Relatives with lower paternity uncertainty are perceived as kinder

According to a large study published in Evolutionary Psychology, people consistently perceive family members as kinder when there is greater certainty of biological relatedness.

Humans often assume that kindness within families is driven mainly by love, shared history, or cultural expectations. Yet evolutionary theories suggest that altruism within families may also be shaped by genetic relatedness. According to kin selection theory, people are predisposed to invest more care and support in relatives who are more likely to share their genes, because such investment indirectly promotes their own genetic success.

One important factor complicating this picture is paternity uncertainty, the fact that, unlike maternity, biological fatherhood is never absolutely certain. Radim Kuba and Jaroslav Flegr examined whether this uncertainty influences how people perceive kindness among different family members.

Drawing on evolutionary psychology and prior findings on parental and grandparental investment, they asked whether relatives associated with higher paternity certainty (such as mothers or maternal grandmothers) are consistently seen as kinder than those associated with lower certainty (such as paternal grandfathers).

The researchers analyzed data from a large online survey conducted between 2016 and 2021. Participants were recruited through a Czech and Slovak Facebook-based volunteer community using a snowball sampling method, allowing the study to reach a broad internet population. Nearly 15,000 individuals began the survey, and after exclusions, 9,128 adult participants who rated at least one family member were included in the final analyses.

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Participants completed an extensive questionnaire and were asked to rate the kindness of various family members, such as parents, grandparents, siblings, and step-relatives, ranging from “strongly disagree” to “strongly agree” in response to statements like whether a given relative was kinder than other people. Importantly, the concept of kindness was left intentionally broad, allowing respondents to draw on lifelong experiences, including emotional support and everyday prosocial behavior.

The findings revealed a clear and consistent pattern: perceived kindness decreased as paternity uncertainty increased. Mothers and maternal grandmothers (relatives with no paternity uncertainty) received the highest kindness ratings, followed by fathers, maternal grandfathers, and paternal grandmothers, who carry one level of uncertainty. Paternal grandfathers, associated with two layers of uncertainty, were rated lowest among biological grandparents. These differences were statistically reliable, even though their size was modest.

Importantly, this pattern did not appear among step-relatives. Step-family members, who share no genetic relatedness and identical levels of paternity uncertainty, were rated similarly to one another, regardless of role. This contrast strengthens the authors’ interpretation that genetic relatedness, and not just social roles or cultural stereotypes, drives the observed differences.

Additional analyses showed that daughters tended to rate their biological parents as kinder than sons did, a pattern consistent with evolutionary predictions about investment through more certain maternal lines.

Overall, this study suggests that even in modern societies, subtle evolutionary pressures linked to genetic certainty continue to shape how people perceive kindness and altruism within their families.

Of note is that the voluntary, non-representative nature of the sample, particularly its relatively high level of education, may limit the generalizability of findings. Further, kindness ratings were subjective and may reflect personal relationship quality rather than purely objective behavior.

The research, “The Evolutionary Roots of Familial Altruism: Paternity Uncertainty Shapes Patterns of Kindness“, was authored by Radim Kuba and Jaroslav Flegr.