per generation. Fraternal polyandry also accomplishes this, but does so by keeping all the brothers together with just one wife so that there is only one set of heirs per generation. This strategy appears less successful the larger the fraternal sibling group is. Some forms of polyandry appear to be associated with a perceived need to retain aristocratic titles or agricultural lands within kin groups, and/or because of the frequent absence, for long periods, of a man from the household. In Tibet the practice was particularly popular among the priestly Sakya class. The female equivalent of fraternal polyandry is sororate marriage. Partible paternity Anthropologist Stephen Beckerman points out that at least 20 tribal societies accept that a child could, and ideally should, have more than one father, referring to it as "partible paternity". This often results in the shared nurture of a child by multiple fathers in a form of polyandric relation to the mother, although this is not always the case. One of the most well known examples is that of Trobriand "virgin birth". The matrilineal Trobriand Islanders recognize the importance of sex in reproduction but do not believe the male makes a contribution to the constitution of the child, who therefore remains attached to their mother's lineage alone. The mother's non-resident husbands are not recognized as fathers, although the mother's co-resident brothers are, since they are part of the mother's lineage. Culture According to inscriptions describing the reforms of the Sumerian king Urukagina of Lagash (ca. 2300 BC), the earlier custom of polyandry in his country was abolished, on pain of the woman taking multiple husbands being stoned with stones upon which her crime was written. An extreme gender imbalance has been suggested as a justification for polyandry. For example, the selective abortion of female children in India has led to a significant margin in sex ratio and, it has been suggested, results in related men "sharing" a wife. Known cases Polyandry in Tibet was a common practice and continues to a lesser extent today. A survey of 753 Tibetan families by Tibet University in 1988 found that 13% practiced polyandry. Polyandry in India still exists among minorities, and also in Bhutan, and the northern parts of Nepal. Polyandry has been practised in several parts of India, such as Rajasthan, Ladakh and Zanskar, in the Jaunsar-Bawar region in Uttarakhand, among the Toda of South India. It also occurs