b), the authors examined the changing relationship between maternal age and child cognitive ability (once more measured by verbal ability at age 10/11) in the three separate cohort studies. Again using pooled linear regression methods, results showed that the relationship changed from negative in the NCDS and BCS to positive in the MCS, potentially driven by changes in parental
characteristics relative to maternal age such as levels of education and household income. Given the previous literature, in this chapter I contribute to the applied empirical literature in two main ways. Firstly, I contribute to the literature investigating the relationship
between SES and child cognitive ability, with this being the third empirical study (after Maika
et al., 2013 and Vallejo-Torres et al., 2014) to apply the CI methodology and use the
decomposition methods of Wagstaff et al., (2003) in the context of child non-health outcomes such as cognitive ability.
Secondly, I contribute to the small literature comparing socioeconomic inequalities in child cognitive ability over time, with this being the first to use dominance analysis, which allows for the difference in the level of socioeconomic inequality to be estimated, given the different sampling structures of the NCDS and the MCS. Although Connelly (2013) also examined the relationship between SES and child cognitive ability over time using both the NCDS and the MCS datasets, this chapter differs from that study in a number of ways. For instance, rather than latent measures of SES and child cognitive ability, I use two distinct measures of SES (parental occupation classification and household income) and a range of measures of cognitive ability in empirical analysis. Furthermore, I use a variety of empirical methodologies related to the CI that are not explored by Connelly (2013).
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In examining the socioeconomic gradient in child cognitive ability over time, I am also the first to indirectly empirically test the hypothesis proposed by Mackenbach (2012), who has argued that the changing composition of the social strata may help explain the paradoxical persistence of socioeconomic inequalities in health in developed countries such as the UK. 4.3 Theoretical Considerations
There are three main hypotheses that I test in the empirical analysis:
a) Are there socioeconomic inequalities in child cognitive ability in the NCDS, BCS and MCS?
b) Has the level of income related socioeconomic inequality in child cognitive ability changed significantly from children born in 1958 to children born in 2000?
c) Have the contributing factors to the level of income related socioeconomic inequality in child cognitive ability changed significantly from children born in 1958 to children born in 2000?
In sub-sections 4.3.1 and 4.3.2 below, I outline the theoretical reasoning behind these hypotheses.