1 Emeritus Faculty talk by Ron Wells, A.M., M.D. on 17th March 2010 How does latitude affect the human birth sex ratio? In June 2009 Professor Kristen Navara of the University of Georgia in Athens reported that 51∙1% of births in tropical latitudes were males while 51∙3% were males in temperate and sub‐
arctic latitudes. (Navara, 2009) That small difference might seem scarely worth noting, but “in science, the grandest revolutions are often triggered by the smallest discrepancies” (Clifton and Ferreira, 2008) and as a certain David Everett said when he was only 7 years old, “from little acorns tall oak trees sometimes grow”. Having studied data on some one and a half billion births occurring in the ten year period 1997‐2006 Kristen Navara reported the 0∙2% variation was statistically highly significant. However as the statistician Michael Joseph Moroney pointed out “the words figure and fictitious both derive from the Latin root fingere. Beware!” He went on to remark that “a statistical analysis, properly conducted, is a delicate dissection of uncertainties, a surgery of suppositions.” Reliability of birth data With any statistical conclusion one uncertainty must be about the sufficiency and accuracy of the data studied; and while Navara’s study was based upon some 1 billion births the data she used was obtained from world factbooks published by the CIA. Unfortunately national birth registration data, even if collated by UN agencies and republished by the CIA, have been far from reliable in some countries. Some of the less developed ones simply have lacked the necessary financial resources or their central goverments didn’t control parts of their territories because of wars and insurrections; but their official returns each year have often been accepted as gospel by researchers totally unfamiliar with the nuts and bolts of collecting and processing such data. When chairman of the WHO Expert Committee on Health Statistics in the 70s and 80s I visited vital statistics staff doing their work from the collection of data in developing nation villages to the presentation of top level national statistics. From those onsite assessments I concluded the vital statistics in several nations were not sufficiently reliable to use in studying factors influencing the sex ratio of live births. In some nations birth registration required payment of a fee which some parents could not afford, or parents delayed registering the births of daughters substantially longer than births of sons, and the longer the delays the more some would not have been registered at all. UNICEF recently estimated that the births of 48 million children under five had not been registered. Female infanticide had been common in East and South‐East Asia – in Japanese‐occupied Korea it was estimated to have been as high as 14% of newborn baby girls, and of course these female births were not registered. Then as facilities for prenatal sex diagnosis became available female infanticide began to be replaced by selective abortion of female foetuses. In nations with conscription for military service some families have avoided registering the births of sons. Women born to Chinese families in the Fire Horse years of 1906 and 1966 were traditionally believed to be dangerous to their husbands: so some of these female births were falsely registered as having 1 2 occurred in the immediately following years, causing strange blips in some 1906/7 and 1966/7 national birth statistics. Some countries with otherwise efficient vital statistics systems only tabulated their births by the dates of birth registration and not by the actual dates of birth, which made their published birth data useless for studying seasonal variations; and in 1964 Takahishi reported that “in Japan parents do not always notify the real date of birth, especially when the girl is born in December, because younger age is profitable for marriage”. (Takahashi, 1964) I later found it was not uncommon for middle aged or elderly Japanese widows to adopt baby boys and falsely register as their biological mothers. Kristen Navarra was of course aware that selective abortion of female foetuses was common in some countries and without going into details she reported the statistical relationship between latitude and birth sex ratio occurred when data from those countries was excluded. Factors sigificantly related to the human birth sex ratio Because of the above problems with accuracy of national birth statistics studies of the factors relating to the human birth sex ratio in sufficiently large and reliable birth series by researchers adequately experienced in international vital statistics have been the rare exceptions rather than the common rule. However there have been sufficient to show about a dozen statistically significant correlations; and some of the first were published early in the 20th century by Sir George Knibbs (1858‐1929) the First Commonwealth Statistician and by the Italian statististician and demographer Corrado Gini. So let’s consider whether three of the best known factors related to the human birth sex ratio may have a common underlying mechanism. 