Research has also shown there to be a "marriage premium" for men with labor economists frequently reporting that married men earn higher wages than unmarried men, and speculating that this may be attributable to one or more of the following causes: (1) more productive men marry at greater rates (attributing the marriage premium to selection bias), (2) men become more productive following marriage (possibly due to labor market specialization by men and domestic specialization by women), (3) employers favor married men, or (4) married men feel a responsibility ethic to maximize income.