A new UK study says granddaughters who live with their paternal grandmothers are likely to live longer.
However, in case of grandsons, the presence of a gran is not a good idea.
According to the research from Cambridge University, the survival rate of a granddaughter living in the same house as her paternal grandmother is higher than if she were just living with her immediate family, reports The Independent.
To reach the conclusion, Molly Fox of Cambridge University and her colleagues analysed the birth and death records of seven populations in Asia, North America, Europe and Africa who had lived in different periods going back to the 17th century.
In the study, they looked at infant mortality in the first three years of life and found that it differed depending on whether paternal or maternal grandmothers were present in a grandchild's early life.
"The presence of a paternal grandmother in all seven of the populations had a harmful effect on grandsons because her presence was linked with an increase in mortality," Fox said.
"Meanwhile, in six out of seven populations, the paternal grandmother's presence in her granddaughter's early life had a beneficial effect in terms of the risk of mortality.
"This difference between paternal grandsons and granddaughters would explain a lot of the inconsistencies in previous studies, where the sex of the grandchild was not considered," Fox said.
"We've only looked at infant mortality, and the mechanism itself remains mysterious. Other studies have given evidence against conscious favoritism towards one grandchild or another," she said.
The study, in Proceedings of the Royal Society B, could help to explain the evolution of female longevity: grandmothers live beyond their menopause to help bring up their grandchildren.