The Sex Ratio at Birth Following Periods of Conflict

Note: this paper was written by UMass Boston undergraduate student Johnny Xu for my Spring 2011 class ANTH 324: “A Biocultural Approach to War.” I asked his permission to post it here. 


Birth Sex Ratio and Infant Mortality: Adaptations or By-products?


1. Introduction

            The purpose of this paper is to provide manifold reasons attempting to explain why the birth sex ratio following war periods tend to rise in favor of males and what this implies in correlation with infant mortality; and, most of all, to answer the following question: is the combination of these findings proposing that this is an adaptive response of the parent to produce the sex with higher survival prospects in the given environment, or is this simply the by-product of environmental forces?


2. Introducing the Trivers-Willard hypothesis

            It has been observed for some time that high birth sex ratios occur following wars in which many men have fallen (James 2009). Trivers and Willard hypothesized that if a female is in good condition and she is trying to maximize her investment in reproductive life, she should favor a male offspring—and, similarly, if she is in poor condition, she would do better, for both the child and her, to invest in a less variable offspring, a female offspring (1973). The implication follows that the reproductive option to preselect an offspring’s sex would be beneficial to human evolution, particularly in war: to make up for the deaths of males. If the Trivers-Willard hypothesis is correct then there should be various mechanisms that would allow for and affect the conception of male embryos naturally and thus prove to be adaptive. These possible factors and “mechanisms” that contribute to differing birth sex ratios—without the conscious intervention of parents (e.g., with ultrasound, abortion)—have been noted in various articles to include a mother’s preconception diet and post-conception diet (Mathews et al. 2008); occupation with regards to economical wealth, status, and stress (Ruckstuhl et al. 2010); war and terror (James 2009; Zorn et al. 2002; Catalano et al. 2006; Bethmann and Kvasnicka 2011; Helle et al. 2009); and climate change (Helle et al. 2009).


2.1.  Preconception and post-conception diet on birth sex ratio

            If it is true that diet (esp. preconception diet) immediately and directly affects the sex of a fetus, then this conclusion may help explain the changes of birth sex ratio following wars. In a study done in the UK, data on 740 British women who were unaware of their fetus’s gender was used to show that fetal sex is most influenced by maternal diet prior to conception ( Mathews et al. 2008). Male fetuses require more nutrients and different conception patterns from female fetuses. The empirical proof for additional nutrients required for male fetuses lies in simple birth patterns in weight, length, and overall size. The implications of diet logically connect with Trivers and Willard’s hypothesis, which states that a pregnant female who is living in poor conditions will do better to have females (Trivers and Willard 1973). If this is true then the results of the study on the 740 British women would show that women who had a higher nutritional intake would also have higher male births. What this simple hypothesis lacks is the categorization of pregnancy periods. Keeping food diaries was the method used to gauge each woman’s nutritional intake during and before pregnancy: all of the 740 women were able to produce these early pregnancy food diaries, 721 of these women were also able to produce food diaries containing their nutritional intake in the year prior to conception, and finally 661 women reported their later pregnancy (28 weeks gestation) food diaries ( Mathews et al. 2008: 1662). The data was broken up into “preconception” and “later pregnancy” (1662). The three common periods where nutritional data was collected were preconception, early pregnancy (16 weeks gestation), and later pregnancy (16-28  weeks gestation) (1662). In additional to these three periods, two factors were used to measure nutrition. Factor one was comprised of diets high in protein, fat, vitamin C, folate, and minerals; factor two included diets high in vitatmin A and vitamin B12 (1662). The results showed that factor one significantly influenced fetal sex in favor of male conception, whereas factor two produced nothing significant (1663). However, this applied only to the preconception period—meaning that neither in early pregnancy nor later pregnancy was factor one influential to a mother’s conception of a male infant (1663). What this implies is that maternal nutritional intake in any period after conception had no significant effect on fetal sex (1663). This even implies that the additional energy a male fetus needs to reach their typical 100g difference in weight (in comparison to females) may not come from the mother while he is in utero but rather sometime when she herself is growing up (1663). The emphasis on preconception diet coincides with studies done on non-human mammals and have shown that early conception diets tend to affect fetal sex (Cameron 2004). Additional research was done to see what kinds of food may alter fetal sex, and it turned out that regular consumption of breakfast cereal, prior to pregnancy accounted for an increase in mother’s conception of males (Mathews et al. 2008: 1665). The results of this study showed that, compared to the 45% of women with the lowest preconception energy intake, 56% of women with the highest preconception energy intake had male infants. In the end, the take-home point is that not only diet but timing too makes up for determining sex ratio.


