This paper examines the source of variation in marriage customary and the mating systems in the two different societies, the traditional hunter-gathering and horticulturalist, namely the !Kung San (San) and the Yanomamo. The San reside in the Dobe area of Botswana at the edge of the Kalahari Desert. They are nomadic and live in small groups of ten to thirty individuals thus enabling them access to resources such as nuts, bulbs and fruits which the women in the group contribute as for the men they primarily hunt (Shostak 1981, p.7). The Yanomamo, on the other hand, resides on the Amazonian tropical rainforest of Brazil and Venezuela. They live in villages of forty to fifty people and rely on cultivation as their primary source of food though the men still go hunting (Chagnon 2009, p.1, 5). Due to the environmental variation, marriage customs and mating system has to adapt in a way fitness is best suited in their respective societies. The Yanomamo is a patriarchal society that is based heavily upon a system of kinship and descent where marriage customary is decided by the older male kin (Chagnon 2009, p.7, 121). The marriage arrangements are usually conducted between bilateral first cousins between men and their Father's Sister's Daughter or their mother's Brother's Daughter (Chagnon 2009, p.7, 121). This inbreeding results in the decrease in fitness of individuals due to the increase of homozygosity (Hamilton 1964; Bittles 2010), but the Yanomamo views this as an increase in closeness between the kinship to reduce the chances of warfare between all individuals or as an aid in forming alliance (Chagnon 2009, p.7, 121). This means that even though a high infant mortality rate exists, deleterious traits of offsprings are negligible because there is higher probability of passing on genetic material of the father's descent (Changnon 2009,; Hughes 1980). In the Yanomamo society, sex-ratio imbalance is common due to female infanticide over the preference for male kin (Chagnon 1968, p.74, 122; Hughes 1980). Another reason for this imbalance could also be related to polygyny, one male taking on more than one wife, this results in other men not being able to find female mates (Chagnon 1968, p.74, 122; Hughes 1980). Other reasons could also be the raids and warfare that results in marriageable women being abducted or even killed. Therefore cross cousin marriages are more suited to the Yanomami society as there is a lack of non-related partners (Chagnon 2009, p.190