<p>This paper contributes an explanation for the rise in zero-sum thinking in the politics of western democratic countries. It sets out two new mechanisms that we argue have respectively played roles in undermining positive sum thinking and boosting the zero-sum kind and so combined to shift the terrain of politics in the zero-sum direction. The first mechanism involves politicians being increasingly regarded as ‘delegates’ rather than ‘trustees’. This reduces the scope for politicians to respond to positive sum opportunities. The second occurs when citizens decreasingly socially identify with economic class towards more essentialist/othering non-economic social identities. This inclines people towards zero-sum thinking and so reinforces the effect of the relative boost towards zero-sum thinking from politicians increasingly becoming ‘delegates’. In our analysis, a decline in trust is associated with the first and an increase in inequality with the second. The two mechanisms combine to lower the material gains enjoyed under democracy. This not only undermines support for democracies, but it also reinforces zero-sum thinking in what can become a zero-sum trap.</p>