The Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW12 HE, UK
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Abstract |
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Key words: adult sex ratios/cross-country comparisons/secular trends/sex ratios at birth
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Introduction |
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Parazzini et al. (1998) reviewed the data worldwide and confirmed many of the above findings. In addition they noted decreases since the 1950s in Greece, Hungary, Poland, Rumania, Portugal and Mexico; and increases in France, Ireland, Spain and New Zealand. Lastly, in the USA 19691995, the live birth sex ratio increased in black births and decreased in white births (Marcus et al., 1998, 1999
). Thus, though decreases have been more common during the past half-century, there have been some increases. In this note I shall consider whether these movements in sex ratio at birth may be (i) related to movements in adult sex ratios, and/or (ii) a cohort phenomenon, and/or (iii) the result of external environmental agents (e.g. some form of pollution). These three hypotheses are not mutually exclusive. Three studies will be described.
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Study 1. The adult sex ratio |
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Material
Most countries publish their numbers of births by sex in annual volumes of vital statistics. In addition, most countries hold censuses of their populations and these are enumerated by sex and age every decade. These two sorts of data are reproduced in the annual Demographic Yearbooks of the United Nations. In regard to data from the USA, the numbers of black residents by age and sex at each census date since 1930 have been estimated (Preston et al., 1998). I am grateful to Professor Preston (of the University of Pennsylvania) for advising me that these estimates may be subtracted from official totals to derive reasonable estimates of comparable white adult sex ratios at those time points. The numbers of births by sex and race in the USA are published in the annual Natality volumes of the US Vital Statistics.
Methods
It is hypothesized that adult sex ratios somehow exert a stabilizing homeostatic control over offspring sex ratios with the ultimate result of keeping the adult sex ratio close to an optimum value (i.e. around 1:1 male:female). If this hypothesis were true, then the movements across time of these two parameters should, in general, show a negative correlation with one another. It is not required to estimate that correlation in any given population, but simply to test whether it is negative in the majority of populations. Since censuses occur every 10 years in many countries, it was convenient to choose five time points, i.e. one for each of the five census times in the past half-century (i.e. since the perturbations in sex ratio at birth associated with the Second World War). For this purpose Spearman's rank correlation coefficient, , was calculated for each of 25 of the 29 populations (Parazzini et al., 1998
); and for eight Latin-American populations (Argentine, Chile, Costa Rica, El Salvador, Guatemala, Panama, Puerto Rico and Venezuela); for five former British colonies (Bahamas, Barbados, Bermuda, Hong Kong, and Mauritius) and for Cape Verde Islands, Guadeloupe and Iceland. In accordance with demographic usage, the adult sex ratio was taken as the number of resident males per hundred resident females. The age range was arbitrarily chosen as 1544 years.
Moreover, if the association between sex ratios in adulthood and at birth were causal, then (depending on the causal nexus) one might wonder whether those populations with high mean adult sex ratios would have low mean offspring sex ratios and vice versa. In other words, one would expect a negative correlation not simply within populations but across populations. For this purpose, mean values of the two parameters were calculated and subjected to correlation analysis in respect of 23 (mainly) Caucasian populations [i.e. Austria, Belgium, Bulgaria, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Netherlands, Norway, Poland, Portugal, Spain, Sweden, Switzerland, England and Wales, Canada, United States (whites), Australia, New Zealand]. The reason for not including the other populations in this analysis is that sex ratio at birth varies by race (James, 1987).
Results
Correlations within populations
Of the 41 correlation coefficients, 23 were negative and 18 positive. Table I gives the distribution of these correlations. Many of them were statistically significant. However, the lack of consistency of sign (+ or ) of the correlations is powerful evidence that the temporal trends in the two parameters are (largely) causally unrelated.
