Economic impact on postnatal ossification.

The magnitude of economic impact on postnatal ossification timing in generally lower-income boys and girls of European ancestry was found to be 0.21 standard deviation units or Zscores for a difference of approximately $2200.00 in per-capita income. Both boys and girls were equally affected. While it is axiomatic that the children of the poor are developmentally delayed in comparison with boys and girls of greater affluence, the magnitude of the relationship between economic status and postnatal ossification timing has not been subject to direct quantification. Indeed, most published data on the age at appearance of the 28 postnatal ossification centers of the hand are given without reference either to family income or to indices of parental education and occupation that relate to economic status. In the present study, therefore, we have made use of postero-anterior hand radiographs of 1790 boys and girls generally from lower-income families, collected from eight states during the 1968-1970 Nutrition Survey. Two of the states were from the Northeast (Massachusetts and New York), two represented Appalachia (West Virginia and Kentucky), two of the states were from the Pacific Coast (California and Washington), and the remaining two represented the old South and the upper Midwest, i.e., South Carolina and Michigan, respectively. For each individual, economic status was calculated as the income-needs ratio, a widely-used measure that incorporates (a) total household income, (b) the number and the age of members of the household and (c) the baseline or “poverty level” income (Orshansky, ’65). Two contrasting groups were then selected for comparison. The first, comprising all of the boys and girls with an income-needs ratio of 1.50 and less, had a mean income-needs ratio falling slightly below the accepted poverty AM. J. PHYS. A N T H R O P . , ~ ~ : 1 4 level of 1.0. The second group, with income-needs ratios of 2.25 and above, was a higher income group, averaging approximately three times the poverty-income level. On a per-capita basis, the mean income for the first group was $724.00, and that for the second group was $2920.00 per capita. The per-capita income range was therefore $2200.00, on a group basis. For all of the children in the study, the age at appearance of postnatal ossification centers was computer-derived from (a) birth date, (b) examination date and (c) presence-absence information as initially recorded on optical scanning cards. The mean value (M) was calculated as the midpoint of a Gaussian distribution, and the standard deviation (a ) was generated from the cumulative frequency curves (cf. Garn et al., ’72). In all, six wrist area centers were selected for comparison, covering the range from the first through the seventh year, since the sample did not allow calculation of age at appearance of early postnatal centers. As shown in table 1, the more affluent boys and girls (income-needs ratios above 2.25) were systematically earlier in age at appearance of wrist area centers than children of poorer families (income-needs ratios up to 1.50), and without exception. This trend was statistically significant by sign test at any reasonable level of confidence, since for 12 pairs of comparisons, the more affluent were advanced in all 12 pairs as against the 6:6 or chance hypothesis (x2 > 8.0). Most individual ossification centers were also significantly advanced at the 5% level of confidence,