When was inbreeding historically understood to cause defects in offspring?

• This is one of those tricky subjects where on the one hand, it would seem that clear, compelling scientific evidence would be readily available, and, OTOH, entrenched societal stigma makes human inbreeding a particularly thorny subject for methodical, scientifically rigorous studies. In short, it's not that data is surprisingly hard to find. Rather, rigorously and appropriately controlled studies are hard to come by.
• It's also important to parse human inbreeding into socially acceptable versus unacceptable. Socially acceptable? Marriages between cousins and uncles-nieces are common, indeed preferred, in many parts of the world, viz., North and sub-Saharan Africa, the Middle East, Central and South Asia (1, 2; see figure below for estimates).

Unsurprisingly, data on the socially acceptable form of inbreeding is more voluminous.

• For e.g., first-cousin marriages and marriages between girls and their maternal uncles are longstanding traditions in South India (the states of Andhra Pradesh, Karnataka, Tamil Nadu, Telangana) (3, 4, 5). 
• In the US, state level control determines whether first cousin marriages are legal or not, with criminal and civil sanctions in 8 and 22 states, respectively  (see figure below from 6; also 7, 8).
• OTOH, Europe has largely left consanguineous unions unlegislated.

In Written Historical Record

• In both Plato's Laws (dialogue) (9) and Xenophon's Memorabilia (Xenophon) (10), Socrates is quoted to mention 'unwritten laws' legislated by the gods. These laws couldn't be disobeyed without natural penalties.
• Socrates describes the incest taboo as one of these unwritten laws since it had a greater likelihood of producing defective offspring.
• Problem is these treatises don't mention why the taboo arose in the first place, i.e., no observed data on health of offspring from incestuous unions compared to non-incestuous ones.

Physical Evidence from Archaeology and Historical Census data

• Early 20th century genealogies of the 18th and 19th dynasty Egyptian kings (pre-332 B.C.) are unhelpful since they show 
  
  • No evidence of reduced reproductive capacity.
  • Little/no recorded proof of physical or possible mental defects in the mummies of royal brother-sister offspring (11).
• OTOH, in Roman Egypt, 19.6% and 3.9% of marriages in the city of Arsinoe were between brother-sister and siblings, respectively (12). This data comes from the census of Egypt held every 14 years during Roman rule. Each household was required to file a return listing all its members, their names, ages and kinship affiliation. While only a tiny fraction of the millions of papyrus census filings have survived, 300 have been published and subjected to demographic analysis.
• Pharoaonic Egypt believed that brother-sister unions strengthened the royal bloodline.
• Recent research reveals incest occurred outside the ruling classes as well (13, 14).
• These historical data don't support the idea that prehistoric or historic human may have observed greater birth defects from inbreeding and thereby instituted incest taboo as a cultural norm.

Edvard Westermarck: first hailed then ridiculed and now, a century later, finally vindicated?
The beginning, 1890 -1921.

• In Europe and North America, first-cousin marriages were commonplace until the mid-19th century.
• Starting in the 1850s, scientists and physicians in Europe and North America began to debate the biological effects of close kin marriage (15). 
• Charles Darwin's marriage to his 1st cousin, Emma Wedgwood, yielded 10 children. By lobbying Sir John Lubbock, he tried to persuade the Parliament of Great Britain and Ireland that the 1871 census should assess the prevalence of 1st cousin marriages. Unsurprisingly, his argument was predicated on sound science, 'When the principles of breeding and of inheritance are better understood, we shall not hear ignorant members of our legislature rejecting with scorn a plan for ascertaining by an easy method whether or not consanguineous marriages are injurious to man' (16). After all, we need robust data before we can conclude benefit or harm of a practice, don't we?
• However, the culture of the time prevailed against any such effort. The stated objections are telling signposts of the profound difficulty in inculcating scientific temper in society, a difficulty that stubbornly persists to this day.
• For example, one committee member, Gathorne Hardy stated, 'he did not see the desirability of holding up families where such marriages had taken place, and the children being anatomised for the benefit of science' (17).
• A generation later, Darwin's son G.H. Darwin managed to extract such data anyway by cleverly performing surname analysis to assess prevalence of first-cousin marriages (18, 19).
• Then in 1889, Finnish sociologist Edvard Westermarck became internationally famous when he published his colossal 3-volume survey, The History of Human Marriage.
• Analyzing the biological data available at that time, he concluded that inbreeding produced high rates of infant mortality, and mental and physical defects (20).
• Westermarck argued that harmful consequences of inbreeding selected for an innate aversion to sex with childhood associates.
• Thus, for Westermarck, observed harms of incest didn't create the cultural taboo against incest.
• Rather, natural selection naturally and inextricably linked the two.
• In modern re-telling, cohabitation during formative years creates mutual de-eroticization in co-socialized children. In simpler words, growing up together inculcates sexual aversion.

