FT251

Boy or girl? The difference between the sexes is an enduring fascination; gender is the main distinguishing characteristic of newborn children. A mass of folklore has grown up over the centuries around what influences the sex of a baby; now science has settled some of the mysteries – but has also revealed more.

There is a mass of traditional beliefs on how to ensure that you conceive a child of a particular gender. Wives were supposed to keep a wooden spoon (with a pink ribbon round it) under the pillow to ensure their child was a girl. Making love when the Moon was full, or on even-numbered days of the month, was also supp­osed to ensure a girl; the odd-numbered days and the dark of the Moon favoured boys. Making love while standing up, or eating more meat, were also supposed to promote male babies… and so on. While this may all seem quite harmless, in earlier centuries a woman might be blamed for failing to produce a son by exerting too much influence on the unborn child. Henry VIII’s troubled reign was just one example of this attitude.

In spite of all attempts to change the odds, the sex ratio, the proportion of male to females in the population, has remained roughly half and half. The reason for this equivalence is far from obvious. From a biological perspective, we wouldn’t seem to need that many males: we might expect that a lot more babies would be born if the population were 90 per cent or 95 per cent female, but evolution has settled on an even mix.

The English statistician Ronald Fisher solved this puzzle in the 1920s. If there were more females than males in a populat­ion, then a male child would have a much greater chance of passing on his genes. A mother with three daughters might average half a dozen grandchildren, whereas one with three sons might have a hundred. Select­ion would favour any genes that tended to produce male offspring. This advantage would only last as long as there was a majority of females, then the balance would swing the other way. So the ratio would always tend to be about even. [1]

Human population statistics work out pretty closely to Fisher’s principle, with around 105 boys born to every 100 girls. Differences in health and life expectancy mean that later in each generation there are more women than men.

We now know that in humans sex determ­ination is entirely down to the chromosomes carr­ied by the sperm that fertilises the ovum. But the environment can make a big difference to which type of sperm is more likely to succeed. X-carrying sperm (which produce females) have a longer lifespan, but Y-carrying sperm (which produce males) swim faster. This has led to the Whelan method and the Shettles method of sex determination, [2] which aim to improve the chances of one or other type of sperm by scheduling intercourse for particular days of the menstrual cycle. These do have some effect, but are far from reliable. Whelan claims that her method gives a 68 per cent chance of securing a boy, compared to the normal 52 per cent.

Scientists have also puzzled over the fact that men in certain occupations tend to have daughters. Studies have found that test pilots, divers, astronauts, anæsthetists and aluminium smelters all tend to have more daughters than sons. Some studies have suggested that hormone levels have the greaest influence. One researcher, Valerie Grant, has found that testosterone levels in the mother are a major influence: women with dominant personalities and high testosterone levels are more likely to have sons. [3] Therefore men who select less dominant women are more likely to have daughters. Perhaps if Henry VIII had married someone who was less compliant, he might have secured a male heir more easily.

There are also more general trends. A Finnish study showed that there were peaks in the number of male births in indust­rialised countries during WWI and WWII, followed by a reduction in the proportion of male births afterwards. [4]

A more recent trend is the decrease in the proportion of male births in industrialised countries; the reasons for this are far from clear. In some localised areas, this seems to be connected with certain pollutants. This might also explain why certain occupations (which might involve exposure to particular chemicals) tend to favour the birth of children of one gender.

Women exposed to PCBs were less likely to have sons. [5] PCBs, formerly used in paints, plastics, pesticides and carbon paper, are now banned. Airborne dioxins have a similar effect [6] and are produced in waste incinerators as well as oil refineries and paper mills. The effect is most notable in northern parts of Canada, where some communities in notably contaminated areas are seeing twice as many girls born as boys.

“They had enough girls for three baseball teams, but not enough boys for even one boy team,” commented Jim Brophy, director of a clinic in Sarnia, Ontario, of one local community. [7]

It may be that lower levels of these or other pollutants are having a much wider effect, hence the changing sex ratio. Or there may be other factors at work. If the old wives’ tales are right, then the rise of vegetarianism and the decrease in the proportion of children conceived while standing up may be having an effect…

NOTES
1 Ian Hardy: Sex ratios, Cambridge University Press 2002
2 Baby Centre advice on determining gender
3 Valerie Grant: Maternal Personality, Evolution and the Sex Ratio, Routledge 1998
4 PH Jongbloet, GA Zielhuis, HM Groenewoud, & PC Pasker-De Jong: 'The secular trends in male:female ratio at birth in post-war industrialized countries', Environmental Health Perspectives 2001 July; 109(7). See also FT183:54.
5 Science Daily, 16 July 2008
6 Piet Hein Jongbloet, Nel Roeleveld, and Hans M M Groenewoud, 'Where the Boys Aren't: Dioxin and the Sex Ratio', Environmental Health Perspectives 2002 January; 110(1)
7 Softpedia news, 31 October 2007