Inclusive fitness is defined as an animal's individual reproductive success, plus the positive and/or negative effects that animal has on its sibling's reproductive success, multiplied by the animal's degree of kinship. In instances of siblicide, the victim is usually the youngest sibling. This sibling's reproductive value can be measured by how much it enhances or detracts from the success of other siblings, therefore this individual is considered to be marginal. The marginal sibling can act as an additional element of parental success if it, as well as its siblings, survive. If an older sibling happens to die unexpectedly, the marginal sibling is there to take its place; this acts as insurance against the death of another sibling, which depends on the likelihood of the older sibling dying.