In 2005, he made a similar observation when he looked at data collected in California after the terrorist attacks of Sept. 11, 2001.
The birth cohort, born four to five months after the attacks, had fewer males.
This was an observation, he said, that corresponds with what biologists know about stress, pregnancy and sex ratios.
Researchers have long known that males tend to be weaker than females, as fetuses, embryos, children and adults. Indeed, it's not until people reach their 90s that men start outliving females, Catalano said.
It's also been shown that more male fetuses and embryos are spontaneously aborted than females. This becomes particularly apparent after six weeks of gestation.