Labor economists estimate that married men earn a wage premium of 10 percent to 50 percent over single men. There is some debate as to whether higher wages are caused by marriage or simply correlated with it. The causal theory holds that employers prefer married men because they perceive them to be more productive and thus single men who choose to marry will tend to receive higher wages. It is also possible that the qualities that make a man a desirable mate also make him a desirable employee, and that higher wages are correlated with but not caused by marriage.
See "Analyzing the Marriage Gap", Hal R. Varian, The New York Times, July 28, 2004