Family Income and Infant Mortality in Modern Societies

The infant mortality decline in the Western world described in the preceding section represents one of the truly outstanding achievements of the twentieth century. At the same time, however, it is very clear that this decline has not altered the nature of the traditional socioeconomic differential. Rather, the findings of a wide variety of studies have consistently shown that the lower income groups in all societies have been and continue to be extremely disadvantaged in the probability that their newborn infants will survive to adulthood. According to a recent effort directed by the present author, the existence of a general inverse association between infant mortality and socioeconomic status has been documented by a great many studies based on widely different population groups, covering different points in time, and using a variety of methods, not only in the United States but in a number of western European countries (Stockwell and Wicks 1981). Moreover, a follow-up investigation documented the existence of such a relationship for all race-sex groups and for all major causes of death (Stockwell, Swanson, and Wicks 1986).

This later study entailed an analysis of infant mortality differentials in a metropolitan aggregate comprising eight of the larger cities in the state of Ohio covering the years 1979-81. The general design of the research was an ecological one in which the primary analytic unit was the census tract of the mother's usual residence. (Census tracts are small geographic areas used in the United States to delimit neighborhoods composed of people possessing relatively homogeneous characteristics. They have no political basis, but are merely statistical aggregates that are used primarily for various planning programs and for research.) The independent variable was defined as the percentage of low-income families in each census tract at the time of the 1980 decennial census. An annual income of $10,000, or roughly 50 percent of the median family income, was selected as the low-income cutoff point.

Table IV.3.1. Infant mortality rates for income areas in metropolitan Ohio, 1979-81

Infant mortality rate

Income area

Total

Neonatal

Postnatal

All areas

16.6

10.6

6.0

I (high income)

8.9

6.3

2.6

II

14.3

9.2

5.1

III

16.4

10.8

5.6

IV

19.7

12.6

7.1

V (low income)

24.3

14.4

9.9

Source: Stockwell, Swanson, and Wicks (1986, 74).

Source: Stockwell, Swanson, and Wicks (1986, 74).

The dependent-variable data consisted of the number of live births in each census tract during 1980 and the number of infant deaths, by age, in the three years centering on the census date (1979-81). The analysis first used the income variable to aggregate the census tracts of the study cities into five broad groups in such a way that an approximately equal number of tracts fell into each income status group. The resulting aggregates were then ranked and compared in terms of infant deaths - total, neonatal (under 1 month), and postneonatal (1 month to 1 year). The results of these comparisons are presented in Table IV.3.1. Inspection of the data clearly reveals the existence of a consistent and pronounced inverse association between infant mortality and family income, with the differential being especially marked for postneonatal mortality, or for those deaths generally considered to be most influenced by the nature of the environment.

Similar observations can be made with respect to both men and women and for both whites and non-whites (Stockwell et al. 1986). There are occasional deviations from a consistent linear pattern, but they are not sufficient to detract from the validity of the overall conclusion of the research: The traditional inverse association between infant mortality and family income, first documented in the United States in 1925 (Woodbury 1925), continues to be very pronounced; and it characterizes both the neonatal and postneonatal components of infant mortality, for both sexes and for both major racial groups.

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