1. LENGTH OF PARENTAL MARRIAGE. In children born to married parents the proportions of male births are highest in the 9th and 10th months of marriage, decline steeply during the next year and then more slowly.This factor has the strongest of all known statistical correlations with the birth sex ratio, and instead of elaborating on the possible explanation I’ll leave each of you to apply Occam’s razor. Several other factors related to the secondary sex ratio are closely associated the length of parental marriage, such as birth order and thus a tendency for consecutive children to be of the same sex. 2. WAR ‐ The overall M/F sex ratios of German, French and English births were unusually high during and immediately after the 1914‐18 war. There have been many myths about war per se causing an increase in the proportion of male births. Thus well before accurate large scale records of births were collected John Arbuthnott (1667‐1735) the Scottish physician to Queen Anne presented a paper to the Royal Society in which he deduced that Divine Providence was causing more baby boys to be born than girls to make up for the excess deaths of young males over females. (Arbuthnott, 1710). This belief that Divine Providence was regulating the sex of births was given professional demographic support by the 18th century German pastor and leading demographer Johann Peter Süssmilch (Süssmilch, 1741), and from there it was only a short step for other reputable demographers to copy Süssmilch and one another until Carl Düsing reported the M/F sex ratio of births had increased in Sweden following the 1789‐90 war with Russia (Düsing, 1883). That puzzled me because it conflicted with my reading about that war and my 2 3 theory about how some wars affected the birth sex ratio – until last year when a new study by William James of the birth data collected by the Church of Sweden during the period 1750‐1805 showed no significant increase in the secondary sex ratio during and after the 1789‐90 war (James, 2009). The national registration of births in Sweden only commenced in 1991! Similarly it was often reported that the ratio of male to female births had increased in France during the Napoleonic wars, but from a statistical point of view this remains uncertain partly because the national registration of births in France had only commenced in 1792 and as in other countries would probably have taken at least two decades to produce adequately reliable data for such a conclusion. While the French data (Brian and Jaisson, 2005) did show an unmistakable increase in the male proportion of births during and immediately after the 1914‐18 war and to a lesser extent with the 1939‐45 war. The Germans had observed a similar change in their birth sex ratios by late 1915 and soon realised the effect was largely confined to children of soldiers’ wives resulting from conceptions when the men were on leave. But with the precise reason still not pinned down two German epidemiologists looked forward to the great natural experiment in the Second World War as giving an opportunity to study this matter more thoroughly (Ludwig and Boost, 1943). However in the last year of the war the birth records the Third Reich had carefully been collecting in a central registry were totally destroyed by the allied bombing before they could be adequately studied. The male/female sex ratio of births increased in Britain during and immediately after the 1914‐
18 war and again during and after 1939‐45 war. It also increased in the USA during the last years of that war, again during the 1950‐52 Korean War and perhaps slightly during the 1965‐73 Vietnam War. However one further clue to explaining this effect of wars is that no change in the male/female ratio of births was observed in the participating countries of the 1854‐6 Crimean war, the 1870‐71 Franco‐Prussian war, the Boer wars of 1880‐1 and 1899‐1901, or in the USA during and following the First World War. 3. AGE OF MOTHER’S HUSBAND ‐ The M/F sex ratios of children are related to the ages of their mothers’ husbands (if they have them) with the highest proportions of males being born to mothers with young husbands. By now I expect each of you will have little doubt about why length of marriage, war and age of husband affect the birth sex ratio. But first a brief aside on why I’ve used the term “mothers’ husband” instead of just “father”. In 1952 I was studying at the British Blood Group Reference Laboratory at the Lister Institute where much of our research was concerned with working out how the dozen or so recently discovered blood groups were inherited. A problem was that in the many hundreds of families we studied it was clear from their other blood groupings that about 10% of the putative fathers were not the actual ones. In my then innocence I was considerably surprised at this finding in view of the ready sexual access most husbands have to their wives during their fertile periods compared with those wives’ relatively brief trysts with other lovers. But in confidential enquiries I found the extent of extra‐
