A natural tendency to see rare events as ominous would insure a basic similarity of response, and any disagreement would quickly be overcome by what Roger Brown calls 'an almost ineradicable tendency for members of a group to move toward agreement.'- Arthur P. Wolf, Incest Avoidance and the Incest Taboos
There is, then, no need to burden the incest taboos with the Herculean task of holding up society [which Freudians and their ilk propose]. They can be fully accounted for as the creations of two aspects of human nature - a fear of events perceived as abnormal or unnatural, and what social psychologists take to be a universal human need to 'belong with' those around us.
[...] Another [similar reaction] is the reaction to twinning. It is far more common than [post-puberty] incest but rare compared to singular births, the result being that it is commonly regarded as an abnormal event foretelling misfortune. [...] [T]he Italian pediatrician Alessandra Piontelli found that while that part of Southeast Asia known as the Golden Triangle was home to many peoples, 'they all shared one thing in common, a loathing of twins.' They were regularly disposed of at birth by strangulation for fear the mother had been impregnated by an evil spirit.
[...] There is [almost] always a consensus condemning incest because most people interpret incest as threatening and the few who do not, accept the majority view because they want to belong.