Optimal Outbreeding in Human Populations

Key Insight

For human beings, there exists an optimal genetic distance for reproduction that maximizes fitness. This 'optimal outbreeding' balances the risks of inbreeding depression with the potential downsides of excessive outbreeding. The ideal scenario is not simply having the most babies, but rather producing the largest number of fit offspring who survive to reproduce a third generation and beyond, maximizing the parents' share of the future gene pool.

The Icelandic population, with its extensive genealogical records, provided a unique natural laboratory to study optimal outbreeding. A study of 160811 couples born between 1800 and 1965 revealed that the most closely related couples (second cousins or closer) had the highest number of children. However, their children reproduced at significantly lower rates than those of less related spouses, leading to fewer grandchildren on average. This second-generation drop in fertility for inbred couples was attributed to higher mortality rates in the first five years of life due to deleterious recessive genes.

The Icelandic study, published in Science in 2008, graphically demonstrated the simultaneous effects of both inbreeding and outbreeding depression in humans. It concluded that the peak in fitness, measured by the number of grandchildren, occurs for matings somewhere between third and fourth cousins. While this specific optimal level may vary slightly across different environments and gene pools, the existence of such an optimum in the middle of the inbreeding-outbreeding continuum is clearly supported, guiding human mating patterns for generational sustainable fertility.