Almost no subject is more steeped in myth and lore than the business of choosing the gender of children. Aristotle and the Talmud both recommended placing the bed on a north-south axis for those wanting boys. Anaxagoras s belief that lying on the right side during sex would produce a boy was so influential that centuries later some French aristocrats had their left testicles amputated. At least posterity had its revenge on Anaxagoras, a Greek philosopher and client of Pericles: He was killed by a stone dropped by a crow, no doubt a retrospective reincarnation of some future French marquis who cut off his left testicle and had six girls in a row."
It is a subject that has always drawn charlatans like blowflies to a carcass: The old wives tales that have answered the pleas of fathers for centuries are mostly ineffective: The Japanese Sex Selection Society promotes the use of calcium to increase the chances of having a son—with little effect: A book published in 1991 by two French gynecologists claimed precisely the opposite: that a diet rich in potassium and sodium but poor in calcium and magnesium gives a woman an 80 percent chance of conceiving a son if consumed for six weeks before fertilization: A company offering Americans gender kits for $50 was driven into bankruptcy after the regulators claimed it was deceiving the consumer:
The more modern and scientific methods are somewhat more reliable. They all rely on trying to separate in the laboratory Y-bearing (male) sperm from X-bearing (female) sperm based on the fact that the latter possess 3.5 percent more DNA. The widely licensed technique invented by an American scientist, Ronald Ericsson, claims a 70 percent success rate from forcing the sperm to swim through albumen, which supposedly slows down the heavier X-bearing sperm more than it does the Y-bearing sperm, thus separating them. By contrast, Larry Johnson of the United States Department of Agriculture has developed a technique that works efficiently (about 70 percent male offspring and 90 percent female:) It dyes the sperm DNA with a fluorescent dye and then allows the sperm to swim in Indian file past a detector: According to the brightness of the sperm s fluorescence, the detector sorts them into two channels: The Y-bearing sperm, having smaller amounts of DNA, are slightly less brightly fluorescent: The detectors can sort sperm at ►oo,000 a second: Early concerns that the dyes might cause genetic damage have been largely allayed by animal experiments and this technique is now being used in the United States, mostly by people who wish to balance the family —have a girl after a string of boys, or vice versa:
Curiously, if humans were birds, it would be much easier to alter the chances of having young of one gender or the other because in birds the mother determines the gender of the embryo, not the father: Female birds have X and Y chromosomes (or some times just one X), while male birds have two Xs. So a female bird can simply release an egg of the desired gender and let any sperm fertilize it. Birds do make use of this facility. Bald eagles and some other hawks often give birth to females first and males second. This enables the female to get a head start on the male in the nest, which enables it to grow larger (and female hawks are always larger than males). Red-cockaded woodpeckers raise twice as many sons as daughters and use spare sons as nannies for subsequent broods. Among zebra finches, as Nancy Burley of the University of California at Santa Cruz discovered, attractive males mated with unattractive " females usually have more sons than daughters, and vice versa. Attractiveness in this species can be altered by the simple expedient of putting red (attractive) or green (unattractive) bands on the male s legs, and black (attractive) or light blue (unattractive) on the female s legs. This makes them more or less desirable to other zebra finches as mates."
But we are not birds: The only way to be certain of rearing a boy is to kill a girl child at birth and start again, or to use amniocentesis to identify the gender of the fetus and then abort it if it s a girl. These repugnant practices are undoubtedly on offer in various parts of the world. The Chinese, deprived of the chance to have more than one child, killed more than 250,000 girls after birth between 1979 and 1984.66 In some age groups in China, there are 122 boys for every 100 girls. In one recent study of clinics in Bombay, of 8,000 abortions, 7,997were of female fetuses."
It is possible that selective spontaneous abortion also explains much of the animal data. In the case of the coypu, studied by Morris Gosling of the University of East Anglia, females in good condition miscarry whole litters if they are too female-biased, and they start again. Magnus Nordborg of Stanford University, who has studied the implications of sex-selective infanticide in China, believes that such biased miscarriage could .explain the baboon data. But it seems a wasteful way to proceed:68
There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died. (This would make little sense; the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older mothers are more likely to have boys. Women with infectious hepatitis or schizophrenia have slightly more daughters than sons: So do women who smoke or drink. So did women who gave birth after the thick London smog of 1952. So do the wives of test pilots, abalone divers, clergymen, and anesthetists. In parts of Australia that depend on rainfall for drinking water, there is a clear drop in the proportion of sons born 320 days after a heavy storm fills the dams and churns up the mud. Women with multiple sclerosis have more sons, as do women who consume small amounts of arsenic."
