|Early Supplementary Feeding and Cognition (Society for Research in Child Development, 1993, 123 pages)|
We now sketch a theoretical model of the linkage that we propose between early nutrition and later development. We recognize that these efforts could be similar to painting broad strokes over a large canvas, representing a reality that may well be far too complex to be tested experimentally. The model is first discussed in general terms, followed by a description of its relevance to the particular findings from the follow-up.
The model builds on well-grounded empirical information that, among nutritionally at-risk children, variations in nutrition status (i.e., protein, energy, and micronutrient deficiencies such as iron and zinc) account for significant variations in physical growth, motor maturation, and physical activity (Husaini et al., 1991; Martorell & Habicht, 1986; Meeks Gardner, Grantham-McGregor, Chang, & Powell, 1990; Rutishauser & Whitehead, 1972; Simonson, Sherwin, Anilane, Yu, & Chow, 1969; Super, Herrera, & Mora, 1990; Vasquez-Velasquez, 1988). In Guatemala, the children in the Atole villages were taller and heavier than those in the Fresco villages, and such differences continued into adolescence (Martorell et al., 1980; Martorell, Rivera, & Kaplowitz, 1990). They were also motorically more mature at 24 months than the children in the Fresco group. Because motor activity was not assessed in the study, we do not know whether the children in the two groups differed in this regard; however, it is likely that this was the case. Both theory and empirical studies suggest that one of the mechanisms available to maintain energy balance among malnourished children is a reduction of energy expenditure in motor activity (Beaton, 1984; Grantham-McGregor, Meeks Gardner, Walker, & Powell, 1990).
We conjecture that the effects of malnutrition on body size, motor maturation, and physical activity are the linkages between malnutrition and delays in behavioral development over time. In particular, we conjecture that small body size, delays in motor maturation, and reduced physical activity contribute to the gradual formation of styles or modalities of social-emotional and behavioral interactions between the malnourished child and the environment that slow cognitive development and educational progress. This proposition is an outgrowth of current theorizing that the associations between particular biological characteristics of children and their environment tend to covary with each other (Plomin, DeFries, & Loehlin, 1977; Wachs & Plomin, 1991). The following is a more detailed analysis of our theoretical proposal.
1. In both industrialized and preindustrial societies, accelerated physical growth and height are associated with comparatively better performance on developmental scales in early childhood and intelligence and school achievement tests in adolescence. While the exact mechanisms behind these associations remain unclear and are likely to differ from one society to another, one possible explanation is that physical growth and body size determine in part ways children are cared for and treated socially. In particular, small children are likely to be treated as younger than their chronological age, and they trigger caretaking behaviors of nurturance and protection at a time when children of a similar age but of average size and maturity are searching for autonomy and independence from their caretakers. The opposite will be true for comparatively larger, more mature-looking children; their physical attributes will induce caretaking behaviors that, on average, are reserved for older children. For example, the mothers of malnourished children maintain more frequent physical contact and closer proximity to their offspring than mothers of well-nourished children of the same age (Graves, 1976, 1978; Lozoff, 1988; Lozoff, Klein, & Prabucki, 1986). Similar patterns of behavioral interactions between caretaker and offspring are also observed in animals; in some species, the frequency of adult-offspring contact correlates negatively with the offspring's age and size (Harper, 1970; Konner, 1976; Moss, 1967; Trivers, 1974).
2. It is now well established that malnutrition delays motor maturation in infants and young children. In keeping with the first proposition, there is also evidence that the timetable of motor maturation influences the nature and range of contacts that the organism has with its physical and social environment. Motor milestones, particularly those of self-locomotion (i.e., crawling, creeping, and walking), transform the child's perception of and physical relations with the physical space and the elements within it (Bertenthal & Campos, 1990; Bremner & Bryant, 1985). These developmental changes lead, in turn, to the acquisition of new perceptual skills (e.g., depth perception). In addition, self-directed locomotion increases social contacts, broadens social experiences, and enhances affective contacts with adults (Gustafson, 1984). Of concern to us here is that, because of delays in motor maturation, malnutrition limits the opportunities that these children have to move about the environment, learn from it, and develop cognitive skills that children who are of a similar age, but more mature, are acquiring.
3. Our last proposition is that physical activity in children is positively related to exploratory behavior - behavior exploring the physical and social environment. We recognize that there is not much empirical evidence to support this proposition; however, the evidence that is available comes mostly from studies of well-nourished groups, where, under ordinary circumstances, the level of motor activity of children is independent of their nutrient intake. In these groups, the critical issues may be the child's capacity to self-regulate and to modulate activity according to the demands of the environment rather than the characteristic level of activity (Wachs, 1990).
