Oliver Hauser and colleagues’ model of economics and game theory uses a technical parameter that they call ‘productivity’ (Nature 572, 524–527; 2019). This introduces an ambiguity that has political implications because it does not align with the usual meaning of productivity when applied to income inequality.

In the model, individuals can each contribute some portion of their allocated resources to public goods that pay out to all participants. The twist is that the multiplier between donated resources and societal payout can vary from individual to individual. This multiplier is referred to as ‘productivity’, a term that, with respect to income inequality, conventionally implies individuals with large economic output. The multiplier in Hauser and colleagues’ model refers instead to returns on the portion of invested resource — and only if they are donated back to create public goods.

Hauser et al. conclude that the optimal configuration of endowments, which results in the largest societal benefit, relies not just on inequality but on the unequal distribution of endowments to specifically favour “more productive individuals”. In other words, the term productivity is used to mean ‘effect of donation to public goods’ but seems designed to imply ‘productive’ in its conventional sense.

The inference is that inequality is a path to optimality, whereas productivity is intrinsic and not related to individuals’ endowments. Such ambiguous use of terminology risks compromising political impartiality and the goals of social equality and welfare.