Past studies have shown that the deaths of siblings in a family are correlated for a variety of reasons and are likely to be independent of the length of the preceding birth interval. Such studies can be traced back as early as the 1930s when Gardiner and Yerushalmy (1939: 31), from New York data of 1936, found higher neonatal as well as still-birth rates among infants bom to mothers with experience of a previous child loss than to those bom to mothers with no experience of such loss. A follow-up of this study carried out by Schlesinger and Allaway (1955: 182-183) also noted that the risk of death during the prenatal period among those who had lost the previous sibling was 2.7 times greater than those who had not experienced such loss.
Hobcraft et al. (1985: 371) argued that studies related to these issues could indicate the level of mortality within a family. Hobcraft et al. using the data from 39 World Fertility Survey countries, noted the elevated risk of death for the index child when the older of the pair of children has died. Stockel and Chowdhury (1972: 119) found a higher probability of death among index siblings in Bangladesh when the older of the siblings was dead. Majumder (1989: 159) also observed that the survival status of the preceding child at age one had a net influence on the probability of death. However, Swenson and Harper (1979: 467) found higher probability of death to index siblings only when mothers in Bangladesh had experienced two or more foetal deaths. In a later study in Bangladesh, Swenson (1981: 302-303) also observed a higher risk of death among infants bom to mothers with experience of two or more previous child losses as compared to those with no child loss or one previous child loss. Swenson found that this pattern was true for all
lengths of preceding birth interval categories in the study. For Korea Kim (1986: 7) found that the risk of death to infants preceded by a dead child was more than two times greater than to infants preceded by a surviving child. These findings based on different studies discussed in this section show that previous foetal or sibling loss experiences among women of reproductive ages increases the probability of death to index siblings bom to them.
The mechanism through which survival status of the preceding child operates in influencing the probability of death to the index sibling seems to be different in a situation where the elder of a pair has survived compared with where it has died. The death or survival of a sibling pair and its relation with the survival status of the index child is explained through the absence or presence of the competition between children for familial resources, care and disease transmission (Koenig et al., 1990: 250). Hobcraft (1987: 10) argued that the increase in the risk factor to index siblings preceded by a surviving child could be the consequence of the competition between the pair for resources. In contrast, Hobcraft also speculated that the increased risk of death to an index sibling preceded by a child who failed to survive could be the reflection of a combination of several biological factors such as prematurity, low birth weight, continuing social, economic and environmental deprivation or poor knowledge and health care.
Das (1975: 449) in a study in India argued that the survival or death of a child in a family determines the frequency of intercourse which in turn is likely to determine the length of birth interval between siblings in a family. The death of an infant could increase the frequency of intercourse as an effort to conceive to replace the loss which in turn is likely to result in a closely spaced birth contributing to elevated risk of death. Pebley and Millman (1986: 71) argued that the death of the elder of a pair may result in a shortened birth interval as a consequence either of the replacement effect or of truncated lactation which in turn could deplete a women's reproductive efficiency and nutritional reserves leading to the birth of a less healthy child and so to the higher risk of death. DeSweemer (1984: 56-59) puts forward a number of alternative hypotheses to explain the adverse influence of the death of the elder of the pair on the survival chances of the index sibling.
DeSweemer argued that this effect could be explained by common causes such as maternal depletion syndrome, higher environmental contamination, poor nutritional practices, prevalence of communicable diseases or diminishing quality of parental care because of grief on the death of the previous child. DeSweemer further argued that survival of the elder of a pair reflects effective parity or favourable conditions that also protect the index child from dying.
Past studies have also noted that the probabilities of dying for children within a family are correlated (Wolfers and Scrimshaw, 1975: 483; Winikoff, 1983: 232; Gubhaju, 1991: 106-118; Majumder, 1989:174-175; Pebley and Millman, 1986: 71). This indicates that a woman who has a history of at least one child loss is at higher risk of losing a succeeding child bom to her. In this context, Winikoff (1983: 232) and Pebley and Millman (1986: 72) suggested that a family that has experienced a previous child loss may have the same biological or behavioural risk since children bom in the same family presumably are exposed to a similar environment. In this respect, Boerma and Bicego (1992: 245-246) argued that interpreting the result of controlling for the fate of the preceding sibling is not an easy task due to two possible counteractive mechanisms. They are:
(1) if mortality risks between successive children are correlated because of common familial characteristics, the effect of birth interval on child mortality could be reduced by controlling for previous child death and (2) if sibling competition is important, then the previous child removes the potential effects of sibling competition and disease transmission; therefore controlling for the survival status of the previous child could be expected to increase the effect of birth intervals on child survival.
5.3
Analytical approach
As in the preceding chapters, analyses in this chapter are mainly confined to cross tabulation of bivariate and multivariate main effects logit models. The cross-tabulation analyses are performed to examine whether or not mortality differentials among index children exist according to the survival status of the preceding sibling, mother's age at childbirth, birth order, and sex of the preceding and index siblings. The main effects logit models are fitted for further analysis.
The survival status of the preceding child at the time of the birth of the index child was employed to examine infant mortality differentials in Nepal (Gubhaju, 1991) and in Indonesia (Hull and Gubhaju, 1986). In this context, Majumder (1989: 135) argued that the survival status of the preceding child at age one could be a better measure for this variable where the data are based on birth history information rather than pregnancy history. Majumder further argued that the survival status of the preceding sibling measured at the time of the birth of the index child from birth history information tends to produce biased results. This is because an index child following more than one pregnancy loss is likely to possess a longer preceding birth interval than where no such losses are experienced. Hobcraft (1987: 10), explaining how the survival status of the preceding sibling influences the survival status of the index sibling, used the results based on the survival status of the preceding sibling at two years of age. This study used the survival status of the preceding sibling at age one for the analysis of infant mortality and at age two for the analysis of child mortality.