Most contemporary political philosophers deny that justice requires giving people what they deserve. According to a familiar anti-desert argument, the influence of genes and environment on people's actions and traits undermines all desert-claims. According to a less familiar – but more plausible – argument, the influence of genes and environment on people's actions and traits undermines some desert-claims (or all desert-claims to an extent). But, it says, we do not know which ones (or to what extent). This article examines this ‘epistemological’ argument against desert. It gives reason to believe that it fails, emphasizing the importance of justice relative to efficiency and attempting to construct a practical way of measuring desert.