The importance of grandmothers in the lives of their grandchildren is underlined in a study published today.
But the research showed that it was only granddaughters who were likely to do better with their paternal grandmothers involved in their early lives. In contrast, the presence of paternal grandmothers had a detrimental effect on the survival of their grandsons.
The discovery supports the idea that grandmothers have played an important role in human evolution and could explain why human females - alone among the animal kingdom - live well beyond their reproductive age.
Molly Fox of Cambridge University and her colleagues tested out the idea by analysing 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.
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," Ms 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," Ms Fox said.
"We've only looked at infant mortality, and the mechanism itself remains mysterious. Other studies have given evidence against conscious favouritism 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.
It is widely believed that the reason human females live well beyond the menopause is so that grandmothers can invest their energy in raising their children's children rather than risking further pregnancies of their own.
The "grandmother hypothesis" suggests that all grandchildren benefit from having either of their grandmothers involved in their early upbringing. But studies have so far failed to support the hypothesis with consistent evidence.
It is normally assumed that all grandchildren share about 25 per cent of their DNA with each of their four grandparents but Ms Fox, a doctoral student in Cambridge's department of biological anthropology, pointed out that the female "X" sex chromosome of grandmothers is not inherited equally between their grandchildren, which could explain why some do better than others with each grandmother.