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There is conflicting evidence from studies on the fitness consequences of plant kin interactions, suggesting that kinship is not the only important factor in determining the outcome of kin interaction.Here, we tested whether density‐dependent kin interactions have evolved from competition to cooperation by kin selection of reduced shade avoidance responses following the introduction of an invasive plant. We measured how plants respond to kin neighbours (the same accession) and simulated vegetation shade (lower PPFD and R:FR ratio treatments using transparent green plastic filters) and determine whether shade avoidance responses affect the outcome of kin interactions using five native (Argentina) and five invasive (USA) accessions of Alternanthera philoxeroides.Our results showed that invasive accessions exhibited constant stem elongation but increased total biomass as canopy density increased, while native accessions exhibited the opposite. Furthermore, invasive accessions reduced stem elongation, but increased biomass and defence allocation in response to an experimental treatment causing a low R:FR ratio, while native accessions exhibited the opposite.Synthesis. Our results support the hypothesis that kin selection against shade avoidance may effectively promote the evolution of density‐dependent kin cooperation. This suggests that kin selection pressures may play a more important role in fine‐tuning plant shade avoidance responses than generally thought. Kin selection against selfish shade avoidance behaviour and release from negative density‐dependent regulation provides a new mechanism to explain plant invasion success for future studies. [ABSTRACT FROM AUTHOR]