marital conception was well known in the blood transfusion services of other countries, including Australia. Of course there’s long been the saying “mother’s baby, husband’s may be”, a scepticism subsequently found to be equally applicable to what we used to call hedge sparrows in England, 3 4 now known as dunnets. But whereas there was no ethical or practical problem in publishing papers about the sexual behaviour of female dunnets blood transfusion services hadn’t published their findings on marital infidelity for fear of frightening away many badly needed blood donors. It may be publish or perish in academia, but it can sometimes be publish and perish in government and commercial programs. Someone else who had not published her knowledge about marital infidelity was Dame Marie Stokes who had started Britain’s first birth control clinics. I’d got to know her well, for upon first encountering the attractive student Nightingale nurses in the hospital wards at Thomas’s I realised urgent action was necessary and recruited Marie Stopes to talk to my fellow medical students about birth control. With us all thus better informed I organised a ball at St. Thomas’s to be attended only by medical students and student nurses. “– boobies think that people come to balls to do nothing but dance: whereas everyone knows that the real purpose of a ball is either to look out for a wife, to look after a wife, or to look after somebody else’s wife.” Robert Smith Surtees, 1805‐64 Marie Stopes, from her long experience in interviewing women with unwanted pregnancies thought extra‐marital intercourse was commonest during women’s brief fertile periods. However surveys to directly measure frequencies of human sexual intercourse have been notoriously unreliable ‐ for example men have tended to report more frequent hetero‐sexual intercourse on average than females. Incidentally I might mention that births due to rape and illegitimate births, that is births without marriage and where there is no legal recognition from at least one of the parents, both have significantly lower M/F sex ratios than so‐called legitimate ones. But, although many claims have been made and are still being made to the contrary, I am not aware of any other reliable statistical evidence that maternal factors have independent effects of the sex ratio of births. In 1970 I became interested by accident in factors affecting the secondary sex ratio. I’d found there was a spate of unnecessary surgical inductions at the Canberra Community Hospital each Thursday resulting in the numbers of births on Thursdays and Fridays averaging almost 40% higher than on Saturdays and Sundays (Wells, 1980a). But a much more intriguing observation was that there had been two annually recurrent concurrent peaks in both the Canberra Hospital birth rates and M/F birth sex ratios. These peaks had occurred nine months after the Christmas New Year holidays and nine months after early June. However as I continued to monitor the ACT birth data I found the coincident February peaks in birth rates and M/F ratios were declining and by the 1980s had entirely disappeared while the concurrent peaks in September/October had continued. Annually recurrent bimodal peaks in births rates had been noted during the late 18th and early 19th centuries in several other countries; and Lowell J. Reed at the Johns Hopkins School of Hygiene and Public Health reported that these peaks and troughs had been flattening out in Boston during the 1900‐21 period and in London during the period 1880‐1921 (Reed, 1925). Huntingdon reported that several counties had seasonal variations in their live birth sex ratios (Huntingdon, 1938); and Herman M. Slatis of McGill University in Quebec showed there had been broadly similar bimodal peaks in the sex ratios of United States livebirths in the periods 1915‐30, 4 5 1931‐36 and 1942‐48 (Slatis, 1953). However prior to my observations in Canberra no one seems to have reported concurrent seasonal variations in birth rates and sex ratios. Now any of you lived in Canberra during the 1950s and 60s will have readily reasoned what Canberrans were getting up to during their Christmas New Year holidays down on the sunny south coast, and those of you who were old enough may have been doing the same yourselves. More frequent unprotected intercourse during the Christmas/New Year holidays seemed likely to have been the principal cause of the higher birth rates nine months later. But those of you who have only come to Canberra in recent years may puzzle about those annual June peaks in conception, why they gradually decreased and now no longer occur. The 1950s birth data had related to a period before the contraceptive pill and when most Canberra homes were poorly heated, poorly insulated and had no electric blankets. So when the nocturnal cold snaps began in late autumn unprotected couples may have tended to cuddle up in bed together for warmth with one thing often leading to another, and more babies nine months later. So the Canberra birth records suggested the rather obvious hypothesis that higher frequencies of unprotected sexual intercourse around the time of conception might also be causing higher M/F birth sex ratios (Wells, 1980b), and this behavioural hypothesis not only seemed to explain all the factors then well established as associated with variations in human birth M/F sex ratios but also to suggest a basic biological explanation and further promising lines of enquiry. But with such a simple explanation I’d at first assumed others would have thought of it before though perhaps not have got the credit in accordance with Stigler’s later Law of Eponymy that “no scientific discovery is named after its original discoverer” (Stigler, 1980), which in accordance with his law he attributed the original discovery to Robert K. Merton. Several hundred theories had been advanced about factors influencing the sex ratio of human births (secondary sex ratio) but after an extensive review of the literature I had not found one that included a sound physiological basis which explained all the known reliable observations and suggested simple practicable experiments to further test its validity. A suggestion in the so called Hippocratic writings that a man