Of the 512 homicide cases closed by the Detroit police between 1972 and 1981, 25 percent of the victims were related to their killers. However, only 6.3 percent of this group were consanguineous relatives while the rest were spouses, in-laws, or stepchildren. This low proportion of genealogical homicides is not unusual in American studies, although levels tend to be higher in other countries. A recent analysis of homicide case reports in 13th century England showed that over two-thirds were perpetrated by two or more offenders. Only 35 percent of the 177 related victims (2 percent of all victims) were blood kin while 65 percent were related by marriage. However, 75 percent of the 567 related co-offenders were blood kin. Other empirical studies and ethnographic research on blood feuds confirm these patterns. While commonality of interest as well as kinship contribute to family cohesiveness, spousal harmony suffers if one party perceives the other as contributing less to the partnership or using joint resources for personal ends. Thus, conflicts caused by steprelationships, adultery, and polygamy may result in homicide. The motives and circumstances surrounding many kin killings manifest evolutionary adaptive logic, as demonstrated in cases from the Detroit sample involving defense of one relative against another and sibling rivalry over property or money. Tables and 41 references are provided.