Reproductive division of labour (RDL), where sterile 'helpers' assist specialised 'reproducers' in transmitting genetic information, has evolved repeatedly at vastly different biological scales. Examples include queen-worker division in eusocial colonies, germline-soma division in multicellular organisms, micronucleus-macronucleus division in ciliates, and genome-enzyme division in cells (enzymes provide catalysis to 'help' genomes transmit information). What drives this repeated evolution of RDL? The standard view posits that a critical driver is high relatedness, which promotes selection for altruistic helpers. Here, we use theoretical modelling to demonstrate the opposite possibility: the evolution of RDL is driven by low relatedness, which promotes selection for cheaters - selfish individuals that avoid cooperation and replicate uncontrollably, reducing group-level productivity. Our results show that RDL evolves as a means to increase relatedness and protect groups against cheater invasion. This mode of RDL evolution occurs only when relatedness is low because the protective advantage of RDL is beneficial only when the risk of cheater invasion is high, a situation requiring low relatedness. Our findings suggest that the evolution of RDL is a cause, rather than a consequence, of high relatedness, challenging a long-standing view on the evolution of RDL.