How willing we are to share a water bottle, sit close to someone or greet them with physical contact is influenced above all by how important those people are to us. This is shown by a new study in the journal PNAS, in which researchers from institutions including the Karolinska Institutet have investigated people's social behaviours in 61 communities around the world.
People need social contact to build relationships. At the same time, close contact carries a risk of spreading infectious diseases. In this study, the researchers sought to understand how people balance these factors against one another.
The researchers collected data from 42,756 people in 61 communities worldwide, ranging from Sweden and other European countries to countries in Africa, Asia, North and South America, and Oceania. Participants were asked to imagine one of four types of person: their closest friend, an acquaintance, a stranger or someone they strongly dislike.
They then answered questions about how comfortable they would be in situations that might involve the transmission of microorganisms, such as drinking from the same water bottle or sitting so close that their legs touched. They were also asked to state whether they would touch the person when greeting them.
The results showed a clear pattern across all the societies included in the study: the greater the value people attributed to a relationship, the more positive they were towards contact involving a potential risk of infection. The correlation was similar regardless of country or cultural context.
"The study shows that people around the world appear to make similar trade-offs between social benefits and the risk of infection. How important another person is to us plays a central role in these decisions. Especially interesting is that the risk behavior is partly modified by the pathogen risk in the environment," says Mats J. Olsson , professor at the Department of Clinical Neuroscience, Karolinska Institutet, and one of the researchers behind the study.
The researchers also investigated whether societal-level factors, such as the historical prevalence of infectious diseases, individualism and social norms, could explain differences in people's behaviour. These factors did have some significance, but their influence was considerably less than the importance of the value placed on the relationship.
An interesting finding was that differences between societies were primarily evident in contact with people whom the participants valued highly, such as close friends. In societies with a higher historical prevalence of infectious diseases, people were slightly less comfortable with contact carrying a risk of infection, even with such individuals. For relationships of lower value, such as those with strangers, the differences were small.
The study was carried out by an international research network comprising researchers from universities and research institutes in 61 societies worldwide, with Karolinska Institutet being one of the participating institutions. See the study for details of funding and any conflicts of interest.
Publication
"Interpersonal value regulates infection-risky social contact across societies", Hongyu Sun et al, Proceedings of the National Academy of Sciences of the United States of America, online xx 2026, doi: xx
