Let's start with the concept of Gotra.
While some opine that Gotra is simply the 'abode of cows' or 'house of cows' indicating a typical paternal familial line, others opine that Gotra is made up of two parts: go meaning the indriyas or senses and tra meaning to protect. Either case, Gotra is a patrilineal familial association, with a Rishi as the originator. In other words, a Gotra is a group of people who all share a single forefather. The Gotra is often synonymous with the Rishi. Even more simply, your Gotra is the same as your father's, his (your father's) is the same as his father's and so on until they reach a root rishi from whom the Gotra originated.
The Gotra is usually used during various rituals to identify the yajamana, or the person conducting the rituals, as well as during marriages to check the familial associations of the bride and groom.
Per Sanatana custom, two related philosophies are prevalent in Bharata: mitaakshara and daayabhaaga
The daayabhaaga vidhi is prevalent in Bengal and portions further East. The rest of India follows the mitaakshara vidhi from Punjab all the way down to Kanyakumari. The primary difference between these two is in terms of succession, and therefore is not relevant to our discussion.
In the mitaakshara vidhi, two lineages of people are identified for every individual: gotraja and bhinna-gotraja.
Typically a gotraja (of any person) is someone who shares a patrilineal line (up to 5 generations, per Hindu Law), and a bhinna gotraja, someone who shares a matrilineal line (up to 3 generations, again per Hindu Law).
In other words, all people tracing their parental lineage to your father's paternal grand father's, paternal grand father are your gotrajas. Similarly, all people tracing their parental lineage to your mother's maternal grandmother are your bhinna gotrajas. Per Hindu common law, marriage to your gotrajas and bhinna gotrajas are forbidden and indeed considered null and void in an Indian Court of Law.
The reasons for this appear to be rooted in ensuring sufficient genetic diversity among the populations and preventing the ills stemming from genetic homogeneity. There appear some instances from ancient past where sa-gotra marriages were normal, even encouraged - perhaps to 'keep wealth within the household', but with the resulting birth defects that occur with such marriages, the practice was not only abandoned, but also outlawed among ancient Indic societies.
There seem to have been 4 gotras originally - kAshyapa, AMgirasa, vasiShTa, and bhrigu (jamadagni). This seems to have increased to 8 in the first stage of additions (adding atri, vishwamitra, and agastya plus kEvala bhrigu) - representing the Saptarishis, eventually ending up with a total of 108 gotras. A statistical count appears to show an equal distribution of gotras across all varnas of people.
Among South Indian states, marriage among cousins is fairly common, and is much misunderstood, especially by other sections of society where this is not permitted, assuming all cousin weddings are allowed.
Before the generic "eww... cousin marriage" reaction, let us understand the underpinnings of why marriages among blood-relatives are disallowed: The very simple reason for this is the need to preserve genetic diversity. A standard boiler-plate note on genetic diversity would read as:
A lack of genetic diversity can make it harder for a species to adapt to environmental changes and survive. Genetic diversity is the raw material for evolution, and species need it, along with reproductive excess and a selective force, to evolve. When there is low genetic diversity, there are fewer differences between individuals, and therefore fewer opportunities to adapt to environmental changes. This can make populations more vulnerable to extinction
More simplistically, among humans, a lack of genetic diversity leads to physical and mental birth defects in children, and eventually to a high infant mortality rate within a few generations. Given that birth defects in the South and North are more or less identical despite marriages among cousins (in fact infant mortality is lowest in the Southern states), there must be something the South does to ensure the requisite genetic diversity.
Obviously, the law is applied differently in the South. Not all ‘cousin marriages’ are permitted. Think of it as gotraja and bhinna gotraja being redefined
Typically a gotraja (of any person in the South Indian context) is someone who shares a patrilineal line, and a bhinna gotraja, someone who shares a matrilineal line respectively.
Patrilineal lineage is a male and his male progeny (your father, his brothers and their sons, your paternal grandfather, his brothers and all their sons and grandsons, and so on).
Matrilineal line is a female and her female progeny (your mother, her sisters and their daughters, your maternal grandmother, her sisters and all their daughters and grand daughters, and so on).
While the patrilineal line is readily detectable (same gotra), the matrilineal line takes some digging.
First up, marriages in the same gotra are disallowed regardless of the number of generations - that automatically removes the father’s male siblings’ children. (In some extreme cases, people do come up with 7 generations and other justifications, but in general sagotra marriages are frowned upon, even leading to exile of the couple, and anyone who associates with them - including parents of the couple - in the villages, and in specific cases that I have known personally). Also disallowed are marriages with your mother's female siblings' children. This also extends to second, third or higher order cousins in the patrilineal line, and second, third or higher cousins in the matrilineal line.
Both these categories (father’s brothers’ children, and mother’s sisters’ children) are often called “cousin-brother” and “cousin-sister” - a term generally not meaningful to those who do not know/understand - or in ignorance, misuse the term for all cousins. In comparison, the other cousins - father's sisters' children and mother's brothers' children are just cousins, distinguishing between the two types of cousins. Think of it as chachera bhai/chacheri behen and mausera bhai/mauseri behen verseus bua ka beta/ki beti and mama ka ladka/ki ladki. bua/mama ke bachche are not called bhai/behen for this reason.
Among first cousins, this leaves only father’s sisters’ children (who are obviously not of the same gotra) and mother’s brothers’ children (also, equally obviously not of the same gotra) as potential permissible spouses.
Generally, though, in modern times, marriages even among permissible cousins - first, second, or third - has decreased significantly, particularly in urban/semi-urban areas.
I tried to make a graphic starting from a male entity to indicate permissible cousin-marriages. This can be extended to a female entity easily by switching the genders of each unit in the graphic.
In the image below, the dotted lines indicate lineage rather than draw out the whole tree. The yellow depictions are contingent on a different gotra from the person in question in the blue circle at the center (unlike the green depictions who are by default of a different gotra, the ones in yellow could be from the same gotra, and hence unallowed). The Reds, of course, are unallowed. I haven’t shown more details since that would take up too much space and make the chart unreadable:
2/8 types of first cousins are permissible
14/32 types of second cousins potentially permissible and so on.
Note that this also works across generations - for example, women marrying their maternal uncles, pretty-much following the same template: not in the same gotra, and not in the female-matrilineal line. Once again, the genetic diversity is thought to be sufficient with such marriages.
The genetic degeneration of from marital relations within the gotra was found out a long time ago, and thus marriages marriage within the same gotra were banned. Apparently, the gotra filter (combined with the forbidden matrilineal [patrilineal automatically comes under the gotra filter] marriages) provides sufficient genetic diversity to stem this degeneration else, one can imagine a high rate of birth defects within the populations. A person's father's sister's (or mother's brother's) children would have their own father's (or mother's) genes and this diversity appears to be sufficient to sustain populations.
Hoping to demystify cousin-marriages in the South for folks who do not understand the concept and think cousin marriages are indiscriminate. One doesn't need to 'eww' at ideas they don't follow without really understanding the science and limitations beneath the superficial.