Finding the logic in this plethora of statistics is beyond most scientists at this stage. William James of the Medical Research Council in London has for some years been elaborating a hypothesis that hormones can influence the relative success of X and Y sperm: There is a good deal of circumstantial evidence that high levels of the hormone gonadotrophin in the mother can increase the proportion of daughters and that testosterone in the father can increase the proportion of sons.'°
Indeed, Valerie Grant s theory suggests a hormonal explanation for the returning-soldier effect: that during wars women adopt more dominant roles, which affects their hormone levels and their tendency to have sons. Hormones and social status are closely related in many species; and so, as we have seen, are social status and sex ratio of offspring. How the hormones work, nobody knows, but it is possible that they change the consistency of the mucus in the cervix or even that they alter the acidity of the vagina. Putting baking soda in the vagina of a rabbit was proved to affect the sex ratio of its babies as early as 1932."
Moreover, a hormone theory would tackle one of the most persistent objections to the Trivers-Willard theory: that there seems to be no genetic control of the sex ratio. The failure of animal breeders to produce a strain that can bias the gender of its off spring is glaring: It is not for want of trying: As Richard Dawkins put it: "Cattle breeders have had no trouble in breeding for high milk yield, high beef production, large size, small size, hornless-ness, resistance to various diseases, and fearlessness in fighting bulls: It would obviously be of immense interest to the dairy industry if cattle could be bred with a bias toward producing heifer calves rather than bull calves: All attempts to do this have singularly failed."'
The poultry industry is even more desperate to learn how to breed chickens that lay eggs that hatch into chicks of only one gender: At present it employs teams of highly trained Koreans, who guard a close secret that enables them to sex day-old chicks at great speed (though a computer program may soon match them"): They travel all over the world plying their peculiar trade. It is hard to believe that nature is simply unable to do what the Korean experts can do so easily.
Yet this objection is easily answered once the hormonal theory is taken into account. Munching enchiladas in sight of the Pacific Ocean one day, Robert Trivers explained to me why the failure to breed sex-biased animals is entirely understandable: Suppose you find a cow that produces only heifer calves: With whom do you mate those heifers to perpetuate the strain? With ordinary bulls— diluting the genes in half at once.
Another way of putting it is that the very fact that one segment of the population is having sons makes it rewarding for the other segment to have daughters. Every animal is the child of one male and one female. So if dominant animals are having sons, then it will pay subordinate ones to have daughters. The sex ratio of the population as a whole will always revert to I:I, however biased it becomes in one part of the population, because if it strays from that, it will pay somebody to have more of the rare gender. This insight occurred first to Sir Ronald Fisher, a Cambridge mathematician and biologist, in the 1920s, and Trivers believes it lies at the heart of why the ability to manipulate the sex ratio is never in the genes:
Besides, if social rank is a principal determinant of sex ratio, it would be crazy to put it in the genes, for social rank is almost by definition something that cannot be in the genes. Breeding for high social rank is a futile exercise in Red Queen running. Rank is relative. You can t breed for subordinate cows, said Trivers as he munched. You just create a new hierarchy and reset the thermostat. If all your cows are more subordinate, then the least subordinate will be the most dominant and have appropriate levels of hormones: Instead, rank determines hormones, which determine sex ratio of offspring."
REASON'S CONVERGENT CONCLUSION
Trivers and Willard predict that evolution will build in an unconscious mechanism for altering the sex ratio of an individual s progeny. But we like to think we are rational, conscious decision makers, and a reasoning person can arrive at the same conclusions as evolution. Some of the strongest data to support Trivers and Willard comes not from animals but from the human cultural rediscovery of the same logic:
Many cultures bias their legacies, parental care, sustenance, and favoritism toward sons at the expense of daughters. Until recently this was seen as just another example of irrational sexism or the cruel fact that sons have more economic value than daughters. But by explicitly using the logic of Trivers-Willard, anthropologists have now begun to notice that male favoritism is far from universal and that female favoritism occurs exactly where you would most expect it.
