The "good sperm hypothesis" is very common in polyandrous mating systems. The "good sperm hypothesis" suggests that a male's genetic makeup will determine the level of his competitiveness in sperm competition. When a male has "good sperm" he is able to father more viable offspring than males that do not have the "good sperm" genes. Females may select males that have these superior "good sperm" genes because it means that their offspring will be more viable and will inherit the "good sperm" genes which will increase their fitness levels when their sperm competes.