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Living with close relatives can be highly beneficial, enhancing reproduction and survival. However, high relatedness is also known to increase susceptibility to pathogens, a phenomenon commonly known as the ‘monoculture effect’. Here we examine if the benefits of living with relatives offset the harm caused by pathogens, and if this is more pronounced in species that live with kin. Using comparative meta-analysis across 56 species (plants n=11, animals n=44, bacteria n=1), we show that high relatedness within groups increases mortality when pathogens are present. In contrast, mortality decreases with relatedness when pathogens are rare, particularly in species that typically live with kin. Examining variation across groups showed that pathogens spread more consistently through groups of relatives, but that rates of mortality are more unpredictable. These effects were only evident when pathogens were experimentally manipulated, indicating that the effects of pathogens may be masked by the benefits of living with relatives in natural populations. These results show that the interaction between kin-selected benefits and the costs imposed by pathogens are important for understanding the evolution of social behaviour and disease spread in natural populations.