Sex is often thought of as a straightforwardly binary categorical variable. Yet there is considerable variation in would-be sex traits; from genitals and hormones to morphology, neurology and behaviour, there is rarely if ever a categorical binary. We introduce a strategy that researchers use to deal with this variation: Individualising Variation (IV). IV involves treating non-binary and gradual variation as idiosyncratic, as individual differences rather than sex-based differences. Using the contrasting cases of sex identification in field ornithology and the debate (...) about sex differences in neuroscience, we illustrate IV and investigate its epistemic and conceptual consequences. We argue that IV stabilises the ontological picture of sex as categorical and binary. While IV can be an epistemically benign research strategy in some cases, we argue that it can also be epistemically detrimental. This is because of its ability to mask evidence that would otherwise challenge related assumptions about the phenomenon of interest, such as what sexes are and what they look like. We also identify an alternative strategy, De- individualising Variation, which works against IV and helps life scientists recognise variation beyond categorical binaries. (shrink)