From The Economist,

Avner Ahituv of the University of Haifa and Robert Lerman of the Urban Institute found that “entering marriage raises hours worked quickly and substantially.” Married men drink less, take fewer drugs and work harder, earning between 10% and 40% more than single men with similar schooling and job histories.

Thanks to Greg Mankiw for the pointer.

The article refers to a paper by Robert Lerman, which says

One revealing comparison is between women in shotgun marriages and women who did not marry before the birth of their first child. All women in both groups became pregnant before marriage. Note that the women entering shotgun marriages experienced a 38 percent higher level of living standards and a 20 percent lower variability of living standards.

Kevin Lang, in Poverty and Discrimination, takes a more skeptical view of the impact of marriage. My sense is that the results depend a lot on how one controls for other factors that affect income. If you do a poor job of controlling for other factors that affect income, then marriage will appear to account for more than if you do a good job.