Callitrichid primates typically give birth to twin offspring that are somatic chimeras of cells derived from two products of conception. Each individual is thus the phenotype of two sibling genotypes, one of which may be more closely related to the germ line of the individual's parents than to the individual's own germ line. Chimerism could therefore help to explain the evolution of alloparental care and social suppression of reproduction in callitrichids. Placental chimerism may also have important implications for understanding kin interactions within the womb: on one side of the coin, the intimate juxtaposition of genotypes provides unique opportunities for antagonistic interactions between embryos; on the other side, chimerism could facilitate cooperation between sibling genotypes.