University of California at Berkeley, School of Public Health, University of California at Berkeley, Berkeley, CA 94720, USA e-mail: rayc{at}uclink4.berkeley.edu
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Abstract |
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Key words: economic decline/human sex ratio/unemployment
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Introduction |
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A second mechanism suspected of connecting population stressors to the secondary sex ratio acts on fathers. Research reports reduced sperm motility among stressed males (Fukuda et al., 1996). Males in populations subjected to ambient stressors therefore may father fewer males.
Among the hypotheses inferred from this literature is that the secondary sex ratio in a human community would decrease when the economy contracted. Proponents of this hypothesis (Trivers and Willard, 1973) cite reports that the sex ratio varies positively with the socioeconomic status of mothers (Shapiro et al., 1968
). These authors reason that the individual experience of being relatively poor simulates the stress experienced by a population enmeshed in a contracting economy. However, at least two large studies have rejected the hypothesis that the sex ratio varies with socioeconomic status (Erickson, 1976
; Rostron and James, 1977
).
The lack of an association between individual socioeconomic status and the gender of offspring does not axiomatically detract from the theory that macroeconomic decline will stress a population sufficiently to affect the sex ratio. Societies typically use public and private charity, although to varying degrees, to ameliorate the worst effects of relative poverty (Janoski and Hicks, 1994). A growing economy should increase the benefit of such charity because all can increase their wealth even when those with more income give, voluntarily or through taxes, to those with relatively less. A contracting economy, on the other hand, implies that simply maintaining the income of the relatively poor will require the remainder of society to realize less income than previously enjoyed, and that everyone will enjoy less than earlier growth had led them to expect. This circumstance comes closer, in my opinion, to the population stress suggested by the research summarized above than does being relatively poor.
This study contributes to the literature by testing the hypothesis that the sex ratio during the collapse of the East German economy in 1991 declined below that expected from both history and the sex ratio in West Germany.
East and West Germany formally united in October 1990. East German workers and firms experienced the full force of free market competition for the first time in more than half a century. Researchers have estimated that 20% of the East German labour force was not working and another 20% worked only a few hours a week in 1991 (Neumann, 1991). Industrial production dropped by half to its lowest recorded value (Neumann, 1991
; Funke and Rahn, 2002
). West Germany, on the other hand, was experiencing relatively good economic times. Unemployment was low and production was higher than at any time in the post-war period (Neumann, 1991
). While large infusions of West German capital eventually reinvigorated the former East Germany, the investments actually added to economic adversity in 1991 by inducing inflation of over 6% (Neumann, 1991
) in an economy suffering its greatest loss of household income since the Second World War.
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Materials and methods |
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Analyses
Analyses were in the tradition of the interrupted time-series test (Box et al., 1994). This approach has the null hypothesis that the sex ratio in East Germany in 1991 will not differ from the value statistically expected from the ratio observed from 1946 to 1990. Time series, however, often exhibit autocorrelation, such as trends, cycles and the tendency to remain elevated or depressed after high or low values, which makes expected values difficult to estimate from history. Researchers typically solve this problem by the purely empirical approach of identifying autocorrelation in the dependent variable and expressing it as an effect of earlier values of the variable itself. Here, the strategy attributed to Dickey and Fuller (1979)
and Box et al. (1994)
was used to identify and model autocorrelation. The strategyautoregressive, integrated, moving average (ARIMA) modellingdraws from a very large family of models to specify autocorrelation in time series.
The German data made it possible to go beyond the purely empirical approach. The fact that the West German economy functioned relatively well during East Germanys collapse allowed the use of the sex ratio in the former as a control variable in the test equation (Catalano and Serxner, 1987). This provides the benefit of the purely empirical approach in that it removes any autocorrelation in the East German sex ratio induced by forces also at work in West Germany. The approach has the added benefit of controlling for confounding variables that exhibit no autocorrelation and affect the sex ratio in both populations. Such confounding variables could induce a type I error if they shifted coincidentally with the collapse of the East German economy.
The test proceeded through the following steps. The East German sex ratio was modelled as a function of that in West Germany. The residuals from step 1 were inspected for a mean different from zero (P < 0.05; two-tailed test) and for autocorrelation. The equation used in step 1 was expanded to include a constant, if a mean were found, and any needed ARIMA parameters. The test equation was specified by adding a binary variable for the economic collapse to the equation resulting from steps 1 and 2. Variable 1 was scored for 1991, and 0 otherwise. The equation resulting from step 3 was estimated, and the error terms inspected for autocorrelation. If any were found, additional ARIMA parameters were added to the equation, and the resulting equation was estimated again.
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Results |
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The possibility that the association discovered in 1991 extended, though attenuated, into 1992 was tested, and it was found that it did not.
To test the sensitivity of the results to the specification of autocorrelation used, the equation was estimated with no ARIMA parameters, as well as with an autoregressive parameter at lag 1 substituted for the moving average parameter. The results did not change.
The routines suggested by Chang et al. (1988) were used to assess the effects of outliers other than the low value in 1991 on our estimated coefficients. The routines detected and controlled an upward outlier in 1979. Controlling the outlier did not change the results of the test.
It is possible, although not argued in the literature, that risk-averse women bear more males than females and that their abstaining from child bearing in the turbulent year of 1991 induced a lower sex ratio. It could also be that women in East Germany opted to abort male more than female fetuses in 1991. These possibilities were tested by adding the number of live births to the test equation. The economic collapse variable remained significantly related to the sex ratio.
Both sex ratios were also transformed to their natural logarithms to reduce any effect that variability in variation might have had on the estimated effects. The results of the test, other than the metric of the coefficient, did not change.
Adding the economic collapse variable to the equation that included the West German sex ratio and a moving average parameter at lag 1 increased the explained variance by 6.8% (61.4 compared with 71.2%). The test in which the sex ratios were converted to their natural logarithms allowed the expression of the discovered association in terms of effect on odds. The antilog of the estimated parameter for the binary collapse variable suggested that the 1991 sex ratio in East German (i.e. 1.044) was 1.5% lower than the ratio of 1.059 expected from history and the ratio in West Germany.
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Discussion |
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In order to induce the association discovered here, an unmeasured confounding variable would have to have operated only in 1991, affected East but not West Germany, and be unrelated to the collapse of East Germanys economy. I can think of no such phenomenon.
To maximize the internal validity of the test, dramatic case of economic decline was used, which also allowed comparison to a genetically similar population. The cost of this strategy is that the results may not generalize to milder cases of economic contraction uncomplicated by the simultaneous reordering of social and political institutions. Only replication in other populations can yield and estimates of external validity.
The results presented here have implications beyond support for the theory that the biological response of men and women to ambient stressors systematically affects the sex ratio, and that economic shocks can trigger these responses. Applied implications range from the possibility that the stress response in pregnant females has outcomes other than spontaneous abortion. Premature delivery resulting in low birth weight, for example, appears associated with maternal stress (Hobel et al., 1999). Declining economies, moreover, have been reported associated with elevated incidence of very low birth weights (<1500 g) in Sweden and Norway (Catalano et al., 1999
). Low birth weight infants are at elevated risk of severe disability that requires expensive medical intervention (McCormick and Siegel, 1999
). Better understanding of the connection between macroeconomic circumstances and the biological mechanisms that affect the sex ratio may yield improved design and allocation of prenatal interventions (McCormick and Siegel, 1999
), as well as improved estimates of the health externalities of economic policy choices (Kapp, 1950
).
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References |
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Submitted on April 29, 2003; accepted on May 22, 2003.