This Gender Mystery Starts Nine Months Before Birth
Is it a boy or a girl? That’s what everyone wants to know from the expecting parents. The ratio of newborn boys to girls (called the secondary sex ratio) is a matter of social importance and, occasionally, even national policy. But there is another ratio that, despite being more obscure, is just as important: The ratio of boys to girls at conception, when the egg is fertilized and development begins, called the primary sex ratio.
While the secondary sex ratio is consistently about 106 boys for every 100 girls (or, just 106 for shorthand), the primary sex ratio has been a subject of persistent speculation over the centuries. Naïvely one might expect it to be 100—balanced, equal numbers of boys and girls. To see why it has attracted such attention, suppose it were 125 instead. Since the cells from which sperm are generated start with one X and one Y chromosome, that would mean that one-fifth of the X chromosomes either never make it into functioning sperm, or are prevented from fertilizing an ovum somewhere along the way. This is a huge bias, requiring a powerful but as yet undiscovered biological mechanism.
Or, if sperm are being sex-selected after they are created, that process would likely extend over time. Changes in the timing of intercourse could then have substantial influence on the likelihood of the resulting child being a girl. (Such effects have frequently been claimed, though inconsistent about the direction of the effect, and never conclusively demonstrated.)
At every age, in almost every time and place, a man is more likely to die than a woman.
The primary sex ratio could also speak to how reproductive technologies, nutritional supplements, medications, and environmental chemicals may be shifting the invisible scales of gender, influencing which embryo lives and which dies. Some major pollution exposures are known to cause dramatic changes in the secondary sex ratio: Small but statistically significant declines in the proportion of males born have been observed across several developed countries, including the Unites States and Canada, and there are suggestions that this may be an indicator of disruptive chemical exposures.
The larger the discrepancy between primary and secondary sex ratio, the more of one sex are being lost through a sex-specific mechanism. Making
