Andrology Centre, Department of Obstetrics and Gynaecology, Slajmerjeva 3, S1-1000 Ljublijana, Slovenia
1 To whom correspondence should be addressed. e-mail: branko.zorn{at}kclj.si
Dear Sir,
We thank Professor James for his letter related to our report (Zorn et al., 2002). He has cited the paper by Ansari-Lari et al. (2002)
and our paper, which both provide evidence of a decrease in sex ratio at birth after the war in Iran in 1980 and in Slovenia in 1991. He states that the findings of these studies contradict previous reports, which found an increase in sex ratios during and just after major wars.
Then, he explains the hypothetical relation between sex-hormonal U-shaped changes during the fertile interval, testosterone and gonadotrophins in males, and estrogens, gonadotrophins and progesterone in females, and the sex ratio. Earlier (or late) fertilizations in the fertile period, due to high coital frequency, result in birth of males, the intermediate ones giving birth to females. Further, he suggests that decrease of sex ratio at birth may also be caused by low testosterone in men as a consequence of either chemical poisoning (Iran) or psychological stress (Slovenia). Finally, he points to the need for additional research to elucidate the real impact of such dramatic events on sex ratio and to confirm the hypothesis he developed.
Since researchers dispose with valid data on fertility and births, they investigate if, and how, sex ratio at birth, which is a relatively constant data for a population, may fluctuate following dramatic events, one of them being wars.
Rare but solid statistical analyses (Graffelman and Hoekstra, 2000) have confirmed an increase in sex ratio at birth during and after world wars, although in Europe the phenomenon was not general. Surprisingly, whereas wars were at least 5 and 7 years long, the increase of sex ratio was observed only at the end and after the wars, not at their beginning. What were the conditions which might have changed throughout the war to explain this phenomenon? Did the frequency of coitus change so much? Why would the air blitz on London at the beginning of the war affect the sex ratio differently than bombing raids at the end of the war?
Different methodologies used in evaluating sex ratio may explain contradictory findings.
Retrospective studies involved different periods and events; many factors with supposed effects on changes in sex ratio were speculated upon. Greater female age, birth order, frequency of sexual intercourse and stress have been suggested to have caused the changes in sex ratio after wars.
Retrospective studies analysing sex ratio after determining the risk factor using quantifiable data are more reliable. When studying newborns of grand grand multiparous women, Almagor et al. (1998) found that the sex ratio did not change with respect to birth order or maternal age.
Fukuda et al. (2002) assessed the smoking habits of parents at the time around the conception. After delivery, 5372 women were questioned about their own and their partners daily consumption of cigarettes during the periconceptional period. They found that parental periconceptional smoking was related to a decrease in sex ratio. The difference was highly significant when the man smoked; no significance was observed when only the female smoked.
However, in that study the duration of smoking and male age were not reported, although these data are important. It is supposed that young men when they begin smoking have higher testosterone levels than non-smokers. On the other hand, testosterone levels declined faster in chronic male smokers, the lowest testosterone levels were found in men who smoked the most (James, 2002).
Another way to elucidate the reason for the sex ratio change is to submit a population to definite conditions and to look for changes in sex ratio using prospective studies. In studying pregnancies among women using natural family planning, Gray et al. (1998) found that the sex ratio did not vary consistently or significantly with the estimated timing of insemination relative to the day of ovulation. Thus, this finding does not support the involvement of earlier fertilization in sex ratio change.
In the 1970s (Zorn et al., 2002) the concept of lower serum testosterone was developed as a possible consequence of stress. Recently, lower testosterone has been hypothesized to be a consequence of testicular dysgenesis due to environmental factors (Møller et al., 1996
).
Because of the interaction of multiple factors, the aetiology of sex ratio changes after stressful events is not expected to be elucidated in the very near future. Today we are not in the position to make definite conclusions regarding the association between stress and sex ratio changes, as the existing studies on the relationship between testosterone and adverse events lack appropriate methodologies. These studies are not controlled, much less are they randomized. This last decade has taught us that the active testosterone is free testosterone, or bioavailable testosterone, and not the total testosterone and its derivates, as considered until then.
More rational research approaches are a prerequisite for a better understanding of the complex problem of wars and sex ratio changes.
References
Almagor, A., Schwed, P. and Yaffe, H. (1998) Male to female ratio in newborns of grand grand multiparous women. Hum. Reprod., 13, 23232324.[Abstract]
Ansari-Lari, M. and Saadat, M. (2002) Changing sex ratio in Iran, 19762000. J. Epidemiol. Community Health, 56, 622623.
Fukuda, M., Fukuda, K., Shimizu, T., Andersen, C. Y. and Byskov, A.G. (2002) Parental periconceptional smoking and male:female ratio of newborn infants. Lancet, 359, 14071408.[CrossRef][ISI][Medline]
Graffelman, J. and Hoekstra, R.F. (2000) A statistical analysis of the effect of warfare on the human secondary sex ratio. Hum. Biol., 72, 433445.[ISI][Medline]
Gray, R.H., Simpson, J.L., Bitto, A.C., Queenan, J.T., Li, C., Kambic, R.T., Perez, A., Mena, P., Barbato, M., Stevenson, W. and Jennings, V. (1998) Sex ratio associated with timing of insemination and length of the follicular phase in planned and unplanned pregnancies during the use of natural family planning. Hum. Reprod., 13, 13971400.[Abstract]
James, W.H. (2002) Smoking, sperm quality and testosterone level. Hum. Reprod., 17, 32753276.
Møller, H. (1996) Change in male:female ratio among newborn infants in Denmark. Lancet, 348, 828829.[Medline]
Zorn, B., Sucur, V., Stare, J. and Meden-Vrtovec, H. (2002) Decline in sex ratio at birth after 10-day war in Slovenia. Hum. Reprod., 17, 31733177.