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, including more Caesarean sections and intensive care units for premature babies. Its primary causes switched from infectious ailments such as diarrheal diseases to congenital conditions and complication tied to childbirth and premature delivery, the researchers said.

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, the researchers said.

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