Dispersal as an Optimal Mating Mechanism

Key Insight

Dispersal, the movement of individuals from their natal territory, is a key mechanism for optimal outbreeding in both animals and humans, helping to avoid both excessive inbreeding and outbreeding depression. While ideal dispersal would involve moving a fixed radius to find genetically optimal mates, real-world environments with uneven resource distribution and threats make it more complex. Overcrowding and competition can also push individuals into less ideal areas.

In most mammal species, males typically disperse, while in most bird species, females leave the natal area. However, individual personalities can influence dispersal likelihood and extent, even among littermates facing the same external pressures. For instance, chimpanzees, our closest living relatives, exhibit female dispersal at puberty, with sexually mature females actively avoiding mating with their maternal brothers due to the costs of inbreeding depression, sometimes leading to violent male coercion.

In humans, while incest avoidance remains paramount (with incest within the nuclear family universally taboo), dispersal primarily serves an optimal-outbreeding function. Historically, human societies predominantly practiced patrilocality, where females dispersed, though matrilocality and neolocality also exist. A Danish study found that optimal fertility occurred at a marital radius of 75 kilometers, with drops at closer and further distances, suggesting both inbreeding and outbreeding depression. This dispersal distance is influenced by personality, with novelty-seeking individuals (linked to the D4 dopamine receptor gene, prevalent in populations with long-distance migration histories) likely traveling further, promoting outbreeding.