Chaos in the home and socioeconomic status are associated with cognitive development in early childhood: Environmental mediators identified in a genetic design
Introduction
For decades, socialization theorists have attempted to draw links between aspects of the environment and early cognitive development (see Wachs, 1996). By far, the most common approach has been to identify educational, behavioral, or societal environmental risk and protective factors that are correlated with cognitive development. These studies have been influential in demonstrating that family-level risk factors have a negative impact on children's cognitive functioning, independent of individual-level factors (e.g., Leventhal & Brooks-Gunn, 2000).
One factor is familial socioeconomic status (SES), which generally correlates between r=.30 and .40 with general cognitive ability (for a review, see Brody, 1992, or Jensen, 1998). In fact, this relationship is so robust cross culturally and across different types of measurement that it is standard practice to include SES as a covariate in studies of ability, rather than to focus on the effect of SES itself (e.g., Andersson, Sommerfelt, Sonnander, & Ahlsten, 1996; Martin, 1995).
The process by which SES impacts cognitive ability constitutes a central focus of inquiry in the developmental literature. Research has suggested that lower economic status is associated with less access to potential goods and services, as well as enhancing experiences, and greater exposure to harmful experiences and substances (e.g., Bradley & Whiteside-Mansell, 1997, Duncan & Brooks-Gunn, 1997, Huston et al., 1994). As a result, researchers have attempted to examine the characteristics of the home environment that mediate the relationship between SES and cognitive outcomes. The HOME (Bradley & Caldwell, 1976, Bradley & Caldwell, 1980, Caldwell & Bradley, 1978, Caldwell & Bradley, 1984) is a set of tester-rated instruments that indexes multiple aspects of the home environment, which includes learning stimulation, parental responsiveness, spanking, as well as more physical aspects, such as the number of books on the shelves, cleanliness of the home, and crowding. Cherny (1994) suggested that this measure correlated r=.22 with general cognitive ability in childhood.
More recently, the degree of organization and calm in the household versus chaos has received attention (Matheny, Wachs, Ludwig, & Phillips, 1995). Using a parent report questionnaire, this construct has demonstrated modest to moderate links with children's general cognitive ability (Pike, Iervolino, Eley, Price, & Plomin, submitted for publication). This finding is particularly striking because the prediction from chaos held when controlling for SES, as well as eight additional environmental ‘risk’ variables such as parenting style and life events.
While the socialization literature has attempted to better understand individual differences in cognitive development by examining the taxonomy of the family environment, behavioral genetic research has attempted to better understand cognitive ability by estimating the relative importance of genetic, shared, and nonshared environmental influences. Numerous twin and adoption designs involving family members living together and apart have suggested that shared environmental influences approach zero by adolescence. However, in early childhood, the shared environment (e.g., growing up in the same home), along with genetics and the nonshared environment (and error), is important to cognitive ability in early childhood (Boomsma, 1993, McCartney et al., 1990, McGue et al., 1993, Plomin, 1986, Plomin et al., 1997). Similarly, behavioral genetic research has suggested that the shared environment, as well as genetics, are important to the stability of cognitive skills in early childhood (see Petrill, 2002, for a review).
Until recently, socialization and behavioral genetic theories of development have employed very different methodologies. Socialization research has typically examined the environment in samples in which family members (parents and children) share genes as well as environments. Thus, these studies have confounded genetic and environmental influences (Rowe, 1994, Scarr, 1992). On the other hand, behavioral genetic studies have estimated the proportion of variance in cognitive ability influenced by genes, the shared (family level) environment, and nonshared (individual level) environment but have not attempted to identify specific genetic and environmental factors that constitute this variance.
There has been the beginning of a convergence between socialization and behavioral genetic designs in the developmental literature (Wachs, 1993). Socialization research has profited by being embedded in research designs that can take genetic influence into account in understanding environmental mechanisms. Behavioral genetic research has benefited by including specific measures of the environment (Plomin, 1994).
Thus, the goal of the present study is to examine the links between environmental markers and cognitive development in early childhood using the genetically sensitive Twins Early Development Study (TEDS). TEDS is a population-based study of all the twins born in England and Wales from 1994 to 1996 (see Trouton, Spinath, & Plomin, 2002). We chose to focus our attention on two factors linked to general cognitive ability in the extant literature, as well as within the TEDS sample (Pike et al., submitted for publication): SES and the level of chaos versus organization in the home environment.
In particular, we examined three issues with respect to the relationship between SES, chaos in the home, and general cognitive ability in early childhood. First, after controlling for genetic influences on cognitive ability, we examined the extent to which SES and chaos accounted for the shared environmental variance in cognitive ability at Ages 3 and 4. Second, we examined the extent to which SES and chaos influenced the stability in cognitive skills from Ages 3 to 4, after controlling for genetic influences on the stability of cognitive skills. Third, we examined whether chaos constitutes an independent source of shared environmental variance, or whether it is completely explained by SES.
Section snippets
Participants
Twins are participants in the Twins' Early Development Study (TEDS), an ongoing longitudinal study examining all twins born in England and Wales in 1994, 1995, and 1996. Following a screen for infant mortality, 16,810 families with twins were identified by the Office for National Statistics (ONS). When the children were 1 year old, a letter was sent via mail to each of these families briefly explaining the project, along with a return-addressed postcard of interest. Those parents who responded
Phenotypic analyses
Table 1 presents the means and standard deviations for SES, CHAOS3, CHAOS4, VERBAL3, VERBAL4, PARCA3, and PARCA4 by gender and zygosity. The means and standard deviations are roughly similar across all comparisons, although males are slightly higher in CHAOS, but lower in VERBAL and PARCA, than females are. Correlations between SES, CHAOS3, CHAOS4, VERBAL3, VERBAL4, PARCA3, and PARCA4 are presented in Table 2. SES is positively associated with VERBAL and PARCA, while CHAOS is negatively
Discussion
The present study explored the roles of SES and chaos within the home environment for young children's cognitive abilities within a genetically sensitive design. These factors were shown to partially mediate the shared environmental influences for verbal and nonverbal measures of cognitive ability at Ages 3 and 4. Furthermore, SES and chaos significantly mediated the stability of verbal and nonverbal cognitive skills from Ages 3 to 4. Finally, chaos within the home was a significant mediator,
Acknowledgements
TEDS is supported by a program grant from the Medical Research Council of the United Kingdom. The authors wish to thank Andrew McMillan for his data management expertise as well as the parents of the twins and all staff members for making this study possible.
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