Why do grandparents tend to play favourites? This question is often asked in different forums, such as advice columns, columns and chats (e.g., Hax, 2015). There are three common features in these writings. First, mothers are typically the ones who complain that some of the grandparents are playing favourites. It is much harder to find fathers writing about the subject matter in these forums. Second, the paternal grandmother is typically the person accused by the mother. Third, in addition to biological paternal grandparents, stepgrandparents (particularly the stepgrandparents of children from the mother’s previous unions) are blamed for preferring their own biological grandchildren. These points direct interest towards factors that are relevant when we want to understand the grandparental tendency to play favourites. These factors are genetic relatedness, sex and lineage.