2.2.  Maternal occupation and psychological stress

            Ruckstuhl et al. conducted a research on mothers’ occupations and their influence on the birth sex ratio and found that economical success, social standing, and maternal age affected birth sex ratio only with regards to the level of stress each factor suggested (2010). They found that women with “high psychological stress” jobs, were more likely to have females thus bolstering the Trivers-Willard hypothesis’ claim that the cost of having a female infant was less demanding than having a male, and, therefore, being in a poor condition, a mother trying to maximize reproductive success would abort males (if they already have males) and have females over males (Trivers and Willard 1973). One problem with this study is that the researchers categorized occupations in regards to stress from a range of 0-10 (e.g., “Sales, services” equaling a 5) (Ruckstuh et al. 2010: 3). Because the researchers could not look directly into physiological stress (changes in hormones either in mothers or fathers), this study does no deal with an objective, physiological definition of stress but a sensational one—which probably is practical too but not as practical (2010: 3). The findings showed that the probability of having a male offspring did lower when maternal job stress increased (2010: 4). The study claims that there was nothing that suggested that maternal age, economical standing, and social standing affected offspring sex (2010:1), but it misses the point in seeing that maternal age does indirectly affect offspring sex, because quite simply, maternal age determines the kind of environment she will be in (the kind of wage she will get and the social status she will be in) and what kinds of jobs are available to her. So only in converting those factors into stress is it possible to see that there is no doubt that psychological stress affects the birth sex ratio.


2.3.  Tying in the above with war and terror

2.3.1.  Famine: Saxony and the Dutch hunger winter

            If preconception diets high in nutrition do affect birth sex ratio in favor of males (Mathews et al 2008), then there is bound to be conflict in the following findings. James attributes to Ploss the findings in Saxony that “high offspring sex ratios were associated with years of famine” (James 2009: 119). It is not clear if the study done by Mathews et al. directly contradicts this because Ploss does not talk about these women’s period of conception—whether they had been relatively well-nourished and pregnant before the famine or not. James, however, does end up providing information that directly conflicts with Mathews’ study. He cites Stein et al. having provided “direct evidence that severe preconceptional maternal undernutrition was associated with significantly high offspring sex ratio following the Dutch hunger winter” (James 2009: 119). If this is the case then nutrition perhaps does nothing to further our understanding of birth sex ratio increasing during wartime crises. James, however, finds a flaw in Stein’s argument: even prior to the Dutch hunger winter, the sex ratio had been peaking (2009: 119). So what exactly causes the rise of high birth sex ratios following wars—and does it still have anything to do with diet?

2.3.2.  War and how it lowers and raises sex ratio

            Following both the Iraq-Iran war (1980-1988) and the 10-day war in Slovenia (June 26-July 7, 1991), the male birth sex ratio lowered in these countries (James 2009: 117; Zorn et al. 2002). In both these wars is a contradiction in the Trivers-Willards’ hypothesis stating that male birth sex ratio increases following a war because it is adaptive; however, there are several larger conflicts that have shown an increase in male birth sex ratio: these two wars are precisely World Wars 1 and 2.

