"Using a sample of professional baseball players from 1871 - 2007, this paper aims at analyzing a longstanding empirical observation that married men earn significantly more than their single counterparts holding all else equal. There are numerous conflicting explanations, some of which reflect subtle sample selection problems (that is, men who tend to be successful in the workplace or have high potential wage growth also tend to be successful in attracting a spouse) and some of which are causal (that is, marriage does indeed increase productivity for men). Baseball is a unique case study because it has a long history of statistics collection and numerous direct measurements of productivity. Our results show that the marriage premium also holds for baseball players, where married players earn up to 20% more than those who are not married, even after controlling for selection. The results are generally robust only for players in the top third of the ability distribution and post 1975 when changes in the rules that govern wage contracts allowed for players to be valued closer to their true market price. Nonetheless, there do not appear to be clear differences in productivity between married and nonmarried players. We discuss possible reasons why employers may discriminate in favor of married men."
It seems like they do a good job using ceteris paribus conditions (but I have not read the whole paper). They seem to be holding player quality or performance constant and they say that the control for selection. There is always the possibility that women prefer to marry men who make more money (in general married men make more but that does not mean that getting married caused their salary to go up). Somewhere they say that the 20% difference only applies to the top 1/3 of players.
For other views and discussions on this issue go to
When labour market research goes to the ballpark
Married MLB players earn more than single MLB players of the same quality?