When Should Animals Tolerate Inbreeding?

We compare, in an initially outbred population, the number of offspring equivalents expected by an individual that avoids all inbreeding with that expected by an individual that tolerates one inbred mating. The model suggests that for most mating systems, the sole factor determining whether inbreeding tolerance spreads is the cost of inbreeding avoidance. Specifically, most forms of polygyny do not increase the payoff to inbreeding; the critical parameter is not the number of matings an individual engages in but rather how many outbred matings are forfeited when an individual chooses to mate with a relative. The model also suggests that dispersal is unlikely to have arisen primarily as a mechanism to avoid inbreeding, and that father-daughter inbreeding should be more common than mother-son inbreeding.