This compilation thesis consists of four distinct studies, each contributing valuable insights into the dynamics of labor supply, income inequality, life expectancy, and the child penalty in the labor market. 

The first study investigates the effects of Belgium's "flexi-job reform," which introduced tax-exempt secondary jobs for near-to full-time workers in specific sectors. Using a difference-in-differences estimation, the study finds a modest increase in secondary working time and wages and significant restructuring among firms in affected sectors. This research identifies a limited potential to boost labor supply at the full-time margin through targeted policy interventions.

The second paper explores the evolution of income inequality in Sweden from 1990 to 2021, revealing a significant increase in inequality. The study decomposes this trend by examining changes in the wage distribution, working hours, capital incomes, and looks at the effects of immigration, taxes, and benefits, to offer a comprehensive view of the factors driving income disparity in Sweden.

The third study delves into the rising income gradient in life expectancy in Sweden. Analyzing registry data from 1960 to 2021, the research uncovers a widening gap in life expectancy between the highest and lowest income percentiles. The study challenges the notion of a direct link between income inequality and life expectancy, suggesting that a "third factor" correlated with both may be driving this divergence, such as healthier lifestyles and a more rapid adoption of medical advancements among high-income earners.

The final paper examines the child penalty in labor market outcomes, focusing on the importance of genetic factors. Using Swedish administrative data and the Swedish Twin Registry, the study applies an ACE-model and relative fixed-effects regressions to assess the child penalty among twins. The findings show a substantial child penalty of 60.9% for genetically identical twins but no evidence of a long-term penalty, challenging the view that children are responsible for the long-term earnings differences between men and women.