wanting a son should “try to the utmost of his strength” may have foreshadowed the theory that frequency of intercourse affects the secondary sex ratio. But I suspect this was either written by one of his two sons Thessalus and Draco or by his son‐in‐law Polybus because it wasn’t quoted in the works of Aristotle who admired and quoted other ideas of Hippocrates. Aristotle, who didn’t begin writing till some time after the death of Hippocrates, was meticulous in recording the originators of other theories about sex determination current at his time. These are included in the 1831 German Aristotelis Opera by Immanuel Becker but were Bowdlerised from several English translations whose sections were also renumbered to conceal this desecration. Another hint of the coition rate theory appeared in the Babylonian Talmud where Raba advised that “one who desires all his children to be males should cohabit twice in succession” (Babylonian Talmud), and the niddah separation regulations still observed by some Ashkenazi Jews may encourage frequent coition around the time of ovulation in that they forbid sexual intercourse for some 13‐14 days from 24 hours before expected menstruation. 5 6 In 1662 Captain John Graunt, the so‐called father of epidemiology was the first to document that on average about 51% of human births were boys. At the time Graunt had become a wealthy London haberdasher who as a hobby studied the London borough bills of mortality and inter alia was able to predict from them when smallpox epidemics were about to occur, thus enabling King Charles II and his court to escape forty miles north to the safer rural environs of Audley End. And if John Graunt was the father of epidemiology, I’m rather proud that my putative ancestor Sir William Petty was the grandfather of epidemiology and medical statistics because he’d first led Graunt as a young man to take an interest in vital and health statistics when employing him to do a demographic survey of two villages in Hampshire. Graunt later repaid this early patronage by buying Petty a professorship in music at Gresham College in Oxford where Petty mainly taught not music at Gresham but anatomy at Brasenose and became famous for resuscitating Nan Green after she’d been hanged in 1650. Two years later Petty was appointed as Surveyor‐General of Ireland which enabled him to snaffle up so many properties there that he became one of wealthiest men in Britain. Following national registration of births and deaths evidence soon became available about relationships between the secondary sex ratio, length of marriage and ages of parents soon became available in the better developed countries; and reliable evidence about the effect of war on the secondary sex ratio became available in the early 1920. But strangely the first explicit suggestion that the frequency of sex intercourse might be relevant was not published till 1952 (Jalavisto, 1952). However, based solely upon an inadequate series of less than 20,000 births in Sweden and Finland and ignoring many published contrary findings Eeva got the relationship upside down. McKeown got it right way up in 1957 (McKeown, 1957), and then in 1971 William James also began to flirt with idea. However he only came round last year (James, 2009) – 38 years later ‐ to accepting that coital rate was one such factor and still has not proposed a rational physiological explanation for it. Now when coition rates seemed to explain my 1970 findings about birth sex ratios in Canberra and more importantly all the well established correlations with length of marriage, birth order, war and age of husband it was clearly impracticable to test this hypothesis directly with a prospective investigation involving the recording of intercourse frequencies in some hundreds of thousands of human couples, though it might have been possible in other mammals. But because of subtle species differences in sexual physiology any results from such experiements could not be applied with any certainty to human. Nevertheless I did discuss the possibility of animal experimentation with that great raconteur and wine buff Bede Morris here at the ANU with whom I shared a commercial interest in breeding Charolais cattle. We had both come to suspect that increasing the ratios of bulls to cows in a paddock increased the M/F ratio of resulting calves. But like most farmers we were more interested in measures to maximise the births of heifer calves, and reducing the ratios of breeding bulls to cows in our herds would have risked substantially reducing the pregnancy rates and thus our herd profitabilities. So the most promising but indirect way to test the coition rate hypothesis seemed to be by investigating whether concurrent fluctuations in rates and sex ratios of births had occurred in 6 7 other parts of the world where from my WHO experience the national birth records appeared to be sufficiently reliable. That requirement greatly reduced the possibilities, and for example I had to reject Hongkong where it was reported that monthly conceptions were 40% higher in January‐
