It was found that a non-significantly greater proportion of siblicides of siblings-in-law were perpetrated by beatings relative to the proportion of siblicides of full siblings. It was found that a significantly greater proportion of accidental deaths occurred among siblicides of full siblings relative to the proportion of siblicides of siblings-in-law. The greater proportion of accidental deaths among sibling-in-law siblicides might be interpreted to reflect a lesser degree of rage or resentment present between full siblings. Siblicide, the killing of one sibling by another, is rare relative to other types of homicide. This study of siblicide could provide insight into sibling relationships and nonlethal sibling conflict. The study tested two predictions about siblicides using a city-level Chicago, IL database of homicides committed during 1870 through 1930. First, a greater proportion of siblicides of siblings-in-law will be perpetrated by beatings, relative to the proportion of siblicides of full siblings. Second, a greater proportion of accidental deaths will occur with siblicides of full siblings relative to the proportion of siblicides of siblings-in-law. Tables, note, references