Contrary to popular belief a preference for boys over girls is not universal. Indeed, there is a close relationship between social status and the degree to which sons are preferred. Laura Betzig of the University of Michigan noticed that, in feudal times, lords favored their sons, but peasants were more likely to leave possessions to daughters. While their feudal superiors killed or neglected daughters or banished them to convents, peasants left them more possessions: Sexism was more a feature of elites than of the unchronicled masses."
As Sarah Blaffer Hrdy of the University of California at
Davis has concluded, wherever you look in the historical record, the elites favored sons more than other classes: farmers in eighteenth-century Germany, castes in nineteenth-century India, genealogies in medieval Portugal, wills in modern Canada, and pastoralists in modern Africa: This favoritism took the form of inheritance of land and wealth, but it also took the form of simple care. In India even today, girls are often given less milk and less medical attention than boys."
Lower down the social scale, daughters are preferred even today: A poor son is often forced to remain single, but a poor daughter can marry a rich man. In modern Kenya, Mukogodo people are more likely to take daughters than sons to clinics for treatment when they are sick, and therefore more daughters than sons survive to the age of four. This is rational of the Mukogodo parents because their daughters can marry into the harems of rich Samburu and Maasai men and thrive, whereas their sons inherit Mukogodo poverty. In the calculus of Trivers-Willard, daughters are better grandchildren-production devices than sons."
Of course, this assumes that societies are stratified: As Mildred Dickemann of California State University has postulated, the channeling of resources to sons represents the best investment rich people can make when society is class-ridden. The clearest patterns come from Dickemann s own studies of traditional Indian marriage practices: She found that extreme habits of female infanticide, which the British tried and failed to stamp out, coincided with relatively high social rank in the distinctly stratified society of nineteenth-century India. High-caste Indians killed daughters more than low-caste ones. One clan of wealthy Sikhs used to kill all daughters and live off their wives ' dowries:"
There are rival theories to explain these patterns, of which the strongest is that economic, not reproductive, currency determines a sexual preference: Boys can earn a living and marry without a dowry: But this fails to explain the correlation with rank. It predicts, instead, that lower social classes would favor sons, not higher ones, for they can least afford daughters. If instead grandchildren production was the currency that mattered, Indian marriage prac tices make more sense. Throughout India it has always been the case that women more than men can marry up, into a higher social and economic caste, so daughters of poor people are more likely to do well than sons. In Dickemann s analysis, dowries are merely a distorted echo of the Trivers-Willard effect in a female-exogamous species: Sons inherit the status necessary for successful breeding; daughters have to buy it. If you have no wealth to pass on, use what you have to buy your daughter a good husband. 79
Trivers and Willard predict that male favoritism in one part of society will be balanced by female favoritism elsewhere if only because it takes one of each to have a baby—the Fisher logic again. In rodents the division seems to be based on maternal condition. In primates it seems to be based on social rank. But baboons and spider monkeys take for granted the fact that their societies are strictly stratified. Human beings do not. What happens in a modern, relatively egalitarian society?
In that uniform middle-class Eden known as California, Hrdy and her colleague Debra Judge have so far been unable to detect any wealth-related sex bias in the wills people leave when they die. Perhaps the old elite habit of preferring boys to girls has at last been vanquished by the rhetoric of equality.8o
But there is another, more sinister consequence of modern egalitarianism. In some societies the boy-preferring habit seems to have spread from elites to the society at large. China and India are the best examples of this: In China a one-child policy may have led to the deaths of 17 percent of girls. Im one Indian hospital 96 percent of women who were told they were carrying daughters aborted them, while nearly 100 percent of women carrying sons carried them to term.81 This implies that a cheap technology allowing people to choose the gender of their children would indeed unbalance the population sex ratio.
Choosing the gender of your baby is an individual decision of no consequence to anybody else. Why, then, is the idea inherently unpopular? It is a tragedy of the commons—a collective harm that results from the rational pursuit of self-interest by individuals. One person choosing to have only sons does nobody else any harm, but if everybody does it, everybody suffers. The dire predictions range from a male-dominated society in which rape, lawlessness, and a general frontier mentality would hold sway to further increases in male domination of positions of power and influence. At the very least, sexual frustration would be the lot of many men:
Laws are passed to enforce the collective interest at the expense of the individual, just as crossing over was invented to foil outlaw genes. If gender selection were cheap, a fifty-fifty sex ratio would be imposed by parliaments of people as surely as equitable meiosis was imposed by the parliament of the genes.
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