[Opinion] Male Births Declining

In nature, the expected ratio of male to female births is believed to be 105. That is, for every 100 girls born, you would expect 105 boys to be born.

However, there are exceptions to this ratio. Darwin, in his books The Descent of Man and Selection in Relation to Sex, cites a sex ratio of 120 boys to 100 girls for Jewish communities in 19th century Livonia. Perhaps Darwin also didn’t have a clue what caused such irregularity, as he didn’t state it in the book.

There are other cases that documents gender imbalance. For example, the proportion of male births is greater in times of war. Some deduce that Mother Nature gives males an advantage at birth because the male mortality rate is higher than that of females at times of war.

The gap between male and female births is narrowing and the number of male births has declined. In the newest issue of Environmental Health Perspectives, Devra Lee Davis, a professor at the University of Pittsburgh Graduate School of Public Health, reports that the sex ratio in America decreased from 105.5 in 1970 to 104.6 in 2001, and from 106.3 to 105, respectively, in Japan. She insists environmental degradation may play a role in lowering the possibility of male gestation.

The reason why environmental toxins may affect the male birth rate is that environmental toxins weaken the survival rate of the male fetus and sperm with Y chromosome. It is the Y chromosome that determines gender. Only males have the Y chromosome. Genetically, the Y chromosome is very vulnerable. Conversely, the X chromosome is far bigger in size and has a longer genome sequence. While the X chromosome includes thousands of genes, the Y chromosome only consists of dozens of genes. That is why the X chromosome is dubbed as the “blessed chromosome.”

A rising sex ratio imbalance is a sign of danger. Some point out that a surplus of men was the implicit cause of the First World War. Wars in ancient times are often comprehended as the struggle of men to afford a spouse. Nowadays, however, the situation is shifting. The development of genome research revealed the vulnerability of the Y chromosome. Someday we might face an era when women should take care of their men for the same reason that caused ancient wars.

Chung Sung-hee, Editorial Writer, shchung@donga.com