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Comment
Thus one may conclude that if secular movements in the adult sex ratio have any effect on the secular movements in the sex ratio at birth, this effect is very small in large populations. In general, it seems that the well-established secular variations in sex ratio at birth must have some other cause. There are one or two caveats to this generalization. For instance one may wonder whether changes in adult sex ratios played any part in the (very small but persistent) rise in sex ratio at birth in Australia during this century; and in the curious rise and fall respectively in the sex ratios at birth of black and white births in the USA across the years 19691995. These examples are not chosen randomly: they all occurred in the presence of movements of the adult sex ratio in the opposite direction. So they are consistent with the hypothesis that movements in sex ratios at birth are partially controlled by movements in adult sex ratios. To test this hypothesis in regard to these three populations, the data need to be submitted to analysis of a kind advanced in the following section. This will suggest that secular changes in the sex ratio of adults played little part in the changes in sex ratio at birth in England and Wales since 1950.
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Study 2. Maternal age-specific changes in sex ratios at birth and in adulthood in England and Wales since 1950 |
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no. of resident women aged x to (x + 4),
where x = 15, 20 . . . 35. (The ages of men and women are different to take account of the mean age difference at marriage.) Table II gives the age-specific sex ratios arranged by cohort, the unweighted means of these, and the unweighted means of the corresponding adult sex ratios.
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In contrast, Figure 1 suggests that secular movements of the age-specific sex ratios at birth are (very roughly) in parallel. Table II
also lends weight to the suggestion that any cohort effect in sex ratio at birth must be very small. For instance, of the maternal cohorts considered, the one that was least squeezed (with the greatest abundance of men of marriageable age) was that aged 1519 years in 19701974. In contrast the following cohort (that aged 1519 years in 19751979) experienced a deficit of marriageable men. Yet the latter cohort produced a higher proportion of daughters than the earlier cohort. These data suggest that the decline in sex ratios was the consequence of a period effect rather than a cohort effect.
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Study 3. Sex ratios at birth in isolated island populations with biased adult sex ratios |
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One last question arises: if mean sex ratio at birth and mean adult sex ratio are (negatively) correlated, why are they? Is it because some factor affects both (rather than that one directly affects the other)? In this context, one may note that communities differ in their propensity to violence and warfare. War and murder certainly reduce the adult sex ratio. Moreover there can be no reasonable doubt that regardless of the causal details, human aggression is correlated with male and female testosterone concentrations. Archer (1995), for instance, wrote: `Overall the correlation between various measures of human aggressive behaviour and testosterone level is about 0.38'. Now according to my hypothesis, high parental concentrations of testosterone are associated with high sex ratios at birth (James 1996). So in principal it is possible to envisage a proximate explanation for such a correlation. If the above reasoning were correct, the surplus males produced at birth in violent societies would die through murder and warfare. The point may be illustrated by the Yanomamo Indians. About 30% of deaths among adult males of this tribe in one region are due to violence: most victims are males, and about 44% of men aged
25 have killed someone (Chagnon, 1988
). The sexes of all reported Yanomamo births were 383 males and 295 females (Chagnon et al., 1979
). This sex ratio, 0.565 (with a standard error of 0.019) is substantially and significantly higher than the sex ratios reported in industrial societies (James, 1987
). Thus an adaptive explanation would be available for a negative correlation between mean sex ratios at birth and in adulthood. But the mere availability of an adaptive explanation is far from a guarantee of its truth.
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Further research |
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It should be possible to identify further samples in which adult sex ratios are substantially skewed in one direction or the other. For instance, what is the sex ratio of births in feminist communes? Or of births to women prisoners (or men prisoners) following conjugal visits? In this context, it may be wondered whether the well-established rise in sex ratio in wartime is due to a low domestic adult sex ratio (men being engaged in war elsewhere). But that phenomenon is susceptible to other interpretations and seems not a useful topic for further speculation here.
If it were correct that offspring sex ratios are affected by the sexes of the adults with whom the mothers cohabit around the time of conception, then one might expect that among children born outside marriage, those whose births are registered by the mother alone would have a different sex ratio from those whose births are registered by both parents (the latter suggesting more stable domiciliary arrangements at the time of conception and subject to a more permanent adult male presence prior to conception).
It would be particularly interesting to see attempts to confirm the finding of Lummaa et al. (1998). Data are presumably available in the reconstructions by historical demographers from parish records of pre-industrial France and Quebec.
The States of the USA have differing adult sex ratios. It would be interesting to see whether this is reflected in their sex ratios at birth.
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References |
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Submitted on October 12, 1999; accepted on February 9, 2000.