Well that’s easy. Who are you related too “by blood” and by marriage? Logically we should define kin based on genetic relatedness and distance. From physical anthropology we know that we tend to do more for and more obligated toward relatives. This is referred to as INCLUSIVE FITNESS. By helping relatives we help to pass on some of our own genes.

So, biologically speaking, it makes sense to help kin over non-kin and closer relatives over distant relatives. For the most part, this is how other cultures define their relationships, but not always.

Take our kinship system. We refer to all aunts and uncles the same way whether they are married into our family or not. We respect them the same way whether we share genes with them or not.

Below is an American kinship chart, which, by the way, is just like the Eskimo system. ▲s represent males, ●s represent females. Ego is the person to which all the kinship terms apply. So for example, if a person in the chart is labeled ‘M’ for mother, it is Ego’s mother.

American and Native Alaskan Kinship Chart:

In the chart above, all green individuals are cousins and yellow indicates siblings. The orange circle indicates mother, the dark blue triangle ego's father, and the purple circles and light blue triangles are aunts and uncles.

In our kinship system we treat all first cousins the same. We have a sibling-like relationship with them and can not marry them. This makes sense genetically because we share 1/8th of our variable DNA with each of our first cousins. Other cultures may distinguish first cousins based on what side of the family they are from, their gender, the gender of their parents, and on their birth order.

In our system, you are allowed to marry more distant cousins. Second, third, etc. cousins are fair game, and so are referred to as “kissin’ cousins”

Let’s look at the Yanomamo example (ignore the colors for now):

Cousins 'D' and 'C' are PARALLEL COUSINS and cousins 'G' and 'H' CROSS COUSINS. In the Yanomamo kinship system, you can marry cross cousins, but not parallel cousins. Even though, genetically, they are all first cousins, the rules regarding the relationships for cross and parallel cousins are quite different. Parallel cousins marriages are not allowed and are considered incestuous. Although cross cousins are related to ego the same genetically, these cousins can be married.

Why? What makes them different? Look at the chart closely. Look at the gender of the parents and aunts and uncles of Ego. Notice a pattern? Cousins born to Ego’s mother’s sister or Ego’s father’s brother are parallel cousins, and so off limits. Mother’s brother’s kids and father’s sister’s kids are cross cousins, and so are marriageable. For Ego to marry a cousin, the cousin’s parent and Ego’s parent have to be opposite genders: Father’s sister’s kids or mother’s brother’s kids. Get it?

This distinction between cross cousins and parallel cousins is common cross culturally. The terms do not always distinguish marriageable cousins from non-marriageable cousins. Sometimes the relationship is merely closer, where ego has more rights and obligations with regards to parallel cousins.

In the Iroquios system parallel cousins (yellow) are treated as brothers and sisters (also yellow) to Ego. Their parents are therefore referred to as mother (orange) and father (dark blue), rather than aunt and uncle.

The Hawaiian system lumps family members even more. In the chart below you’ll notice that all first cousins are referred to and brothers and sisters (all yellow). All those people we call aunts and uncles are mothers (orange) and fathers (dark blue) for the Hawaiians. So even though one shares ½ of their variable genetic material with a sibling and only an 1/8th with a cousin, in the Hawaiian system they are treated the same. Prehistorically Hawaiians did marry their genetic first cousins (aka brothers and sisters). When Cook and the missionaries arrived in Hawaii and inquired as to whom individuals were married, they were shocked when the Hawaiians answered, “my brother” or “my sister”. The Hawaiians had married the cousins they referred top as brother and sister, not their actual brothers and sisters.

Every culture in the world has an incest taboo. How they define incest and marriageable kin, and non-marriageable kin varies, however. The rules are often based on need and population size. A smaller population may find it necessary to marry closer relatives than a larger population.

For example, some Amish populations have strict rules about marrying within the culture. Some settlements started with about 1000 people and grew to around 60,000 people. There may be only five last names in the entire community. This has resulted in recessive alleles (genes) showing themselves frequently. For the Amish a common trait is polydactalism (extra fingers and toes). Because of genetic factors, people will tend to marry the most distantly related individuals they can while still maintaining the population.

Some cultures do marry off genetic brothers and sisters. This is relatively rare and usually happens only in royal families. An example of a culture who married off brothers and sisters was the royal families of ancient Egypt.