A rational person can have a negatively-sloped labor supply curve.

A rational person can have a negatively-sloped labor supply curve due to the income and substitution effects. Initially, as wages increase, the substitution effect may dominate, leading the individual to work more (supply more labor) in place of leisure. However, after reaching a certain income level, the income effect can dominate, where the individual values leisure more than the additional income from working extra hours, thus choosing to work less as wages increase further. This results in a negatively-sloped portion of the labor supply curve.