Ten Hypotheses about Socioeconomic Gradients and Community Differences in Children's Developmental Outcomes - February 2003

4. Summary and Discussion

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This report suggests a definition of socioeconomic gradients, and sets out ten hypotheses about gradients and community differences relevant to policy research in the areas of child development, education, and population health. Weaved within the discussion of the ten hypotheses is a presentation of a set of analyses concerning children's vocabulary development, based on analyses of data from Canada's National Longitudinal Study of Children and Youth and the Understanding the Early Years survey. The findings of these analyses have implications for social policy, and for future academic research. The principal findings are summarised in Table 2, and discussed further below:

Table 2 Ten Hypotheses regarding Socioeconomic Gradients for Children's Receptive Vocabulary

Hypothesis	Accepted or Rejected?	Comments
Socioeconomic Gradient	Accepted	Strong evidence
Community Differences	Accepted	Strong evidence
Diminishing Returns	Rejected	Consistent with research on schooling outcomes
Converging Gradients	Rejected	Inconsistent with research on youth literacy skills
Double Jeopardy	Rejected	Needs to be examined further with smaller units of analysis
Relative Deprivation	Rejected	Needs to be examined with spatially-related measures
SES by Group-Status Interactions	Accepted	Results apply only to immigrants versus non-immigrants; needs to be examined further with other measures of group status
Family and Community-Level Mediators	Accepted	Family and community factors mediate the relationship between vocabulary skills and SES, but largely operate as independent factors
Spatial Auto-Correlation	Rejected	Needs to be examined further with a wide range of outcomes and covariates
Stable Gradients	Not tested	Related research suggests gradients are relatively stable, but can be altered through public policy and the people's efforts

1. Children's receptive vocabulary is related to socioeconomic status. On average, across 23 Canadian communities, the slope is 4.57. This means that a child of low SES (e.g., with a family SES score at -1.0, or about the 16th percentile) would have an expected score that was about 9 points lower than a high SES child (e.g., with a family SES score of 1.0, or about the 84th percentile.) This is a large difference — a difference of 9 points in receptive vocabulary could have a substantial effect on children's skills upon entry to school.

   Although there is a strong relationship, a child's socioeconomic status is far from being deterministic of a child's receptive vocabulary. For the full NLSCY sample, which is representative of all Canadian children, SES explains less than 9% of the variation in children's vocabulary scores. Figure 1 shows that while there are many children living in low SES families with low scores, the majority of them have scores above 85, which is commonly seen as a vulnerability cut-off score. Similarly, there are several children from high SES families whose scores are quite low. Indeed, the majority of children with scores below 85, are from average and above-average SES families. These results emphasize the need for universal interventions aimed at improving the early literacy skills of all children, rather than targeted interventions aimed particularly at low SES families. (See also Willms, 2002a).

2. There are large and statistically significant differences among Canadian communities in the level of children's vocabulary skills. The gap in vocabulary scores between the lowest and highest scoring communities was more than 10 points. This difference is not simply attributable to sampling error, as there were sufficiently large samples in each community to achieve accurate estimates. Moreover, a difference of about the same magnitude remained after controlling for SES and sampling error (see Figure 2).

   These differences among communities are very large, and as with differences associated with SES, a difference of this magnitude could have substantial effects on children's learning during the schooling years. The analyses which follow in the paper explain some of these differences, but overall there is not a good explanation about why communities differ to this extent. These findings call for further research that examines children's outcomes across a range of outcomes, at differing ages, and across time.

3. Socioeconomic gradients are linear in nearly all communities. The average socioeconomic gradient for these communities was slightly curvilinear, but the extent of curvilinearity was not statistically significant. Therefore, the hypothesis of diminishing returns is rejected, and it is not possible to identify a low SES threshold that could be used to target certain families for interventions. Moreover, the relationship between vocabulary scores and SES was also linear in nearly all communities.

4. The gradients do not converge at higher levels of SES. Although analyses of literacy skills for youth have indicated a pattern of converging gradients, the gradients for early vocabulary skills are remarkably parallel. Thus, one cannot identify communities which have particularly low early literacy scores for low SES children but not high SES children, or vice-versa.

5. There is no evidence of double jeopardy in children's early vocabulary skills. The hypothesis was that children from low SES families who also lived in low SES communities would have lower vocabulary scores than comparable children living in high SES communities. This hypothesis did not hold; in fact, the effect of community SES was in the opposite direction, indicating that on average children's vocabulary scores, after adjusting for SES, were higher in low SES communities than in high SES communities.

6. There is no evidence of a relative deprivation effect for children's early vocabulary scores. Children's vocabulary scores were not related to the amount of variation in SES within each community. This relationship could be examined further, with measures of deprivation assessed using spatial techniques at the local level.

7. Children whose families had immigrated within the past five years scored on average about 14.6 points lower than children in non-immigrant families. The scores for children whose parents had immigrated more than five years previously were only 4.2 points lower than those in non-immigrant families. This is a substantial gap, which could result in some children having a relatively slow start during the first few years of elementary school. The findings suggest that the gap may be greater for low SES families, but the interaction term was not statistically significant. This is a case which calls for a targeted intervention, and many school districts have special programs for children whose first language is not the language of instruction.

8. The four most important family and community factors related to children's early vocabulary skills, aside from SES and number of siblings, were the amount that parents read to their child, the extent to which the family functioned as a cohesive unit, the degree of social support in the neighbourhood, and the stability of the neighbourhood. These factors operated mainly as independent factors, alongside SES; that is, they only partially mediated the SES gradient. They also show that it is not possible to identify a single factor that can be the focus of social policy at the municipal, provincial, or national levels. Rather there are several factors which by themselves have a fairly small effect, but taken together can have a rather substantial effect on children's vocabulary.

9. There is no evidence of spatial auto-correlation at the neighbourhood level, after accounting for SES. One might expect that there are factors such as effective literacy or parenting practices that are diffused from one neighbourhood to its neighbouring communities, which would result in a strong observed autocorrelation. However, this was not the case. The results suggest that neighbourhoods operate largely independent of one another. It may be that the neighbourhood, defined using enumeration areas, is the wrong unit of analysis. At a more macro-level, such as provinces, there is certainly a spatial correlation for many childhood outcomes (Willms, 2002a). Further analyses are required which take a more macro approach to examining spatial variation across communities and provinces.

10. Gradients are not immutable; they can be altered through policy and reforms, and through the efforts of families and children. This paper did not examine longitudinal trends in SES gradients. However, gradients can be altered through public policy and reform, and through the efforts of concerned citizens. In New Brunswick for example, the provincial government launched a comprehensive program of early childhood reforms, including: prenatal screening and intervention; postnatal screening and intervention; preschool clinics at 3.5 years of age; home-based early intervention services; integrated daycare services; social work prevention services; and home economic services. A detailed analysis of changes in socioeconomic gradients for children's developmental outcomes in New Brunswick indicated that the prevalence of low birth weight, prenatal complications, and the motor and social development of New Brunswick's babies decreased during the first few years of the program (Willms, 2000). The analyses in this report point to several factors that are related to the level and slope of socioeconomic gradients, suggesting that it is feasible to direct social policy for children at raising and leveling gradients, at local, provincial, and national levels.