1. Introduction
Children whose parents have low incomes and low levels of education, or are unemployed or working in low-prestige occupations, are more likely to exhibit behaviour problems and have poor cognitive development during the early years than children growing up in families with high socioeconomic status (SES) (Hertzman, 1994; Hertzman & Weins, 1996; Willms, 2002a). When these children enter school, their problems tend to worsen: they are less likely to do well in academic pursuits, or be engaged in curricular and extra-curricular school activities (Datcher, 1982; Finn & Rock, 1997; Johnson, Crosnoe, & Elder, 2001; Voelkl, 1995). In the longer term they are prone to leaving school early (Cairns, Cairns, & Neckerman, 1989; Crane, 1991; Ensminger & Slusarcick, 1992; Janosz et al., 1997; Rumberger, 1995) and their relatively poor literacy skills make it difficult to successfully enter the labour market or pursue post-secondary training (Raudenbush & Kasim, 1998). Adolescents who are from low SES families are more likely to be obese (Willms, Tremblay, & Katzmarzyk, 2002), and to participate in a range of activities such as smoking, drug use, and unsafe sexual practices that can compromise their later health status (Duffy, 2000; Elliott, 1993; Jessor, 1992; Raphael, 1996). Adults who have relatively poor socioeconomic circumstances are more likely to experience mental and physical health problems, and ultimately die at a younger age (Hertzman, 1999; Kunst & Machenbach, 1992; Marmot, et al., 1991; Power, Manor & Fox, 1991; Wilkinson, 1992; 1996). There is clearly a relationship between SES and a wide range of social outcomes, which are evident at birth and persist throughout the life cycle.
The study of the relationships between children's outcomes and the SES of their parents has a long tradition in the sociology of education (White, 1982). One strand of this research has been concerned with whether certain kinds of schools or school programs are successful in educating children with differing family circumstances (Murphy, 1985; Raudenbush & Willms, 1995). Another strand has been concerned with individuals' academic and occupational attainment, and the extent to which these are determined by the socioeconomic positions of their parents (e.g., Bielby, 1981; Sewell & Hauser, 1975). The relationship between health outcomes and SES also has a long history. Much of the interest was focused initially on the effects of income, or more generally on the effects of poverty and poor living conditions (Deaton, 2002; Gordon & Townsend, 2000), but in the past two decades researchers have emphasized other aspects of socioeconomic status, especially social class. The celebrated study of Whitehall civil servants in the United Kingdom found that illness and mortality were related to occupational grade, and that better health was associated with each successive increase in social status (Marmot et al., 1991). The term "gradient" is used to describe the relationships between social outcomes and SES, as it emphasizes the notion that the relationship is gradual and increases across the range of SES (Adler et al., 1994; Marmot, 2002).
The focus of policy research in child development, education, and population health is on identifying the underlying processes that give rise to socioeconomic gradients, and how these are shaped and constrained by institutions and the wider communities in which people live. Researchers have strived to identify risk and protective factors that mediate or moderate the socioeconomic gradient, or have effects on social outcomes in addition to the effects associated with SES. One can describe socioeconomic gradients with simple descriptive statistics, such as the correlation between a social outcome and SES, or the difference in percentages of people from low and high SES groups who experience a particular social outcome. However, the strength of socioeconomic gradients and their functional form (e.g., linear or curvilinear) depends on the unit of analysis used to estimate the gradient (e.g., the individual or the community) and much more can be learned about the underlying processes that affect social outcomes through a careful examination of gradients at each level of analysis.
This paper has four aims. The first is to define socioeconomic gradients and suggest a standardised method for presenting them. This could make it easier for researchers and the policy community to compare gradients across studies and monitor changes over time. The second aim is to set out ten hypotheses about socioeconomic gradients and community differences, describe the statistical models for testing them, and discuss their implications for social policy. The third aim is to provide examples of the tests of these hypotheses using data describing children's early literacy skills, derived from two Canadian surveys — the National Longitudinal Survey of Children and Youth (NLSCY) and Understanding the Early Years (UEY). Although the focus of the paper is largely expository, the substantive results have important policy implications. Therefore, the fourth aim of this paper is to discuss the policy implications of these findings specifically, and more generally the implications of findings pertaining to the ten hypotheses.
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