It's mentioned in the abstract: 'We deal with the potential endogeneity of the decision of how many hours to work by using the instrumental variable estimation

It's mentioned in the abstract: 'We deal with the potential endogeneity of the decision of how many hours to work by using the instrumental variable estimation technique.' Essentially they use 'instruments' which are just other variables to control for this issue. Page 7 has the variables that they're using.