Altruism among siblings vs. friends in a contemporary affluent population

Inclusive fitness theory predicts that altruistic behavior is more common among kin than non-kin. Using population-based data from younger and middle-aged Finns (n = 1254), the present study investigated childcare help, financial support, and practical help received from full siblings and friends. The results showed that childcare help was substantially more likely to be received from siblings than from friends (45.5% vs. 36.7%). Moreover, financial support was slightly more common among siblings than among friends (4.5% vs. 3.4%), although the difference was not statistically significant. In contrast, practical help was substantially less likely to be received from siblings than from friends (45.1% vs. 58.3%). Childcare help represents a higher-cost form of help than practical or financial support, as it typically requires substantial physical and mental effort, significant time investment, and a high level of responsibility and commitment. At the same time, when provided by siblings, such help may substantially enhance the helper's inclusive fitness. The present findings suggest that while a large share of people receive lower-cost help from friends, higher-cost help is predominantly received from kin.