Palaeodiet of humans can be determined using stable carbon isotope ratios of bone collagen. Differences in δ13C‐values between individuals in a population may be due to real differences in diet, or alternatively, they could be due to age‐ or sex‐dependent differences in the physiologically controlled fractionation of carbon isotopes between food and collagen. The dependence of this fractionation on age and sex was tested in a study of 50 individuals of differing ages from a prehistoric population of bison hunters. The total variation in δ13C for bone collagen was found to be ± 0.3‰, indicating that variations larger than this, observed in other populations, are due to real dietary differences between i