Initially when the People's Republic of China occupied Tibet political systems in many regions of Tibet remained unchanged. Then starting between 1959 and 1960 political reforms changed the land ownership and taxation systems. Professor Melvyn Goldstein believed this had a direct impact on Tibet's traditional marriage system. With the change of the social stratification as a result of land ownership and taxation systems, the du-jung and the mi-bo lower classes were the first to avoid the intramarriages that characterized the older society. As part of its population control measures, the Chinese government also forbid polygamous marriage under its new family law, thereby drastically impacting Tibet's social structure and her traditional marriage system.
Even though it is still officially illegal, after collective farming was phased out and the farmed land reverted in the form of long-term leases to individual families, polyandry in Tibet is de facto the norm in rural areas for economic reasons.
The anthropologist Melvyn Goldstein, however, questioned both of these explanations. He argued that female infanticide was never institutionalized in Tibet, and that women in Tibetan society had considerable rights. Moreover, there is no demographic evidence of a relative shortage of females in the literature on the subject. Goldstein also disputed the second theory, arguing that polyandry was widely practised only among the land-owning families, and not among the poorest classes where starvation was more likely. He, therefore, counterproposed that polyandry could be better understood in relation to the social stratification of Tibet. In fact, family structures and marriages in Tibet were inextricably tied to a social organization that was distinctly characterized by clearly-defined social classes.
The family structure and marriage system of tre-ba were characterized by two fundamental principles:
A "stem family" is one in which a married child is inextricably linked to his natal family in a common household. The "mono-marital principle" dictates that for each and every generation, one and only one marriage is permitted collectively among all the male siblings, and the children born out of this marriage are members of the family unit who have full legal rights.
The family organization was based on these two patterns to avoid the partitioning of their estates. A generation with two or more Conjugal families was seen as unstable because it could produce serious conflicts that could divide their corporate family land. As a matter of fact, Tibetan inheritance rules of family land, mainly based on agnatic links, did provide for each generation to partition the land between brothers, but this was ignored to prevent the estate unit from being threatened. Polygamous marriage, therefore arose as a solution to this potential threat.
To elucidate, let us consider a family with two or more sons. Tibetan inheritance rules gave all males of the family, the right to claim a part of the family estate, so if each son took a different bride, there would be different conjugal families, and this would lead to the partitioning of the land among the different sons' families. To avoid this situation, the solution was a fraternal polyandrous marriage, where the brothers would share a bride. Bi-fraternal polyandrous marriages were more common than tri-fraternal or quadri-fraternal polyandry, because the latter forms of marriage were often characterized by severe familial tensions. Different mechanisms were employed to reduce the number of sons within a household, such as making one son a celibate monk, or sending away a son to become an adoptive bridegroom to a family without male children.
Another kind of marriage, although uncommon, is the "polygynous marriage". In a family where all the children were female, sisterly polygynous marriage represented the most common choice. In traditional inheritance rules, only males had rights over the land, but where there were no males to inherit them, the daughters had the right over the corporation’s land. To maintain the familial estate unit, the daughters would share a bridegroom who will move matrilocally (as opposed to the patrilocal principle where the brides move into the husband's family) and become a member of his wife's family.
Bigenerational polygamy was present as an application of the mono-marital principle. Let us consider a family in which the mother died before the son was married. If the widower remarried another woman, two conjugal families would have been created, leading to the eventual partition of the estate. Bigenerational polyandry, whereby the father shared a wife with his son, was therefore the solution to avoid this problem. Conversely, when a woman with no male offspring was widowed, she would share a husband with her daughter ("bigenerational polygyny"), thus avoiding land partitioning.
In these mono-marital stem families, the family head, who had a dominant role in the family, was called trong bey abo (or simply abo). The abo who managed the property and resources of the family unit, was always a male, and almost invariably the oldest male of the elder generation in power. Sometimes, a younger brother would assume the abo role when the eldest male retired.
In taxpayer families, polyandrous and monogamist marriage were the more common forms of marriage, while less widespread was the polygynous marriage. Bigenerational forms of polyandry were, however, very rare.
The householder family structure — unlike the taxpayer families — lacked the single marriage per generation requirement to avoid land parceling. When a son married he often established a new household and split off from the original family unit. If taxpayer sons married that created succession for the family corporation and bound them to the estate for patrimonial and land reasons. Householder marriages did not incur that responsibility, and they generally married for love and were more often monogamist. The small number of polyandry cases within the householder class were limited to only the wealthier families.
All children were treated equally, and a "father" is not allowed to show any favoritism, even if he knew who his biological children really were, as biological paternity was not regarded as important. Similarly, the children considered all their uncles as their fathers, and a child avoided treating members of the elder generation differently, even if they knew who their biological father was.
Divorce was quite simple. If one of the brothers in a polyandrous marriage felt displeased, he only had to leave the household. Polyandrous marriages were often characterized by tensions and clashes for a variety of different reasons. Conflicts may, for example arise because a younger brother wanted to contest the authority of his eldest brother, or sometimes, sexual favoritism might occur, generating tension among the male partners in the marriage, especially so, when there are significant age differences among the brothers.