Charles Darwin and his son might have a few ideas
An illustration of the dire effects of genetic inbreeding can be found in the history of the Habsburg dynasty of Spanish and Austrian royalty. The family’s two centuries reign ‘survived’ nine consanguineous marriages of third cousins or closer. It came to an end when Charles II (1661-1700), who suffered severe mental and physical disabilities, including a deformed chin, the so-called ‘Habsburg jaw’ that prevented him from chewing his food, died without an immediate heir. Apart from their chronic infertility, the dynasty also experienced a debilitating rate of infant mortality with half of their offspring dying before age one. Spanish geneticists calculate the inbreeding coefficient for Charles to have been as high as that of from a parent-child or a brother-sister relationship, reducing the dynastic survival rate by as much as 18 per cent.
Closer home, when Eugène Casalis (1812 – 1891), Director of the Paris Evangelical Missionary Society and founder of the French Protestant Mission in Lesotho, visited his colleague, Isaac Bisseux, in Wellington in 1834, he was struck by the close family relations and assumed effects of intermarriage that he observed.
“In the district encompassing the Valley of Charron (Wellington), Drakenstein, Franschhoek, and Paarl, you are in the heart of the French refuge. We were amazed to hear French family names spoken all the time. Before us, there were D'Ailly, Cellier, Duplessis, Dutoit, Faure, Dupré, Jourdan, Joubert, Le Roux, Malan, Malherbe, Lombard, Lefebvre, Sabatier, Sénéchal, de Villiers, Prévôt, Pinard, Niel, Ménard, Taillefer, and so on. Their roots have faithfully remained in the areas initially assigned to them, where they have thrived extraordinarily.
These colonists are very proud of their heritage, and as if their names did not already indicate it, they always take care to point out that their complexion is darker and their hair has a different shade than that of their neighbours, the Van Wyks, Van der Walts, and other "Vans" of all kinds. However, in terms of ideas, language, customs, and habits, the colonists sent by the VOC in the Netherlands all resemble each other.
Families here have a very patriarchal footing. They are generally very close-knit, often even too much so. The habit of marrying cousins, which prevails especially among the descendants of the refugees, results in many cases of "phthisis" (a form of pulmonary tuberculosis) and other consequences of blood impoverishment”.
[Eugène Casalis: Mes Souvenirs – départ pour le Sud de l’Afrique & l’arrivée au Cap, Librairie Fisch Bacher, Paris, 1884]
Casalis’s observation confirms the notion that the quest for social exclusivity and/or wealth was so potent that it could disregard the opposition of John Calvin and the Reformed Church establishment to first cousin marriages. This historical account is not meant to imply that Afrikaners are an inbred people. On the contrary, the seminal research conducted by UP genetics professor, Jaco Greeff, affirms that Afrikaners today have inbreeding levels comparable to present-day European populations. Greeff attributes this phenomenon to their robust population growth (five times faster than North European populations) and their diverse ancestral origins.
Relative humidity, indeed.