SES and CHAOS as environmental mediators of cognitive ability: A longitudinal genetic analysis☆
Introduction
It is now commonly accepted that both genetic and environmental influences contribute to individual differences in general cognitive ability. On average, about 50% of the variance in intelligence is due to genetic differences, whereas shared environmental influences (factors that make family members more similar) account for around a quarter of the variance, with the remaining 25% of the variance accounted for by nonshared environmental influences and error (Jensen, 1981, Plomin et al., 2001, Vernon, 1997). However, these estimates are not static. Genetic influences on intelligence increase as a function of age, starting at around 40% in childhood, to 60% in adulthood, to around 80% by old age (McGue et al., 1993, Pederson et al., 1992). Conversely, shared environmental influences are most important in early childhood and decline to zero by adolescence; thus, studies have begun to identify measures of the environment that affect intellectual development early in life. Previous phenotypic research on cognitive development has consistently shown that deprived family environments (e.g., low quality of home environment, poor non-maternal care, low family income, low mother aptitude, etc.,) can lead to increasingly poor cognitive functioning (Bradley and Caldwell, 1980, Carlson and Corcoran, 2001, Feinstein and Bynner, 2004, Neiss and Rowe, 2000, NICHD Early Child Care Research Network, 2001, Petrill et al., 2004, Petrill et al., 2004). These findings are consistent across age and ethnic groups (Bradley et al., 1989). More specifically, children from low-SES groups are more likely to experience academic problems, such as low test scores, increased course failure, and retaking of grades, as well as lower levels of educational attainment (Ellis and Bonin, 2003, Gutman et al., 2003, Jimerson et al., 2000, McLoyd, 1998, Rumberger, 1987).
Beyond SES, researchers have begun to look for other characteristics of the home and family environment that account for individual differences in cognitive functioning (van den Oord & Rowe, 1999). One such environmental mediator under recent study is the concept of “environmental confusion” (i.e., noise, crowding, and traffic within the home), which can be measured by a parental questionnaire entitled “The Confusion, Hubbub, and Order Scale” (CHAOS; Matheny, Wachs, Ludwig, & Phillips, 1995). Phenotypic studies have shown that adolescents from low income families experience a higher amount of environmental confusion at home, and this CHAOS mediates the SES effects of socioemotional development (Evans, Gonnella, Marcynyszyn, & Salpekar, 2005). Additionally, Dumas et al. (2005) showed that the CHAOS scale measures a construct that is distinct from known destructive social and psychological mediators (e.g., parenting stress, neighborhood attributes, etc.).
While these phenotypic studies have provided an important foundation, genetically sensitive designs have the power to extend our understanding of the etiology of the impact of measured aspects of the home environment on early cognitive development. In particular, Petrill, Pike et al. (2004) examined whether SES and CHAOS account for a proportion of the shared environmental variance associated with general cognitive ability in a longitudinal study of twins assessed at ages 3 and 4 years. SES and CHAOS mediated a small but significant portion of the shared environment of the cognitive measures at both ages. Specifically, the total measured environment accounted for about 6% of the total variance at ages 3 and 4 years for both verbal and nonverbal measures of cognitive ability. Also, this study showed that CHAOS predicted independent shared environmental influences on cognitive ability separate from SES, and therefore influenced a separate portion of the shared environmental variance of the cognitive measures. Furthermore, this study showed that SES and CHAOS contributed to stability in cognitive performance from ages 3 to 4 years.
However, results in Petrill, Pike et al. (2004) were based on the parent-report component of the Twins Early Development Study (TEDS), where all data (CHAOS, SES, and cognitive ability) was obtained from parent-report questionnaires. Thus, it is unclear whether the shared environmental overlap between SES, CHAOS, and cognitive ability was a function of “true” shared environmental influences or a function of the fact that all measures came from the same informant (generally the mother). Moreover, it is unclear whether the effects of SES and, in particular, CHAOS are generalizable beyond the very young age of the TEDS sample.
For this reason, the goal of the current study was to examine the links between SES, CHAOS and cognitive ability in a sample of kindergarten and 1st grade twins using the Western Reserve Reading Project (WRRP; Petrill, Deater-Deckard, Thompson, DeThorne, & Schatschneider, 2006). The current study has three primary goals. First, we examine whether SES and CHAOS account for a significant portion of the shared environmental variance that affects general cognitive ability. Second, we examine whether SES and CHAOS account for a portion of the longitudinal stability in general cognitive ability. Finally, we test whether CHAOS explains a portion of shared environmental variance affecting cognitive ability independent from SES. Given Petrill, Pike et al. (2004) and the larger literature, we hypothesize that the SES and CHAOS will each predict a significant and independent proportion of the shared environmental variance in cognitive ability, and the effects will be stable across the measured years.
Section snippets
Participants
Participants were drawn from the Western Reserve Reading Project (WRRP), an ongoing study of 350 twin pairs. WRRP is a longitudinal genetically sensitive project looking at the development of reading and other cognitive skills across four annual assessments. The twins are from the Greater Cleveland, Columbus and Cincinnati metropolitan areas, with a small minority coming from other areas of Ohio and Western Pennsylvania. The twins were recruited primarily through school nominations (n = 273
Socioeconomic status
Socioeconomic status (SES) was measured using mothers' highest level of education reported in the initial demographic questionnaire. This was a scale from 1–9 (1 = less than high school, 9 = post-graduate degree).
Chaos
Chaos in the home was measured via maternal questionnaire in both waves 1 and 2. This questionnaire was derived from the “Confusion, Hubbub, and Order Scale” (CHAOS; Matheny et al., 1995, Petrill et al., 2004). The questionnaire consisted of six questions concerning the level of chaos in
Phenotypic analysis
Means and standard deviations for SES, wave 1 and wave 2 CHAOS, and wave 1 and wave 2 Stanford-Binet SAS are presented in Table 2. Pearson correlations between wave 1 and wave 2 Stanford-Binet SAS, SES, and wave 1 and 2 CHAOS, are presented in Table 3. Wave 1 and wave 2 CHAOS were reverse coded so that all variables were in the same direction (high score = positive). Wave 1 Stanford-Binet SAS was highly correlated to wave 2 Stanford-Binet SAS (r(409) = .72, p < .01). Similarly, wave 1 and wave 2
Discussion
The goals of this study were to: 1) examine whether SES and CHAOS accounted for a significant portion of the shared environmental variance in general cognitive ability; 2) examine whether SES and CHAOS accounted for a portion of the longitudinal stability in general cognitive ability; and 3) examine whether CHAOS explained a portion of shared environmental variance in cognitive ability independent from SES.
As expected from previous research, both genetic and shared environmental influences were
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This work was supported by NICHD grant HD38075 and NICHD/OSERS grant HD47176. The authors also wish to thank the twins and their families for making this research possible.