Darwinian Grandparenting Redux: a Pre-registered Replication and Extension

Mothers’ fathers consistently invest more in their grandchildren than fathers’ mothers. This pattern was explained by Laham et al. in Personality and Social Psychology Bulletin , 31(1), 63-72. ( 2005 ) via the preferential investment hypothesis —the idea that fathers’ mothers invest less in grandchildren than mothers’ fathers because the former typically have more certain alternate investment outlets available. In two studies of prolific workers (combined N = 4086), we first failed to replicate the findings of Laham et al. and then successfully replicated them. In the combined sample, mothers’ fathers received more positive ratings than fathers’ mothers when participants had cousins through fathers’ sisters, but this difference between grandparents disappeared when participants did not have cousins through fathers’ sisters. We also found that people spent more time with their maternal and paternal grandparents to the degree that they were not maternal grandparents to someone else, which mediated the closer feelings.