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In many cultures, parents want at least one son, since a son may be needed to perform certain rites and traditional ceremonies and to carry on the family line. But the proportion of families with zero sons will of course increase as couples have only two or three children (or fewer) instead of four or five children (or more), as couples did in earlier generations. Under its one-child policy, China gave stiff penalties to families with multiple children, making it all the more likely that parents would want a male birth from the first pregnancy.
In regions with shrinking family size along with a strong preference for sons — that's basically every region in the world today with a skewed sex ratio at birth — most sex selection occurs through abortion, Hvistendahl writes. An ultrasound test midway through the second trimester will usually reveal the sex of a fetus. Then, where the technology is available, parents who want a boy may abort a female so that they can try again for a male without increasing the size of their family.
In fact, among many populations with an atypical sex ratio at birth, the ratio for first births is still close to its natural level: somewhere around 105 males for every 100 females. (Nature almost always produces slightly more males because they tend to have higher death rates than females at all ages; the surplus at birth leaves roughly one man per woman at reproductive age.) However, the proportion of births that are male tends to rise with birth order. An article published online in May in the medical journal The Lancet shows that Indians who first give birth to a girl are more likely to use sex-selective abortion during a second or subsequent pregnancy — presumably to ensure that their family eventually includes at least one son. Sex-selective abortion is illegal in India, but the procedure appears increasingly common.
Next:The Surprising Roots of Sex Selection
Greater and greater numbers of boys are being born for every girl. In her new book, Unnatural Selection: Choosing Boys Over Girls and the Consequences of a World Full of Men, Beijing-based journalist Mara Hvistendahl investigates what’s driving the sex imbalance.