Gender inequality across the world is associated with disproportionate death rates among girls under five years old, according to an analysis of data from 195 countries, including India.
The study, published in the journal BMJ Global Health, suggests that the unequal treatment of females in society could be interfering with the natural biological advantage they have over males for survival.
Explaining some of the reasons behind the disproportionate death rates among girls, the researchers draw on examples, such as India, where girls have lower odds than boys of receiving certain types of health care, including vaccinations, and are vulnerable to female infanticide.
"The more unequal a society, the more girls are penalised in terms of their survival chances, particularly in lower-middle income countries," said Valentina Gallo from Queen Mary University of London in the UK.
"Gender inequality then perpetuates itself through the generations via these unfair odds of survival," Gallo said.
She said because of a sexist ideology which values boys over girls, young girls are often at greater risk of mortality through diminished access to health resources, as well as through heightened exposure to health risks.
"These girls are also further exposed to this risk via their mothers, who may themselves be penalised and valued less than mothers of sons, and less able to provide for their daughters," said Gallo.
In 2015, a total of 5.9 million children under the age of five years died.
More than half of these early deaths were preventable and the result of socio-economic factors and barriers to healthcare.
Children are at a greater risk of dying before the age of five if they are born in rural areas, within poor households, or to a mother denied basic education, researchers said.
Previous studies have shown that greater gender inequality is associated with higher infant mortality, but up until now, it was not known whether boys or girls were more affected.
The study compared countries' sex-specific under-five mortality rates from the UNICEF database, with their Gender Inequality Index (GII).
GII is a measure of gender inequality that takes into account reproductive health, empowerment, and economic status, which varies from a minimum of 0.040 in Switzerland to a maximum of 0.767 in Yemen.
Overall, boys have a slightly higher mortality than girls worldwide, which is thought to be due to a relative biological advantage of girls over boys, researchers said.
However, the higher the gender inequality in a country, the more girls would die compared to boys, counteracting the girls' innate biological advantage, they said.
The researchers argue that global policy should extend its focus beyond economic development, and work to reduce gender inequality across a variety of domains including reproductive health, political empowerment, educational attainment, and participation in the work force.