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Grandparental investment, which is a worldwide phenomenon, is relatively neglected by Asian researchers. Although the amount of research on grandparental investment in Europe and Northern America has been increasing, there is less research on how grandchild factors influence grandmothers' investment in grandchildren. Drawing on sex chromosome relatedness theory and the hypothesis about phenotypic similarity cues, we designed an experiment to test how two grandchild factors (sex and resemblance) influence grandmothers' biased investment. In the experiment, 80 childless female undergraduates (M age = 19.62 years) who had to imagine being a grandmother were presented with an array of children's faces (which had been morphed to resemble theirs to varying degrees) and were asked to make hypothetical grandparental investment decisions. We showed that (1) hypothetical paternal grandmothers were more likely to invest in granddaughters than hypothetical maternal grandmothers; and (2) hypothetical paternal grandmothers were more likely to favor self-resembling grandchildren than hypothetical maternal grandmothers. These findings extend existing evolutionary accounts of grandparental investment by specifying how grandchild factors influence grandmothers' investment. [ABSTRACT FROM AUTHOR]