Grandparents cannot help having favorites and prefer the grandchildren with whom they share the most DNA

The phenomenon is known as sexually antagonistic grandparental care and explains earlier studies showing a paternal grandmother benefits granddaughters' health more than grandsons. This is because a woman passes around 31% of her genes to her son's daughters but just 23% to her son's sons – suggesting a protective effect for the grandchildren they are most closely related to.