Scientists Discover Why Parents May Favour One Child Over Another

© iStock

An international team that included Prof. Marina Butovskaya from HSE University studied how willing parents are to care for a child depending on the child’s resemblance to them. The researchers found that similarity to the mother or father affects the level of care provided by parents and grandparents differently. Moreover, this relationship varies across Russia, Brazil, and the United States, reflecting deep cultural differences in family structures in these countries. The study's findings have been published in Social Evolution & History.

From an evolutionary perspective, humans—like any other biological species—are driven to pass on their genes to future generations. This principle underlies the concept of inclusive fitness: we unconsciously strive to ensure the survival and well-being of those who share our genes. As a result, relatives’ willingness to care for a child and to invest resources such as time, effort, and money varies according to the degree of genetic relatedness.

Since shared genes cannot be directly observed, the human brain relies on physical resemblance as a quick and convenient marker. Unconsciously, we interpret similarity in appearance as a reliable signal: ‘if a child looks like me, they likely carry my genes, and it is worth investing time and resources in them.’ For a long time, this mechanism was thought to be especially important for fathers and paternal relatives, who seek confirmation of biological relatedness. Mothers and maternal relatives do not require such evidence, since the woman carries and gives birth to the child.

An international team of scientists from Russia, the USA, and Brazil, including researchers from HSE University, set out to test how well this theory holds and how it aligns with patterns of childcare in different countries.

In 2019, the researchers conducted online surveys in Russia, the USA, and Brazil, collecting responses from 302 Russians, 308 Americans, and 605 Brazilians.

To determine how much participants resembled their parents, the researchers asked them to self-assess similarity based on five criteria—such as 'overall resemblance' and 'hair colour and texture'—using a six-point scale from 0 to 5.

A similar scale was used to assess the maternal and paternal direct care participants received as children. They rated the accuracy of statements such as 'I felt that my mother was emotionally supportive' and 'My father usually helped me solve problems.' The objectivity of this method has been confirmed in previous studies.

Marina Butovskaya

'Our methodology is objective and has previously been validated on a large dataset. In total, respondents answered a battery of 21 questions, which were then grouped into three scales: parent–child resemblance (10 questions), parental effort (8 questions), and grandparent assistance (3 questions). In terms of reliability, this questionnaire is not in any way inferior to other psychological assessment tools,' explains Marina Butovskaya, Corresponding Member of the Russian Academy of Sciences, Doctor of Sciences in Anthropology, and Professor at HSE University.

The contribution of the older generation was assessed through questions about the frequency and closeness of participants’ contact with their grandparents during childhood.

Using Bayesian methods, the researchers analysed participants’ responses to identify patterns common across all countries, as well as culturally specific differences. Their findings confirmed the hypothesis regarding the influence of inclusive fitness on fathers and paternal relatives: the more a child resembled the father, the greater the investment from him and his side of the family. An unexpected finding was that greater father–child resemblance was associated with reduced effort from the mother and maternal grandparents. At the same time, mothers were more attentive and caring toward children who resembled them more. This suggests that the mechanism of recognising kinship through resemblance is not unique to 'paternity-uncertain' fathers—it also operates in mothers.

'Even with absolute certainty of motherhood, the degree of biological relatedness between a mother and her children is not entirely uniform. For a mother and her relatives, therefore, physical resemblance is not a meaningless signal, but an unconscious cue that helps guide care and attention toward the child who likely carries more of their genetic material,' Professor Butovskaya explains.

The researchers identified not only universal biological patterns but also cultural variations in childcare involvement. Participants from the USA showed the highest levels of paternal engagement. In Russia, maternal grandparents emerged as the primary caregivers, making the most significant contribution to grandchild-rearing among all types of relatives across the three countries.

© iStock

The data from Brazil also showed distinctive patterns. Grandparents in this country—particularly paternal grandparents—were the least involved in childcare. The researchers attribute this to rapid urbanisation, significant migration, and the weakening of traditional intergenerational ties. In Brazil, childcare support is often provided by government institutions, as well as godparents and neighbours.

The study demonstrates that even in modern, urbanised societies, people are still influenced by ancient evolutionary mechanisms. The willingness to care for children is not unconditional and can be shaped by unconscious cues, such as physical resemblance. The observed cultural differences can also inform the design of social programmes. For example, in Russia, it is important to acknowledge and support the significant role of grandmothers in raising grandchildren by providing social support systems or programmes for the older generation. At the same time, efforts should be made to strengthen fathers’ sense of parental responsibility.