Several factors contribute to consanguinity. These are economic and cultural factors, encounter, social and cultural isolation. In Bedouin, the reasons given for favouring cousin marriages were clan solidarity, interpersonal compatibility, preservation of family property, parental authority, and social protection for women.⁽³⁾ Bener found in Al-Ain, UAE that the only significant variable was the husband’s education and parent’s consanguinity. ⁽⁴⁾ The frequency of consanguinity in this region was found to increase significantly with the improvement of the husband’s educational level; however, this does not seem the case in other regions such as Jordan.⁽⁵⁾

Social Concepts and Consanguinity
The practise of consanguinity is usually cultural rather than religious and this was demonstrated by Tirosh who studied the practice in his survey of infants with visual deficits in Northern Israel.⁽⁶⁾ Consanguinity was present in three of the ethnic groups studied. The rates recorded were Druze (85%), Christian (77%) and Muslim (72%) thus demonstrating that religious observances in some countries were outweighed

Social and Geographical Isolation
In addition to this form of consanguinity, is that where groups of people live in small isolated pockets which results is social isolation. Under these circumstances, the group from which a mate must be selected is small; therefore the proportion of first cousins in it is higher than in larger populations and random mating will more often result in consanguinity. This has been demonstrated in varying degrees of consanguinity for such isolated groups e.g. in an alpine community in Switzerland (11.5%), a district in Northern Sweden (6.8%) and a Brazilian group (20%). ⁽¹⁾ Other examples of genetic isolates are the populations of Newfoundland and Labrador. ^{(7), (8), (9)} where inbreeding is common, and also in islands such as Cyprus where the whole island has become practically one large family despite the fact the first cousin marriages are forbidden by the Greek Orthodox Church. ⁽¹⁰⁾ In the former, it was suggested that the continuing high prevalence of genetic blindness in this province could be attributed to the genetic structure of the population, which derives from natural increase by settlers who arrived from highly circumscribed areas of Southwest England and Southern Ireland before 1835 and there is very little migration into these communities.⁽⁹⁾ Thus, frequencies of specific recessive disorders may be increased owing to inbreeding from mating between distant relatives and the frequency of a recessive disorder is reasonably attributed to ‘founder effect’; a finding similar to Fraser’s group of sex-linked myopia in South Australia. ⁽¹¹⁾

Genetic Non-Relatedness
Consanguinity and genetic non-relatedness cannot be sharply distinguished from one another. There are many people descended from common ancestors who are unaware of the fact that they are related. In most geographical areas, man does not reproduce within pedigrees which are completely isolated from one another but, rather in a network of relationships which joins all, or most strains together in a single reproductive unit. This is the case even where branches of a society seem to be separated. Over generations, prohibitions and barriers to intermarriage break down, particularly as both legitimate and illegitimate unions lead equally to an interchange of genes. That being the case, tracing of pedigrees of any group of apparently unrelated individuals of similar territorial origin will reveal that many of them possess a common ancestor. Therefore, if two such people, who are apparently unrelated, marry, they do contract a distant consanguineous marriage.⁽¹⁾

Changing Trends and Non-Acceptance
Many societies now prohibit marriage between close relatives, perhaps as a result of the observations on the progeny of consanguineous marriages. These prohibitions vary in degree and in some countries include marriages between second cousins. In addition to national laws governing marriage between relatives, there are also religious laws that decree the levels of consanguinity allowed. ⁽¹⁾

In addition to marriage between relatives, it is also custom, or law, for marriages between unrelated persons to be prohibited. These have included marriage between a person and his/her step-parent or between a person and his/her deceased uncle’s/aunt’s spouse. These prohibitions are based either on biological misconceptions or on non-biological grounds.⁽¹⁾

Inbreeding is becoming less common worldwide except among genetic isolates and regions where consanguinity is practiced. In Europe the change started from the beginning of the last century. In Norway this became evident from the 1920s onward. ⁽¹²⁾

Even in social isolates, the fear of unhealthy offspring may also limit consanguineous marriages as found in the Swiss group above which had a lower frequency of cousin marriage (0.7%) as opposed to the Swedish group (6.8%) although the size of the two groups were similar. ⁽¹⁾ This would seem to demonstrate that certain communities show a particular aversion to such marriages even within their isolated communities.

In Japan, ⁽¹³⁾ the trend of consanguineous marriage has been declining and was found to be 8 times lower in younger groups than the oldest groups. The overall rate of first cousin marriage in 1975 was reported at 2.23% in rural districts and 1.77 in urban areas. No consanguineous marriage was recorded in 32% of the 253 administrative units surveyed (There was also interregional variation with the Kyushu district showing the highest figures of inbreeding). Similarly, low consanguinity rates are found in the European countries, ^{(12), (14)} among the indigenous communities.

A brief outline in necessary because of the cultural concepts and the misconceptions about various subtypes. This would be important in advocacy. (Table 9.1)

In some communities particular emphasis is placed upon certain forms of relationship such as type-A of cousin marriage ie the children of two brothers, which is the commonest type of cousin marriage, the highest being in Yemen. It is considered as the duty for the male to marry his cousin and an obligation for the female to accept.⁽¹⁾ In Palestine, 20% of marriages were between first cousins.⁽¹⁶⁾ The same pattern was reported in Pakistan where 60% of marriages are between first and second cousins.⁽¹⁷⁾ Type-B is the second most common ie between the children of two sisters and it is believed that such marriages do not constitute a close family marriage as the sisters are from different family from that of the male cousin. In the Jewish society marriage between maternal uncle and niece was also practised.⁽¹⁸⁾ This practice has parallels with a Chinese regulation whereby marriage between the children of a brother and sister or of two sisters is acceptable as a consequence of the social custom which assigned a woman to the family of her husband and thus regarded children as ‘not belonging’ to the biological family of the mother. However, the children of two brothers were considered to be of the same family and were therefore prohibited from marrying, despite that their genetic endowment is the same.⁽¹⁾ Similar cultural practices and myth regarding the marriage between the children of a brother and sister or of two sisters exist in Middle Eastern communities, but marriages between the children of two brothers are favoured as mentioned above.⁽¹⁾

Prevalence of Consanguinity

Higher proportions of first cousin marriage than the proportion in the general population have been reported in schizophrenia, (48) several congenital heart defects such as septal defects (atrial, ventricular and atrio-ventricular). Also reported are pulmonary stenosis and atresia, (49) cystic fibrosis, cystinosis, nephronophthisis, spinal muscular atrophy, albinism, achromatopsia, ⁽⁵⁰⁾ hearing disorders, ⁽⁵¹⁾ and central nervous system anomalies,⁽⁵²⁾ congenital anomalies, physical handicap, mental retardation and malignancies.⁽⁵³⁾ In Kuwait, higher incidence of Meckel syndromes, phenylketonuria, and familial Mediterranean fever have been documented in cousin marriage.⁽⁴³⁾

There is also an added risk of infant and child mortality.⁽⁴⁵⁾ Khlat and Khoury ⁽⁵⁴⁾ found that inbreeding generally increases pre-reproductive mortality; crude mortality increases with inbreeding in proportion to the mortality rate. Morbidity increases significantly with inbreeding. Infant and under-5 mortality is higher in first cousin marriage in Turkey.⁽⁵⁵⁾ Consanguinity can also influence the age of menopause.⁽⁵⁶⁾