|Early Supplementary Feeding and Cognition (Society for Research in Child Development, 1993, 123 pages)|
The findings that we have reported are most prominent in the cohort of maximum exposure. Thus, unless otherwise specified, the discussion that follows focuses on this group.
The coherence of the results summarized at the close of Chapter VI is perhaps their most striking feature. Among them, the most conspicuous is the significant interactive effects of treatment x SES on five of the seven tests (except reading and literacy) included in the psychoeducational test battery. Consistently, the subjects who benefited the most from the Atole were those at the lowest levels of the SES continuum. A second major finding is the interactive effect of treatment x grade: those who had reached the highest grades in school and had received the Atole fared better than all other groups on all three tests tapping reading skills. In the information-processing test battery, there were main effects on memory and paired associates tasks; none of the interactive terms were significant.
The breadth of effects was coupled with their modest size. The maximum R² accounted for by the Atole treatment was 5% (e.g., on the vocabulary test). On some tests, although maximum R² was statistically significant, it represented only about 1% of the variance (e.g., numeracy). However, because these modest effects were generalized over a wide range of mental abilities, they are likely to have resulted in significant differences between the behavioral repertoire of the subjects in the Atole and the Fresco villages. The coherence of these findings extends back, at least in part, to the earlier longitudinal study. At 4 and 5 years of age, the interactive term treatment x SES accounted for significant portions of the variance in the cognitive Factor 1 (derived from the battery of tests administered during the preschool period). As is the case in adolescence, the subjects who benefited the most from the early supplementary feeding were those at the lower end of the SES distribution within the Atole group. Thus, the activation of the processes that resulted in the developmental advantage of the low-SES group must have occurred early in life.
In contrast to what was observed at 4 and 5 years of age, the findings at 3 and 6 years showed neither main nor interactive effects. The absence of effects at 3 years of age agrees with the theoretical model that we propose below, one that incorporates the notion that the probabilities of detecting effects increase as the child grows older. Recall that the effects of treatment during the first 2 years of age on the Composite Infant Scale were restricted to the motor scale at 24 months of age.
The lack of either interactive or main effects at age 6 is not an aesthetically pleasing finding and cannot readily be explained. The personal characteristics and history of the subjects who were included in the statistical tests run for this age group do not differ in any substantive way from those subjects who were assessed at 4 and 5 years of age. The data do not offer any suggestive evidence to explain the finding.
In many respects, the subjects at the lower end of the SES distributions who benefited from the Atole in adolescence were disadvantaged compared to the rest of their community. For instance, on average, the mothers of these children had no more than 1 year of elementary schooling. The fathers had the lowest levels of occupation, and their housing was of the poorest quality across villages (e.g., small, without toilets, thatched roofs). During their preschool years, these subjects performed poorly on the cognitive test battery, and they were also shorter and lighter than other children of the same age.
The rate of physical growth, performance on preschool tests, and SES background of the low-SES subjects who benefited from the Atole suggest that their early development had been at high risk both in absolute terms and also by the standards of their own community. This high risk, moreover, was not derived solely from poor nutrition; it also stemmed from the many other adverse conditions of their environment (e.g., poverty and disease). An assessment of risk during these subjects' early formative years would probably have predicted poor test performance during adolescence. Yet their test performance in the follow-up was comparable to that of subjects at the highest SES level, and it was better than that of a comparable SES group in the Fresco villages.
The protective effect of the Atole conforms with results from longitudinal studies of exposure to early biological (e.g., low birth weight) and SES stress factors in which characteristics of the caregiving environment appear to shield the development of children against the adverse effects of exposure to such risk factors. For example, in a study of children in Kauai (Werner, 1986), the availability of alternate caregivers, the mother's work load (e.g., steady employment outside the household), and the amount of attention given to the children by the primary caregiver proved to be factors that protected the development of children considered to be at high risk in early life.
The significant interactions between treatment and maximum grade attained on outcomes such as the achievement tests suggest that the effects of the Atole were also modified by particular characteristics of the subjects. Recall that the relation between maximum grade attained and the Interamerican reading and vocabulary scores was positive for Atole but not Fresco subjects; Atole children who were in the upper percentiles of grade attainment scored significantly higher than Fresco children.
Intuitively, the interactions between treatment and grade seem to conflict with the interactions between treatment and SES, where those who were worse off were more likely to benefit from the dietary treatment. However, in the context of a rural society living in poverty, the differences in the developmental implications of SES and of maximum grade attained resolve such an apparent discrepancy. SES is a carrier variable of family conditions that were relatively stable over the lifetime of the subjects in the study and that, even among the families who were better off, pointed to a state of unmet basic human needs. On the other hand, maximum grade attained reflects increased exposure to a favorable environment as the children broaden their educational opportunities from one year to the next. This distinction in the developmental meaning of SES and maximum grade attained explains, as discussed below, why those at the lowest end of the SES distribution and those with the highest levels of formal education benefited most from Atole.
Further light on the developmental significance of the two sets of interactions that we have discussed is shed by a consideration of the three-way interaction that emerged in the analysis of the reading achievement test. As noted, the SES X grade interaction was significant for the Fresco but not for the Atole villages. Within the Fresco villages, associations between SES and achievement were positive and significant for both low and high levels of grade attainment. Conversely, these associations were not significant in the case of Atole.
Such a differential pattern in the interactions suggests that the truism that SES affects cognitive test performance is fulfilled in the Fresco group, independent of whether the focus is on opposite sides of the distribution of maximum grade attained. This is not the case with Atole: the truism is challenged since SES has no effect on test scores at either extreme of the school grade distribution - in order words, the high nutrient supplement is acting as a social equalizer.