First approximations

The point in each case is that while changes in taxes or transfers may induce changes in how much people work, when you assess these changes you have to bear in mind that, to a first approximation, workers are paid their marginal product. This means that if increased transfers induce some people to work less, it also causes them to earn less, so that the rest of society isn’t any worse off; if lower taxes induce high earners to work more, it also means that they’re paid more, so that the rest of society doesn’t reap any of the gains.

The marginal product changes substantially for the part time worker when adding transaction costs.