The barren years, 1934-1962.

• Shockingly, from the turn of the 20th century until the 1960s, Westermarck's ideas were actively derided and shunned by a literal who's who of 20th century biology, anthropology and psychology. This, even though early supporters of his ideas included the likes of Edward Burnett Tylor and Alfred Russel Wallace.
• Problem for Westermarck was that he wasn't only a Darwinian but also a steadfast empiricist. Thus, for him incest's biology, psychology and culture were inseparable.
• In the first instance, many simply didn't believe that close inbreeding was harmful.
  
  • Lord Raglan accused Westermarck of arguing inbreeding is harmful 'in the face of all the evidence' (21).
  • Bronisław Malinowski argued 'biologists are in agreement that there is no detrimental effect produced upon the species by incestuous unions' (22).
  • Robert Briffault wrote 'there is not in the records of breeding from domesticated animals a single fact . . . which indicates, much less evidences, that inbreeding, even the closest, is itself productive of evil effects' (23).
  • The final nail in Westermarck's coffin came from Sigmund Freud. In an essay titled Totem and Taboo (1910), Freud challenged the Westermarck hypothesis saying, 'the earliest sexual excitations of youthful human beings are invariably of an incestuous character'. He thus argued that the incest taboo only existed to keep this natural propensity at bay.
  • Later luminaries such as Leslie White in 1949, Claude Lévi-Strauss in 1950 and David Aberle in 1956 continued denying that close inbreeding is injurious.
• Second, through much of the 20th century, Westermarck's ideas were actively shunned in favor of those advocated by Freud and Levi-Strauss, i.e., that humans lacked sexual imprinting so cultural taboo of incest became necessary.
  
  • Coined by Konrad Lorenz, sexual imprinting is the notion that many animal species avoid mating with close kin such as siblings.
  • In other words, the prevailing Freudian idea implied that a priori early human developed cultural taboo around incest based on rational observation of its harmful effects, i.e., greater chances of defective offspring.
  • It's interesting to speculate why Freud's take on the reason for the incest taboo prevailed for almost a century. Hill Gates claims (24) it's due to anthropology's 'embrace... of Freud's Oedipus complex', i.e., the narrative that humans need to repress their selfish sexual desires to fully realize their potential.
  • In other words, our need to believe, contrary to copious evidence, that our rationality triumphs. Against such a dramatic narrative arc, what chance for Westermarck's hypothesis that early life association inhibits sexual attraction? After all, it's positively anticlimactic. 
• Problem is the Freudian idea prevailed in absence, not because, of incontrovertible scientific evidence.

The tide reverses, 1962-Present.

• Starting in the 1960s, a slow tide of studies revived Westermarck's ideas for two reasons,
• One, studies showed close inbreeding is injurious.
  
  • For e.g., studies from Norway (25), Turkey (26), Israel (27), Pakistan (28) showed that compared to unrelated parents, children of first cousins had twice the population baseline probability of congenital malformation and/or genetic diseases.

• Two, sexual imprinting exists in humans. Now called the Westermarck effect.
  
  • In 1962, Robin Fox wrote that the notion of sex among people who'd experienced close bodily contact as children ranged from disgusting to unthinkable to indifferent (29).
  • In 1964, Yonina Talmon studied sexual relations among community-reared children in two Israeli Kibbutz. Neither biologically related nor denied permission to marry, nonetheless, they didn't do so because they didn't feel sexual attraction towards one another (30). Joseph Shepher's 1971 follow-up study (31) confirmed Talmon's findings. 

• In turn-of-20th century Taiwan, 3 types of marriages were practiced.
  
