People are intensely interested in factors underlying earning power and what can be done to enhance it. A recent study using monozygotic male twins adds to knowledge in this area. Based on data from 136 twin pairs, the investigators favored a correlational hypothesis, namely that simply being married raises salaries by 19 to 26%. Following a review of this research, summaries of twin studies presented at the biennial meeting of the International Society for Human Ethology in Ghent, Belgium (July 27–30, 2004) and at the annual meeting of the Human Behavior and Evolution Society in Berlin, Germany (July 21–25, 2004) are presented. The final section of this article provides information of general interest to twin researchers, physicians and others working in twin-related fields.