March when the mean temperature was about 15C than in May‐September when the mean temperature was 25‐30C” (Chang et al., 1963) because my enquiries indicated that some Chinese families in Honglong were practicing selective female infanticide, though to a far lesser extent than in mainland China. I also followed up some investigations by my former NH&MRC colleague Victor Macfarlane. Some of you may fondly remember Victor when he was at the ANU. and after moving to the Waite Institute in Adelaide he reported in 1969 on the seasonality of conception in various parts of Australasia, the Indian sub‐continent, Europe and North America. He concluded the highest conception rates occurred “during physically equable and comfortable weather”. In contrast Victor reported that “in Northern Australia and India when ambient temperatures approached or exceeded 28‐31°C the rates were at their lowest” (Macfarlane, 1969). However I was unable to match the European and North American birth rate data Victor had used with accurate and contemporary information on birth sex ratios. I knew birth sex ratio data for the Indian sub‐continent were too unreliable for my purposes; and there were far too few births in Northern Australia to provide statistically significant results. Nevertheless the Bureau of Statistics did make a special tabulation for me from which I found that the M/F sex ratio of non‐
indigenous births was lowest across the tropical north of Australia where, for what it is worth, there were actually more female than male births. So I concentrated instead on Europe where I suspected there might be some truth in Tennyson’s couplet. “In the Spring a livelier iris changes on the burnish’d dove; in the Spring a young man’s fancy lightly turns to thoughts of love.” A French Government representative on one of my WHO committees helped me gain access to the 1970‐74 birth records for each of the French provinces, and as spring moved north through France from Le Cote d’Azur to Le Pas du Nord there were concurrent peaks in birth rates and birth sex ratios nine months later. Peaks in the Greek birth rates had occurred each year from 1956 to 1979 were reported to have occurred nine months after the movable festival of Easter (Athanassenas, 1985). Trading again on my WHO connections I obtained access to the original and already somewhat musty Greek birth registration records for 1988 and found there had also been coincident peaks that year in the Greek birth rates and M/F birth ratios nine months after Easter. Two major reasons for these findings in Greece were that Easter was (and I understand still is) the most popular time for weddings and twenty years ago there was a strong religious objection in Greece to use of the contraceptive pill. The French study had also shown that asyncronous regional seasonal variations could be concealed when amalgamated into overall national statistics, and the Greek study had emphasised the importance of understanding the culture of a region when attempting to understand its vital statistics. But paradoxically, as more nations have become sophisticated enough to produce reliable birth statistics these have tended to become less useful in studying seasonal variations in 7 8 birth rates and sex ratios because more effective contraception and family planning have tended to smooth out or even eliminate what might otherwise have been naturally recurrent peaks and troughs. But while the coition rate theory had survived two small and indirect tests based upon the studies of French and Greek seasonal variations in birth rates and birth sex ratios, the much deeper question of a physiological explanation remained. I might mention at this point that unravelling the physiology of human sex determination has taken many false turns. Histological misinterpretation of early foetal gonads in the 19th century led to the widely held view that fertilisation produced many more male than female embryos with most of this excess of males dying off before birth. But this zeitgeist changed 180 degrees when Theophilus Painter obtained fresh testicular tissue in 1921 from three inmates of the Texas State Insane Asylum after they were castrated “because of excessive self‐abuse” and it was then shown that meiosis produced equal numbers of X and Y chromosome spermatozoa (Painter, 1923). However some adherents to the older theory of differential sexual mortality before birth then applied Maier’s Law “if facts do not conform to the theory, they must be disposed of.’’ (Maier, 1960) and speculated that if equal numbers of male and female embryos were conceived the female products of conception must be dying off before birth slightly more quickly than the male ones. With such a volte‐face and so many other hypotheses based solely upon mere speculation or induction from faulty or inadequate observations it’s perhaps not surprising that by the 1930s research into human sex determination had acquired a poor academic reputation – a reputation which was further justified by many manifestly false claims of methods to produce male or female children as required. In the second half of the 20th century academic research and publications on the mechanisms causing variations in the birth sex ratio were influenced to an extraordinary extent by two men who became the first ports of call to referee others papers on the subject submitted to the leading journals. One was William James from the Galton Laboratory at University College in London who has covered many statistical aspects of the subject in over 50 papers from 1965 to the present day, and the other was Nathaniel Meyer Victor, the 3rd Baron Rothschild who became recognised in the 60s and 70s as the leading world expert on the physiology of spermatozoa and was firmly convinced that they just swam about at random and did not show chemotaxis. Chemotaxis of spermatozoa was subsequently demonstrated