在历史上,曾经出现过长子继承权制度。请大家分析这种制度安排当时所面临的约束条件,是什么导致了长子继承权的弱化?
 背景知识:
 http://info.datang.net/Z/Z0465.htm
长子继承权
 奴隶社会和封建社会的名位和财产由长子继承的制度。中国的长子继承制与宗法制密切相关,确立于西周。在周代的等级制度下,名位和财产由嫡长子继承。长子蕃衍的家族称为大宗,长子的兄弟所蕃衍的家族称为小宗。周王由嫡长子继承是为大宗,嫡长子的诸弟分封为诸侯国,对周王来说为小宗。诸侯的职位也由其嫡长子继承,在其封国内又为大宗,诸侯嫡长子的诸弟再分封为卿大夫。周代层层分大宗小宗的继承制,是典型的长子继承权。汉、唐以后,封建帝王主要实行长子继承制。中国历史上朝鲜族、苗族、白族和纳西族的大部分地区,流行由长子奉养父母,长子在财产继承上享有优先权的习俗。蒙古族的贵族一般也实行长子继承制。在东南亚,如缅甸的王位由长子继承。到19世纪,新西兰的毛利人仍由长子继承酋长。
 http://www.bartleby.com/65/pr/primogen.html
primogeniture 
 in law, the rule of inheritance whereby land descends to the oldest son. Under the feudal system of medieval Europe, primogeniture generally governed the inheritance of land held in military tenure (see knight). The effect of this rule was to keep the father’s land for the support of the son who rendered the required military service. When feudalism declined and the payment of a tax was substituted for military service, the need for primogeniture disappeared. In England, consequently, there was enacted the Statute of Wills (1540), which permitted the oldest son to be entirely cut off from inheriting, and in the 17th cent. military tenure was abolished; primogeniture is, nevertheless, still customary in England. In the United States primogeniture never became widely established. For other traditional types of inheritance, see gavelkind; borough-English. 
The Columbia Encyclopedia, Sixth Edition. Copyright © 2001-05 Columbia University Press. 
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