Children have a stronger relationship with grandparents on their mother's side, a new study suggests.
Maternal grandparents go the extra mile, say the researchers
Researchers asked grandparents how often they had face-to-face contact with their grandchildren.
More than a quarter of maternal relatives questioned said they had contact several times a week, while the paternal figure was only about 15%.
The findings, by the universities of Newcastle and Antwerp, are published in the journal Evolutionary Psychology.
A sample of more than 800 grandparents from Holland was studied to establish the amount of social interaction between extended family members.
For grandparents living within 19.5 miles (30 km) of their grandchildren, more than 30% of the maternal grandmothers and 25% of grandfathers had contact daily or a few times a week.
In contrast, only about 15% of both paternal grandparents could say the same.
Thomas Pollet, from Newcastle University, said: "Even in families where there has been divorce, we found consistent differences - grandparents on your mother's side make the extra effort.
"We believe there are psychological mechanisms at play because throughout history, women are always related by maternity whereas men can never be wholly certain they are the biological father to their children."
This certainty suggests that maternal grandparents, especially maternal grandmothers, may go the extra mile to visit their grandchildren, he added.