(Agencies)
Updated: 2008-03-25 09:43

WASHINGTON - Infant boys are more likely to die than infant girls in industrialized countries, although the disparity has narrowed since the 1970s, US researchers said on Monday.

Mortality data spanning 15 countries -- 11 in Europe plus Canada, the United States, Japan and Australia -- showed this gap was at its widest in 1970 when boys had about a 30 percent higher chance than girls to die by age 1, the researchers said.

In the past three decades, the gap has closed a bit, with boys this decade having roughly a 20 percent higher chance of death by age 1 than girls, according to Eileen Crimmins of the University of Southern California, one of the researchers.

This narrowing has occurred due to medical practices that seem to have helped more infant boys survive, including more Caesarean sections and intensive care units for premature babies, the researchers said.

The study found that during the past century, as infant mortality dropped in these countries, its primary causes switched from infectious ailments such as diarrheal diseases to congenital conditions and complication tied to childbirth and premature delivery.

Boys are 60 percent more likely than girls to be born prematurely and to have conditions tied to pre-term birth such as neonatal respiratory distress syndrome, a condition that makes it difficult for a baby to breathe, the researchers said. This syndrome can occur in infants whose lungs have not yet fully developed.

Infant boys also face a higher risk of birth injury and mortality due to their larger body and head size, they said.

Crimmins said higher mortality rates among men often are attributed to their riskier behavior such as smoking or violent acts. But behavior is replaced by biological explanations among infants, she added.

"Males and females can have very different mortality at an age when behavior is not a factor," Crimmins said in a telephone interview. "We tend to think that males have higher mortality at all ages because they behave worse, basically. But this (infant mortality) is a case where they don't behave any differently."

"It was so large, so consistent across so many countries, and it couldn't be explained by behavior," Crimmins added.

The findings were published in the journal Proceedings of the National Academy of Sciences.