Scientists believe that the sex ratio is equal at conception, explained Steven Orzack, president and senior research scientist of the Fresh Pond Research Institute in Cambridge, Massachusetts. But more than half of all human conceptions die during gestation, and this results in a sex imbalance at birth.
“Overall, more females die during pregnancy than do males. So that’s why there’s an excess number of males at birth,” said Orzack, who has published research on this issue.
Ray Catalano, a professor in the school of public health at the University of California, Berkeley, explained that the process of natural selection in utero is why deaths occur during gestation.
A mother’s biology spontaneously aborts some conceptions in utero but not others. The factors that filter out who “gets through” from conception to birth include chromosomal or genetic abnormalities of the fetus or the mother’s stress response to changes in her environment, Catalano said.
Because the ovaries of a female fetus carry all the eggs she will ever possess, the possibility of genetic defects being found in a female (and her eggs, which represent her potential children) are greater than the possibility of defects in a male fetus, which carries only his own genes.
Worldwide, the newborn sex ratio averages 103 to 106 males born for every 100 females, Catalano explained. In part, this is a result of the fact that a male infant are “a relatively frail organism,” he said.
“For every society, for every year, the human being most likely to die [prematurely] is male infants. And that’s true for every society that we have data for,” Catalano said.
The reasons why are not understood, but some scientists believe that boys are biologically weaker WEand more susceptible to diseases and premature death.
The general theory as to why the sex ratio is not equal at birth is that if you want the sex ratio to be 50:50 by reproductive age, you want a few more males than females at birth because more males than females will probably die in early childhood, Catalano said.