Some complex human traits like the physical traits of facial appearance or voice, or the psychological traits of leadership or genius appear highly genetically-caused, as shown by relatedness, while still being extremely rare and unpredictable. A proposal for resolving this is emergenesis: such genetic traits are highly nonlinear and non-additive (epistasis or dominance/recessive), depending on many traits simultaneously, and failing otherwise, yielding familialness but rarity/unpredictability—identical twins or clones will be highly similar, but siblings little more similar than the general population.

“Truly exceptional individuals, weak or strong, are, by definition, to be found at the extremes of statistical curves…Since each individual produced by the sexual process contains an unique set of genes, very exceptional combinations of genes are unlikely to appear twice even within the same family. So if genius is to any extent hereditary, it winks on and off through the gene pool in a way that would be difficult to measure or predict. Like Sisyphus rolling his boulder up to the top of the hill only to have it tumble down again, the human gene pool creates hereditary genius in many ways in many places only to have it come apart in the next generation.”

E.O. Wilson⁠, On Human Nature 1978