to occur in several mammalian species both in vitro and in vivo, and both mature human oocytes and their pre‐ovulatory surrounding cumulus cells in the follicles were shown to secrete chemo‐attractants for human spermatozoa (Fei Sun et al., 2005). Artificial insemination has an amusing history which unfortunately I cannot go into here, and when AI became commonly used in domestic livestock there was soon commercial interest in measures to produce X‐chromosome enriched sperm so as to increase the proportion of female offspring. Partial fractionation was successfully achieved by several different methods, including differential active progression of spermatozoa through gels and viscosity gradients, and similar 8 9 methodology was then applied to achieve some degree of sex selection in humans with artfiicial insemination. In both human other mammalian species the Y‐chromosome spermatozoa were reported to actively propel themselves through gels and against viscosity gradients quicker on average than the X‐chromosome forms. So whether or not human spermatozoa exhibited chemotaxis in the female genital tract it seemed to me that such a differential swimming speed would increase the proportion of Y‐chromosome forms in the advancing front of a fresh sperm swarm actively moving up the Fallopian tubes to the normal sites for fertilisation in the ampullae. But as a general rule, faster active progression, whether it be running or flying or swimming, consumes more energy than slower progress both per units of time and distance travelled. So although spermatozoa mainly depend for energy upon metabolising fructose they absorb from the surrounding media it seemed likely to me that faster swimming Y‐spermatozoa might lose their fertilising capacity quicker than the slower swimming X‐chromosome forms. The combined effects of such differential swimming speeds and durations of fertilising capacity would be that an ovum released to encounter the oncoming front of a single fresh sperm swarm would have the highest probability of being fertilised by a male forming Y‐chromosome spermatozoon, while an ovum released to meet the remains of a sperm swarm nearing the end of its fertilising capacity would, if fertilised, have a higher chance of becoming a female embryo. Similar reasoning would suggest that when unprotected intercourse is repeated in humans at intervals shorter than the maximum fertilising viability of spermatozoa (about 48 hours) the sperm swarms in the female genital tract would overlap and keep topping up those still capable of fertilisation in the ampullae with fresh spermatozoa containing higher proportions of Y‐
chromosome ones. So the higher the frequency of unprotected intercourse the higher the likelihood that a resulting conceptus would be male. As far as I’m aware neither the possible differential active progress of X and Y‐chromosome spermatozoa to the ampullae of the oviducts nor the possible differential X and Y losses of fertilising capacity have as yet been directly investigated in humans or any other mammal. But it would now be technically feasible to do so both in humans and other mammals by examining the proportions of X and Y spermatozoa in the ampullae of the oviducts at various times after natural intercourse and/or artificial insemination using the double label fluorescent in‐situ hybridisation (FISH) technique developed by Tie Lan Han and his colleagues at the Queen Elizabeth Hospital in South Australia (Tie Lan Han et al., 1993). However it was only when thinking about this talk that I realised there was another powerful and hitherto unpublished argument for the physiological explanation. The eminent French statistician Bertillon suggested a method for estimating the proportions of identical (monozygotic) and non‐identical (dizygotic) twins based on the assumption that half of the latter would be different sex twin pairs (Bertillon, 1874). However as another example of Stigler’s law of eponymy a substantially similar version of Bertillon’s method is usually referred to as “Weinberg’s differential rule” (Weinburg, 1902). There are many problems in identifying with certainty whether same‐sex pairs are monozygotic or dizygotic, but a rigorous study by William James showed a highly significant excess of same‐sex 9 10 over opposite‐sex dizygotic twins (James, 1971); James then collated results from 6 published series in which the zygosity of twins had been determined at birth solely on the basis of autosomal markers and he reported 725 were same‐sex pairs and only 609 were opposite‐sex twins, an apparent 19% excess of same‐sex pairs (James, 1979). Such significant and substantial excesses of same‐sex pairs provide important evidence for a physiological hypothesis to explain the coition rate theory because they suggest that (i) when two ova are fertilised in the same cycle this is likely to be by sperm containing substantially more same‐sex than different‐sex chromosome spermatozoa; and therefore (ii) as freshly ejaculated semen contains equal numbers of X‐chromosome and Y‐chromosome spermatozoa there must be some fractionating process between semen deposition at the external os and arrival of spermatozoa at the ampullae of the oviducts where fertilisation usually occurs; and further (iii) as the secondary sex ratios of singletons and dizygotic twins are generally similar, some of the sperm fertilising pairs of ova leading to dizygotic twins must contain an excess of X‐