  • In 'major marriage', the bride and groom's parents arranged the marriage, and typically bride and groom didn't meet until the day of marriage.
  • In 'uxorilocal marriage', the groom and bride's parents arranged the marriage, typically bride and groom didn't meet until the day of marriage, and the couple lived with the wife's family.
  • In 'minor marriage', the sim-pua (little bride), aged from a few days of age to 4 years old, was betrothed in infancy and raised with her future husband in his house. Post-puberty, they were married (saq-cue-tui or 'pushing together') in their mid-teens. 
  • Westermarck's hypothesis would predict that early association would create sexual aversion, despite the prevailing cultural norm.
  • Indeed, the Stanford anthropologist, Arthur Wolf, found minor marriage couples had much higher divorce and adultery rates, and fewer children per year (32).
  • Taking advantage of the meticulous demographic records the Japanese government kept during their occupation of Taiwan between 1895 and 1945, Wolf collected and analyzed the adoptions, births, deaths, divorces and marriages among the island's inhabitants, i.e., data on >14000 women from 26 villages and 2 towns.
  • Using fertility, divorces and extramarital affairs as proxies for sexual inhibition and aversion, he found that extramarital affairs were 16.5% for women whose marriages were arranged by their and the groom's parents but 37.7% for women from minor marriages.
  • Wolf was also able to rule out competing hypotheses such as health disparities between women in minor versus arranged marriages.

Data from Incest (copulation with parents, offspring, siblings)

• Problem is two-fold
  
  • Few studies exist (see figure below from 6 using data adapted from 33).
  • They're plagued with design flaws that engender confirmation bias and thus preclude firm conclusions about the data. These include
    
    • Incestuous mothers tend to be very young; published studies didn't/couldn't adequately control for that.
    • Incest group typically comprises greater physical and mental handicaps.
    • For e.g., the 1971 Czech study by Seemanova (see table below) had 141 females in the incestuous group. Of these, 20 were intellectually handicapped and, in addition, 2 each were deaf-mutes, had congenital syphilis or were epileptics. In addition, two others were deaf-mutes and three were schizophrenic. OTOH, in the control group of 46 mothers, only 2 were intellectually handicapped (one additionally a deaf-mute), and 2 other deaf-mutes.
    • Same gap exists in the fathers as well. Of 138 fathers in incestuous pairings, 8 intellectually handicapped, 13 chronic alcoholics, 2 with syphilis, and 4 suicides. OTOH, the nonincestuous group of 52 fathers had none who were intellectually handicapped, 2 chronic alcoholics and 1 with polydactyly.

• OTOH, the data is clearer and cleaner in animal studies.
  
  • Mother-son incest is rare in Rhesus macaques (34). 
  • Incest is rare in other primates, mammals, birds, amphibians and even insects (35).
  • In fact, incest is uncommon throughout the animal kingdom (36, 37), rare exceptions being eusocial mammals such as naked mole rats (38).

Irony dictates it's appropriate to end with Freud's Oedipus complex.

• Separated at birth from his mother, Jocasta, Oedipus ends up bolstering rather than demolishing Westermarck's hypothesis.
• After all, early separation undermines natural adaptation for incest avoidance.
• More recent government data also supports this idea, again ironically because the goal of the bureaucratic intervention in this instance was something else entirely.
• In 1975, Britain enacted the Access to Birth Records Act.
• This Act enables adopted persons >18 years of age to trace their biological kin.
• Maurice Greenberg and Roland Littlewood studied reunited kin (39).
  
  • They estimate >50% of reunited kin experience strong sexual feelings.
  • They also suggest incest may be frequent.
• While there are many similar anecdotal reports in the literature, Anaïs Nin's experience with her father, Joaquín Nin (40), is one of the most famous examples that supports Westermarck's hypothesis that early association (attachment?) is essential in engendering incest aversion.

Also, the all-important caveat: Science hasn't yet uncovered the all-important how as in how early life association and incest aversion mechanistically intersect at the biological (genetic/epigenetic) level, i.e., how genetic selection shapes incest aversion over time and how early life association mediates this effect.

Bibliography

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2. Bittles A.H. and Black M.L. (2015). Global Patterns & Tables of Consanguinity. Main Page - ConsangWiki - Consang.net
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