We conjecture that there is a positive relation between physical activity and exploratory behavior in those situations where the motor activity of children is partly determined by the intake of energy and micronutrients (e.g., iron). This postulate stems from the notion that one of the mechanisms available to the organism to maintain energy balance is to reduce the expenditure of energy through a decrement of motor activity. Thus, under these circumstances, the average level of activity in children is lower than the level of activity they would display if their nutrient intake would meet their physiological requirements. This low average level of activity is what we believe is associated with the reduced amount of exploration of the environment evinced among malnourished children.
The importance of exploratory behavior has been discussed at length in the developmental psychology literature (Berlyne, 1966; Bruner, 1968; Piaget, 1952). In fact, early exploration of events, people, and objects is seen as the foundation of cognition. With the development of perceptual systems and the maturation of new action systems, the young child discovers the particular attributes of each new physical and social stimulus that are relevant to the context and to his or her developmental stage (Gibson, 1988).
In sum, we propose that the effects of malnutrition on body size, motor maturation, and physical activity mediate the relation between malnutrition and delayed cognitive and behavioral development. In particular, there are three basic propositions: (1) Children who are small because of their nutritional history induce caretaking behaviors and social responses from others that are generally reserved for children of a comparatively younger age. (2) Their slow motor maturation delays the acquisition of particular cognitive abilities and social behaviors. (3) Their low level of motor activity that results from the need to maintain energy balance limits the exploration of the physical and social environment. These three sets of effects are not limited to a particular age; they operate and express themselves throughout the child's early, middle, and late childhood and adolescence. They activate mechanisms that span years in the lives of children who live under social, economic, and physical circumstances that are similar to those in rural Guatemala. The final outcome is the attainment of a level of cognitive competence that is behind the level of other children and adolescents of the same chronological age with a history of having received adequate nutrition.
The definition of effects that we propose is substantively different from that which assumes that malnutrition has direct effects on the central nervous system and, therefore, on cognitive function. However, we do not deny that such direct effects might indeed exist; the two explanatory models are not mutually exclusive. At issue now is to fit the model to the data that show no main effects on the psychoeducational tests but do show significant interactions: in one case, between treatment and SES and, in the second, between treatment and maximum grade attained.
Within the context of a poor rural society in Guatemala, the families with the lowest economic and social resources must face the most severe constraints to meet the health (e.g., medical care, hygiene, and sanitation), nutrition (e.g., dietary quality), and developmental (e.g., educational opportunities) needs of their children. This was also the case among the severely impoverished families who benefited the most from the Atole; in such a context, the Atole compensated in part for the existing constraints and pushed the children in the study toward faster growth and healthier development. In particular, their comparatively larger size, earlier motor maturation, better health, and, possibly, increased exploratory activities must have induced particular caretaking and social responses and led to the development of behaviors and the early acquisition of cognitive skills that compensated somewhat for the severe limitations of their impoverished households, which failed to meet their developmental needs. On the other hand, these effects did not occur at the other end of the SES distribution because, within the limits of a poor, rural community, even in the family and household environment of those who were better off there is not much that can be offered to a healthy child. In other words, there is a ceiling in the provision of potential benefits given to the children in the context of rural family poverty.
A natural extension of this argument is that, if the children who received Atole would have been exposed to an environment without such a ceiling, then the benefit of the nutritional supplement would have been greater. This postulated incremental effect is what we think explains the significant interaction between treatment and maximum grade attained. Despite all their limitations (Gorman & Pollitt, 1992), the schools in the villages made possible the effects of the Atole because they provided opportunities for intellectual growth. Accordingly, the higher the grade attained - that is, the greater the exposure to schooling - the greater the effects of the nutritional supplement.
The explanatory model also accounts for the effects observed among the children included in the late exposure cohort because nutritional supplementation accelerates growth velocity even after the second year of life, albeit at a lower rate than what is observed when it is implemented before the second year. Modest physical and health advantages among those in the late exposure cohort who received Atole are likely to have activated the same process that we have described in connection with the children in the maximum exposure cohort. The difference between the advantages of the two cohorts is a matter of degree of effects.
An empirical test of the model that we propose is not feasible because we lack the necessary data concerning motor development milestones and physical activity measurements; moreover, even if these data were available, the samples would become too small to permit analysis once all the relevant variables were taken into consideration. At this stage, this model remains strictly conjectural.