The making of individual differences
The making of individual differences

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The making of individual differences

4.3 Small babies: the sequel

Evidence that some of the consequences of being born prematurely were enduring was discussed in Section 3.2. However, what emerges from Section 4.2 is that the quality of maternal care can alter the course of development. So the question arises as to whether the course of development of premature babies can, likewise, be altered by the quality of maternal care? The question is straightforward; obtaining an answer is not.

Activity 8

Using the information from Section 4.2, identify three things that make answering the question just raised particularly difficult.


One reason is that babies cannot be randomly fostered between mothers with different maternal skills. A second reason is that there are other members of the families to consider (fathers or partners cannot be removed from the situation to make the investigation less complicated). Thirdly, prolonged, intrusive observation is not possible, making evaluation of human maternal behaviour problematic.

The first two difficulties can be overcome, at least in part, by carefully selecting participants, so that the babies and their families match as closely as possible on as many criteria as possible, except for the crucial variable of maternal care. One investigation that achieved this was undertaken by Feldman and colleagues (Feldman et al., 2002). Feldman found two hospitals whose catchment area was similar and level of care was similar. (The details of which criteria were matched are not important to the present discussion, but some are presented in Box 1.)

Box 1 Criteria for comparison used by Feldman et al. (2002)

‘Infants in the two groups were matched for gender, birth weight, gestational age, and medical risk. … All families were middle class … and matched for maternal and paternal age, education, parity … and maternal employment. Mothers were all married to the child's father and in all families at least one parent was employed. None of the mothers reported smoking or using drugs during pregnancy.’

‘The nurseries in the two hospitals were level 3 referral centres with a comparable number of admissions, case mix, physician level and experience and nurse-patient ratios. … The physical environment in both nurseries, as to light and noise levels, was comparable. In both units, parents had unlimited privileges and were encouraged to participate actively in infant care routines.’

The third problem was overcome, to some extent, by finding two hospitals whose regime differed on the aspect of maternal care of interest: one hospital had a policy of kangaroo care, the other did not. Kangaroo care (so called because of a superficial similarity to a kangaroo carrying her baby in her pouch) is full body, skin to skin contact between mother and infant. The infant would be removed from its incubator, undressed (except for a nappy) and placed between its mother's breasts. Infants remained attached to a cardiorespiratory monitor and were observed by nurses. The criteria for inclusion in the study were that the parents agreed to give their premature infants at least one hour a day of kangaroo care for at least 14 days, before discharge from hospital. These infants were matched with premature infants in the other hospital who received a comparable amount of maternal care, over a similar period of time, but did not receive kangaroo care.

Activity 9

Which was the experimental and which the control condition?


The experimental condition is the one in which the infants received the kangaroo care, whilst the control condition is the one in which the infants did not receive kangaroo care.

To evaluate the effect of kangaroo care on the babies, infant cognitive development was assessed at 6 months of age. This was an adjusted age. It was not six months after they were born, but six months after the date on which they would have been born had they been born at full term.

Why might the use of an adjusted age be important when considering cognitive development?


The adjusted age allows a comparison to be made between babies who have had the same period of time in which to develop, irrespective of the length of gestation.

The cognitive scales used to evaluate infant development have been derived from studies of full-term babies; so the adjusted age allows meaningful comparisons to be made between babies born at different gestational ages.

Infant cognitive development was assessed by a trained psychologist who did not know to which groups the children had been assigned. The psychologist used a standard test (The Bailey Scales of Infant Development) which produced values for both psychomotor development, a psychomotor index (PMI) and for mental development, a mental development index (MDI). The results are presented in Table 2.

Table 2 Effect of kangaroo care (KC) during infancy on two measures of infant development at 6 months of adjusted age

KC (N = 66) Control (N = 67) Probability the difference between KC and the control mean arose by chance
mean SD mean SD
Birth weight/g 1245 328 1289 358 (no difference)
Gestational age/weeks 30.4 2.5 30.8 3.0 (no difference)
Mental development index (MDI) 96.39 7.23 91.81 9.80 p<0.01
Psychomotor index (PMI) 85.47 18.42 80.53 13.33 p<0.01

Look at Table 2 and compare the two conditions. Notice first that at birth, the babies in the two groups were of very similar weight and very similar gestational age.

What effect did kangaroo care have on mental performance at 6 months?


The mental development index is significantly higher for those infants who received the kangaroo care than for those infants who did not; kangaroo care improved mental performance.

What effect did kangaroo care have on psychomotor development at 6 months?


The psychomotor index is significantly higher for those infants who received the kangaroo care than for those infants who did not; kangaroo care improved psychomotor performance.

This example illustrates that one environmental intervention, kangaroo care, can alleviate to some extent, the effects of another environmental perturbation, prematurity.

The developing organism is subject to an enormous number of environmental factors, each of which exerts its influence on the course of development of the individual organism. Some effects are seismic (e.g. the impact of testosterone) others are more subtle (e.g. the effect of maternal care), but they all contribute to the unique phenotype. What has not been considered so far, is how an environmental factor can influence development; to understand that it is necessary to consider how the nervous system itself develops.


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