            Decreases in sex ratio in Slovenia have been attributed to psychological stress in men and the negative effects of it on sperm motility (Zorn et al. 2002). (Psychological stress has already been studied, as mentioned earlier, to affect a woman’s involvement in lowering the birth sex ratio of males—this time the stress relates to men in particular.) The study on the 10-day war in Slovenia compared the sex ratio of Slovenia and its capital, Ljubljana, in 1992 compared to 1991. The data was suppose to capture the impact of the war on the first possible group of infants (born January-March 1992) whom their mothers had during the war (June 26-July 7, 1991); the period is a span from 6 to 9 months. The findings conclude that there was a significant decrease in male birth ratios following the war: in general Slovenia, 0.504 in 1992 versus 0.518; in Ljubljana, 0.483 in 1992 versus 0.537 in 1991 (2002). Semen analysis of the war group compared to the (normal) control group showed that following the war, there was a significant fall in semen motility (2002: 3174). Similarly, another study (already referenced earlier) including this one both alluded to a study done on the effects of the Kobe earthquake on sex ratio; the findings can be carried over into this one: both in the case of the Kobe earthquake and the 10-day Slovenia war, there was the occurrence of psychologically stressed men who showed signs in reduced sperm motility (Ruckstuhl et al. 2010: 1; Zorn et al. 2002: 3173) and a reduced male birth sex ratio. Zorn et al. mentions that the increase of X-bearing (female sex chromosome) sperm can be an indirect result of psychological stress (2002: 3176). Ruckstuhl et al. reported from another study that Y-bearing (male sex chromosome) sperm were faster but less resilient to unfavorable conditions in the mother’s reproductive system, and that X-bearing sperm were slower but survived longer (2010: 1). The implications of how war may reduce the male birth sex ratio can be said to involve both psychological stress and more specifically its effect on sperm motility. Perhaps, motility did not decrease, per se, in this study or in other studies, but that the concentration of sperm in war group males and stressed males were now more highly made up of X-bearing sperm (which are slower); this would mean that psychologically stressed males were, in fact, producing more X-bearing sperm, which goes hand in hand with what Zorn et al. said earlier. Similarly, Zorn et al. also referenced Rose et al. in noting that soldiers who were conscious of an imminent combat showed a lower amount of testosterone in their urine when compared with the normal population (2002: 3174). All this reinforces the point that female sex ratios would be higher—male sex ratios would be lower.

            On the increase of male birth sex ratio, James states that the possible causes can be found in Kanazawa’s hypothesis and coital rate (2009: 119-123). Kanazawa’s hypothesis or Cartwright’s “the returning soldier effect” states that men who survive wars tend to be taller, and that taller men are reported to have higher offspring sex ratio than short ones (2009: 119, 120). This would mean that an increase in sex ratio is directly and inevitably a result of surviving soldiers, if it follows that low sex ratio non-survivors (those who would have mainly sired daughters) were being depleted in the progression of the war (2009: 120). James attributes coital rate as a major cause of increasing sex ratio. Basically, his idea holds time and timing of coitus (sex) as essential to sex ratios: “boys are conceived at either end of the fertile interval, and girls in the middle of that interval” (2009: 120). Soldiers who were granted leaves had only the equivalence of one menstrual cycle’s time with their partners, and the challenge to achieve pregnancy in that short time is proven by the resulting higher sex ratio (2009: 120). James states that “on average, high coital rates must have preceded the conceptions of partners of service men on brief embarkation leave if they were to conceive at all” (2009: 120). To achieve pregnancy would imply higher coital rates. Results showed that these higher coital rates resulted in an increase sex ratio. The empirical evidence James provides proves that higher coital rates did influence sex ratio after the war, as in the case of England and Wales. “Illegitimacy” rates or temporary liaisons in England and Wales normally in the 1930s was 41 per 1000; during the war years in 1945, these rates grew to 93.3 per 1000 (2009: 120). Along with this increased illegitimacy rate, there was also an increase sex ratio in males—and it follows because the illegitimacy rates imply an increased coital rate. Further evidence shows that as the illegitimacy rates fell gradually, as the war became past history—52.9 in 1947, 47.5 in 1953—so did the increased sex ratio (2009: 120).


