Every human being has four grandparents: Paternal grandfather, paternal grandmother, maternal grandfather, and maternal grandmother. Think of your four grandparents now. And, regardless of whether they are currently alive, think of which of the four grandparents you are or were closest to in your life. Try to rank-order your four grandparents in terms of how close you are or were to them. Which grandparent are you closest to? Which grandparent are you least close to?
If you are like most people, regardless of whether you are a man or a woman, you’d probably conclude that you are or were closest to your maternal grandmother, followed by your maternal grandfather and paternal grandmother, and that you are or were least close to your paternal grandfather. This is in fact a universal pattern. A study of a large sample of undergraduate students at the University of Michigan shows that they are emotionally closer to, spend more time with, and receive more resources from maternal grandmothers than either paternal grandmothers or maternal grandfathers, and they are in turn closer to, spend more time with, and receive more resources from maternal grandfathers and paternal grandmothers than paternal grandfathers.
The pattern is reciprocal as well. From the perspective of the grandparents, a study of bereaved parents shows that maternal grandmothers grieve the death of their grandchildren more than either paternal grandmothers and maternal grandfathers, who in turn grieve more than paternal grandfathers. Similar results are found in studies conducted in Canada, Germany, and Greece. Other studies show that we generally feel closer to and have more contact with our maternal relatives in general than paternal relatives, despite the fact that we on average live closer to our paternal relatives than to our maternal relatives. I am not aware of any study of grandparents and their relationships with grandchildren that violates the order: maternal grandmother > paternal grandmother = maternal grandfather > paternal grandfather. Now what explains this peculiar but precise and universal pattern?
The key to understanding this pattern are the twin concepts of paternity uncertainty and maternity certainty. Mothers are always certain of their maternity, whereas fathers can never be completely certain of their paternity. (The twin concepts are well expressed in the common saying “Mommy’s baby, Daddy’s maybe.”) Because maternity is always certain, maternal grandmothers are always certain to be related to their grandchildren; there is no possibility of cuckoldry (and thus a break in the genetic chain) anywhere in their relationships to their grandchildren. In sharp contrast, paternal grandfathers have two reasons that they may not be related to their grandchildren: They may have been cuckolded by their wives, or their sons may have been cuckolded by their wives, or both. Maternal grandfathers and paternal grandmothers fall in between these two extremes; both categories of grandparents each have one reason that they may not be related to their grandchildren.
It therefore follows from evolutionary psychological logic that maternal grandmothers have more contact with, are closer to, and invest more heavily in their grandchildren than either maternal grandfathers or paternal grandmothers, who in turn have more contact with, are closer to, and invest more heavily in their grandchildren than paternal grandfathers. Of course, none of this is conscious on the part of either the grandparents or the grandchildren. We know we feel closer to our maternal grandmothers than to our paternal grandfathers, but we don’t know why. Maternal grandmothers love their grandchildren much more than paternal grandfathers do, but they don’t know why. Nor am I aware of any theoretical perspective besides evolutionary psychology which can explain the precise ordering: maternal grandmother > paternal grandmother = maternal grandfather > paternal grandfather.