Primogeniture is the practice of passing on the majority or entirety of an inheritance
to the deceased's eldest child (frequently gender
-qualified as eldest male child). Rules of primogeniture were most common among "medieval
" and "feudal
" societies, and were often encouraged as promoting stability and order in a society. Primogeniture has two main effects, as follows:
First, primogeniture prevents dilution of inheritance - if each descendant inherited an equal amount, each generation would receive less and less. Even if a particularly enterprising individual trebled his holdings during his lifetime, this gain in "family wealth" would be neutralized if he had but three (equal) heirs. Primogeniture allows the collected wealth, and attendant influence and power, of a family pass on intact to (a member of) the next generation, and as such, favors the establishment and maintenance of dynasties. This is especially important to cultural stability in highly stratified societies dominated by a landholding elite. Of course, other descendants are typically not left to wither and die, as this wouldn't be particularly good for the strength of the family either. Members of the appropriate gender (usually female) are often married into the richest families possible (viewed in this respect, the giving of a dowry is somewhat like the purchasing of an annuity), and younger children who would otherwise inherit are typically directed towards self-sufficiency, by learning a useful craft, joining a profession like the military or the local religion's clergy, or the like. If the family is rich enough, younger children may be given smaller amounts of wealth to use as capital in founding an enterprise, or even given a stipend with which to support themselves, although this latter option is not viable long-term, for that descendant's descendants and so forth. In any case, the inheritor is usually considered to have some degree of obligation towards the support of their less-fortunate siblings.
Second, primogeniture prevents infighting over the issue of who is to receive what part of an estate, especially in times and places lacking a coherent (and applied) code or rule of law or widespread literacy, in which provably establishing one's wishes in a will is less feasible, or in the case of inherited positions or titles which cannot be split among offspring. Family bonds are possibly the strongest coherent force in any society, and it is in every society's best interest to promote familial harmony. If all the European family conflicts, taking place against a background of strong primogenitive tradition, seem excessive, imagine what they would have been like without. Despite attempts to import one from China, Japan long lacked a rule of primogeniture, and inheritance disputes were common sources of unrest. Even were a rule of primogeniture to be strictly adhered to, there are still situations to be addressed like that in which an eldest son predeceases his parents, leaving both younger siblings and children of his own - there is some variation among individual customs as to how inheritance should proceed, but even in societies with plans for such eventualities, power struggles were more common under such conditions.
These are both good reasons to codify a single-heir system, but why exactly select the eldest? Why not the smartest, or healthiest, or whichever child the parents favor and indicate before death? Well, age is one of the simplest and least subjective properties to classify humans by, but then why not postremogeniture, inheritance by the youngest? The matter is that the eldest son is (here we assume a male-dominated society, which historically is most of them) in theory the most likely to be capable of making productive use of the inheritance and of protecting it from the taxmen, bandits, local strongmen, lords, and the like who might seek to appropriate it for their own ends. Naturally, this would not be true in all cases, and most formal or informal codes of primogeniture allowed parents to designate another beneficiary if the eldest was particularly ill-suited to the task or expressed other interests that they had been unable to discourage. If this was done against the wishes of the nominally entitled, however, it was likely to arouse significant anger and stood a good chance of igniting an intra-family feud.
Of course, primogeniture does nothing to address the problem that arose when all of the deceased's descendants were too young, incapable, or of the wrong gender to take an inheritance, or just or nonexistent, which at all levels of society could prove ruinous for a family, and at the highest ones ruinous for the country. Problems will also tend to arise when rules of succession are simply ignored, as when something as fundamental as sibling rivalry or as important as the rule of a country is at stake, there is a great temptation to do.
Primogeniture promoted stability by discouraging intrafamily conflict, and promoted progress by concentrating and retaining leverageable power in the hands of a powerful minority (a condition, to be sure, true to some degree of all known societies, but here further and formally encouraged). These two effects, if not necessary to the establishment and growth of feudal societies, at least had a significant positive effect, and helped to create the (relatively) ordered societies that were the predecessors of modern states. Primogeniture remains the default in some modern countries, although most give precedence to the specific intentions of the deceased as laid out in a will or other legal instrument. Many western societies, however, have replaced it with a rule of equal division of assets, reflective of a general tendency towards egalitarianism and higher social mobility, and the rise of (divisible) capital and concurrent decline of land as a primary source of power.