3. Discussion and conclusion

            The consideration that sex ratios increase after wars does not need to be either a forced environment response or an adaptive one (Helle et al. 2009: 1226): it can be both. The literature involving increased sex ratios or decreased sex ratios following wars present this challenge of reconciling adaptive forces and environmental forcing. If everyone were to follow the Trivers-Willard mindset, he would likely believe that a decreased sex ratio implies an adaptive change the same way an increased sex ratio does the same—there is never a way getting out of this circle of tautologies. Reduced conception of males and increased male fetal deaths would be altogether synonymous with one another, and both would be seen to be adaptations rather than possible external influences just forcing stress changes in conception. This would imply that males born in a high sex ratio would live shorter than males born in a low sex ratio, because males who are born in low sex ratios have survived the stage where their weaker male peers would have been spontaneously aborted because they were “frail” (Catalano et al. 2009: 273, 274) and as the Trivers-Willard hypothesis implies, a female in the same condition would have a better rate of survival and be a better and less costly investment (Trivers and Willard 1973). This further implies that males born in high sex ratios will have fewer male offsprings (Catalano et al. 2009: 277). If this theory were to be juxtaposed with Kanazawa’s hypothesis, there would be a problem in adaptation in favor of human reproduction. Taller men who supposedly contributed to a higher sex ratio would only give birth to weaker males who would in turn give birth to fewer (male) offspring. In a way, if natural selection is at work to keep the sex ratio at a balance, this would be one of the ways—but it’s not adaptive in favor of humans, as Trivers and Willard suggest, but just the balanced sex ratio population in general (105-107 males per 100 females). However, this theory involving the fecundity of low sex ratio males versus high sex ratio males has not yet been tested; as Catalano says, Only when researchers are able to compare the fecundity of males born in a high sex ratio with males born in a low sex ratio would there be evidence at all (2009: 277).


Works Cited

Bethmann, Dirk and Kvasnicka, Michael. “War, Marriage Markets, and the Sex Ratio at Birth.” JEL Classification J12, J12, N34 (2011): 3-13.

Cameron, E.Z. “Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: evidence for a mechanism.” Proceedings of the Royal Society B 271 (2004): 1723-1728.

Catalano, Ralph, Ahern, Jennifer, Bruckner, Tim, Anderson, Elizabeth, and Saxton, Katherine. “Gender-specific selection in utero among contemporary human birth cohorts.” Paediatric  and Perinatal Epidemiology 23 (2009): 273-278.

Catalano, R., Bruckner, T., Marks, A.R., and Eskenazi, B. “Exogenous shocks to the human sex ratio: the case of September 11, 2001 in New York City.” Human Reproduction 21 (2006): 3127-3131.

Helle, Samuli, Helama, Samuli, and Lertola, Kalle. “Evolutionary ecology of human birth sex ratio under the compound influence of climate change, famine, economic crises and wars.” Journal of Animal Ecology 78 (2009): 1226-1233.

James, W. H. “The variations of human sex ratio at birth during and after wars, and their potential explanations.” Journal of Theoretical Biology 257 (2009): 116-123.

Mathews, F., Johnson, Paul. J., and Neil, Andrew. “You are what your mother eats: evidence for maternal preconception diet influencing foetal sex in humans.” Proceedings of the Royal Society B 275 (2008): 1661-1668.

Ruckstuhl, Kathreen E., Colijn, Grant P., Amiot, Volodymr, and Vinish, Erin. “Mother’s occupation and sex ratio at birth.” BMC Public Health 10 (2010): 1-11.

Trivers, R.L., Willard D.E., “Natural selection of parental ability to vary the sex ratio of offspring.” Science 179 (1973): 90-92.

Zorn, Branko, Sucur, Veselin, Stare, Janez, and Meden-Vrtovec, Helena. “Decline in sex ratio at birth after 10-day war in Slovenia.” Human Reproduction 17 (2002): 3173-3177.

3 thoughts on “The Sex Ratio at Birth Following Periods of Conflict

  1. Pingback: Boys, war and the prescient mosquitofish mother : Sex, Genes & Rock

  2. Pingback: Rob Brooks: What Mosquitofish Mothers Can Tell Us About the Boys Born After a War | Living Biology

  3. Haha, you could say that Matthews et al. is literally cookbook science! The only replications were the three postwar studies, and they seem to contradict each other. That should ring alarm bells about the conclusions of the unreplicated studies. Surely this is well-trampled ground with rat and pig subjects?

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