chromosome spermatozoa and some an excess of Y‐chromosome ones, and (iv) such a biphasic fractionation of spermatozoa would require the interaction of two physiological processes, such as the proposed differential chemotactic motility to the follicular factors released at ovulation (Ralt et al, 1991) and differential loss of fertilising ability. The above physiological hypothesis would explain both the observed excess of same‐sex over different‐sex dizygotic twins and the way coition rate affects the secondary sex ratio, because the more frequently coition is repeated the more likely an ovum ready to be fertilised will encounter the oncoming Y‐chromosome enriched front of an advancing sperm swarm; and if coition is repeated within two days the second and later coitions would each top up the proportion of viable Y‐chromosome spermatozoa at the normal site of fertilisation. Furthermore, as with any useful scientific hypothesis, this one could easily be directly tested, for example in female dogs and pigs by using the double label fluorescent in‐situ hybridisation (FISH) technique to examine the ratios of X and Y‐chromosome spermatozoa in the ampullae at various times after insemination around the period of ovulation. However similar tests in women might nowadays encounter ethical difficulties, though mobile spermatozoa have previously been demonstrated within 30 minutes of coitus at the uterine ends of the oviducts in volunteer women requiring laparotomies (Rubenstein et al., 1951), and in similar women a team led by Diane Fordney Settlage found mobile spermatozoa at the uterine ends of their oviducts within 5 minutes of artificial vaginal insemination (Settlage et al., 1953). That would have been far faster than they could have swum all that way from the vagina unaided and implied active peristaltic sperm transport had occurred in the uterus. It was later shown that around the time of ovulation inert particles placed near the cervix were quickly moved towards the uterine end of the oviduct on the side where ovulation occurred. But as far as is known this process would move X and Y‐
chromosome spermatozoa at equal speeds towards the oviducts where the spermatozoa would then have to swim up the oviducts to the ampullae under their own steam. While ethical considerations for experimentation are now quite rightly much more stringent than when the Rubenstein and Settlage teams were doing their investigations, it might still be 10 11 practicable to find a few suitable volunteers to have artificial insemination 30 minutes to 48 hours before laparotomy. However ethics committees might well object to repeating the Rubenstein methodology involving insemination by natural intercourse a few minutes before laparotomy. Intercourse, conception and births in the tropics Many of you must have made love in the hot tropics and found that without air‐conditioning it could be a hot and sweaty business, and instead of cuddling up in bed companionably afterwards as lovers may in cold climates and then perhaps soon doing it again, couples without air‐
conditioning in the tropics may prefer to sleep a little apart to minimise the heat radiation received from each other. In the 1950s I was told the wives in Malay kampongs tended to sleep next to the open windows while their husbands slept near the inner walls – a practice not conducive to frequently repeated copulation ‐ though this custom appeared to be so that the short flight mosquitos would do their pit stops on the women just inside the windows and spare the Malay husbands. So to reconsider Kristen Navara’s report in the light of the coition rates hypothesis and Victor Macfarlane’s findings, and even if some of the CIA birth data she used may have been somewhat unreliable, her conclusion was probably correct that the M/F ratio of births tends to be lower in tropical latitudes than in temperate and sub‐arctic countries. Summary A lower male/female ratio of human births in the hot tropics than in the cooler temperate and sub‐artic regions provides further support for the coition rate hypothesis linking the frequency of unprotected intercourse around the time of conception with the male/female ratio of resulting births. It’s impracticable to test the coition rate hypothesis directly in humans; and because of family planning and the widespread use of contraceptive measures it has now become difficult if not impossible to accumulate large enough new series of reliable birth records to test the hypothesis indirectly by comparing concurrent seasonal variations in rates and sex ratios of births. However an observed excess of same‐sex over different‐sex dizygotic twins supports a proposed physiological mechanism to explain the coition rate hypothesis and could now be easily tested in experimental animals and also with appropriate ethical controls in volunteer women prior to abdominal surgery by measuring the X/Y ratio of spermatozoa in the oviduct ampullae at various times after artificial vaginal insemination in the peri‐ovulatory period. References Arbuthnott, J. 1710. An argument for Divine Providence taken from constant regularity observ’d in the births of both sexes. Philos. Trans. R. Soc. 27, 186‐180. Athanassenas, G. A. 1985. Seasonal variation of births in Greece. Chronobiologica, 12, 351. Babylonian Talmud. Seder Tohorot. Translated into English. Edited by Rabbi I. Epstein. Tractate Niddah 31a‐31b. Published by Soncino Press, London, 1948). Bertillon, M., 1874. 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