The study examined birth and death records in seven populations across different countries and time periods. In six out of the seven, a paternal grandmother's presence benefited granddaughters, making them less likely to die in early life. But in all seven, a paternal grandmother actually made her grandsons more likely to die. Maternal grandmothers apparently had no effect. The study may explain why the "grandmother hypothesis" — that women live past menopause because they improve the survival rates of their children's children — hasn't been supported in research. Grandmothers and grandchildren don't all share the same amount of genetic material: maternal grandmothers share about 25% of genes with both grandsons and granddaughters, while paternal grannies share 23% with grandsons and 31% with granddaughters. So grandmothers seem to have a protective effect on the grandchildren they're most closely related to, and, surprisingly, a negative effect on their most genetically distant descendants.

So are grandmothers actually smothering their grandsons in the crib? Are they consistently saving the last cookie or, say, swine flu shot for their granddaughters to the extent that the poor boys actually croak? Scientists aren't sure. Study author Molly Fox says, "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." This is even creepier in a way — grandmothers are apparently unconsciously doing something that makes their grandsons more likely to die. Also kind of creepy: the scientists speculate that grandchildren give off "signals" that show how genetically related they are to their grandmas. These signals might be a special smell, or just facial resemblance.

It's hard not to be a bit skeptical of these results, and they may not apply in a society like ours where children often live far away from their grandparents. We usually think of this as a bad thing, but maybe seeing Grandma only over the holidays actually keeps Junior safe. Just to be sure, we should probably start crafting special "granny masks" for babies so they can fake facial resemblance to their grandmothers. That won't be scary at all.

Grandmothers: Good For Girls, Bad For Boys [Independent]
Why Grandmothers Prefer Their Son's Daughters [Telegraph]

Tennessee's High Infant Death Rate [ABC]