This analysis of previously collected data examined four fitness-relevant issues for their possible role in marital conflict. These were sex, finances, division of labor, and raising children, selected in light of their pertinence to sex differences in reproductive strategies. Over 2,000 couples in five diverse cultures were studied. Marital conflict was assessed by the Problems with Partner scale, which was previously shown to demonstrate measurement invariance across cultures and genders. All four issues were significantly related to perceived marital problems in almost all cases. Thus, conflict tended to arise around issues relevant to reproductive strategies. A few cultural idiosyncrasies emerged and are discussed. In all cultures, wives reported more problems than husbands. Another important issue was kindness. The results suggest that a key factor in marital success or failure may be kindness necessary to sustain this prolonged and intimate relationship of cooperation for raising one's offspring.