Throughout recorded history, birth order has affected diverse aspects of social, political, and economic life, and this influence continues to manifest itself today in many traditional societies. Especially in previous centuries and in non-Western portions of the world, societies have generally sanctioned practices whereby parents favor some offspring over others. Such patterns of parental favoritism are often associated with birth order. For example, when a child is deformed or when an older child is still breast-feeding, most traditional societies permit infanticide; but no society condones the killing of the older of two SIBLINGS.
Through differences in parental investment, birth order sometimes affects the general health and well-being of offspring. Laterborns, for example, are less likely than firstborns to be vaccinated, and in developing countries laterborns tend to be shorter and to suffer higher rates of childhood mortality than do their older siblings. Birth order also appears to influence intelligence and personality, doing so through differences in parental investment, as well as through sibling interactions. These intellectual and behavioral differences affect various aspects of life achievement.
One study of the role of birth order in thirty-nine non-Western societies found that the birth of the first child generally has special significance, stabilizing the marriage and raising the status of the parents. In these same non-Western societies, firstborns were typically found to receive more elaborate birth ceremonies than laterborns, to have power over their younger siblings, to acquire and control more parental property in adulthood, and to carry greater authority with nonfamily members.
INHERITANCE practices have often been linked to birth order. The system of primogeniture has generally been followed in societies where wealth is largely dependent on land ownership and where land is a scarce resource. Under this system, firstborns or eldest males inherit the bulk of the parental land and property–a practice that limits the subdivision of estates and thereby helps to preserve the patrilineal family and its patronymic. James Boone's 1986 genealogical study of the leading families of medieval Portugal found that eldest sons were 1.6 times more likely than younger sons to leave descendants over a period of two centuries, whereas younger sons were 9 times more likely than firstborns to father offspring out of wedlock. Among daughters, laterborns were often destined for convents and, as a consequence, left fewer descendants than did earlierborn daughters.
Some countries, including Portugal, recognized landless younger sons as a potential threat to political stability. Expansionist military campaigns as well as the Crusades were in part undertaken as a means of exporting these younger sons to distant lands. There, while attempting to distinguish themselves, these younger sons often died in combat or from diseases.
Male primogeniture has long been the custom in the transfer of political power among royalty. Instituted in feudal times, this practice helped to curb the sometimes bloody conflicts among siblings that had previously punctuated royal succession. Although leaders of the PROTESTANT REFORMATION–in the spirit of egalitarian reform–successfully called for the abolition of primogeniture in parts of Germany, this practice remains the accepted succession policy in those European countries that still have monarchies.
Other discriminatory systems of inheritance are also known, including secondogeniture (whereby the secondborn child or son inherits the majority of the parental property), and ultimogeniture (the practice of leaving property to the youngest child or son). This last inheritance practice is most commonly found in societies that impose heavy death taxes, because ultimogeniture lengthens the period between taxations.
Sometimes parents have opted for an equitable distribution of their assets to progeny. In medieval Venice, where wealth was based on commercial speculation and where prosperity was often a matter of unforeseeable circumstances, parents generally subdivided their estates in order to maximize the likelihood that at least one offspring would achieve commercial success. In short, although inheritance practices associated with birth order have exhibited considerable historical and geographic variability, the specific form chosen by each family and society has usually made local economic sense.
One of the most remarkable discoveries in the field of psychology during the last several decades has been the finding that siblings who grow up together are almost as different as people plucked at random from the general population. Behavioral geneticists have shown that only about 5 percent of the variance in personality from one individual to another is associated with the shared family environment–that is, growing up in the same home. About 40 percent of the variance in personality appears to be genetic in origin, and another 20 percent is associated with errors in measurement. The remaining 35 percent of the variance is attributable to the nonshared environment (unique experiences that are not shared by siblings).
One important conclusion from this behavioral genetic research is that, for the most part, the family is not a shared environment. One possible source of such nonshared experiences is birth order, since children of different birth orders vary in age, size, and family roles. In addition, siblings compete with one another for parental investment (including love, attention, and scarce resources), and parents sometimes favor one child over another even when they try not to do so.
Darwinian theory predicts such competition among siblings, which has been widely documented among animals, fish, insects, and even plants. The principles of genetics help us in understanding this particular form of Darwinian competition. On average, siblings share only half of their genes, so they are twice as related to themselves as they are to another sibling. Based on the theory of kin selection, siblings are expected to act selfishly toward one another unless the benefits of sharing scarce resources are greater than twice the costs. Siblings therefore tend to develop context-sensitive strategies for optimizing parental investment– sometimes at the expense of other siblings–and these strategies are influenced by differences in age, size, power, and status within the family. Birth order is an excellent proxy for these differences.
Prior to about 1800, fewer than half of all human offspring ever reached adulthood, so even slight differences in parental investment, or in the competitive advantages developed by siblings, were sufficient to tip the balance in determining who survived and who did not. By cultivating unique and useful family niches, siblings increase their value within the family system. Firstborns have customarily adopted the role of a surrogate parent, which causes them to be more parent-identified and conservative than younger siblings. Because laterborns cannot baby-sit themselves, they generally seek to develop alternative and unoccupied niches within the family system, a process that seems to involve a predilection for experimentation and openness to experience.
Birth-order research, which encompasses more than two thousand studies, has established a consistent pattern of birth-order differences in personality. These differences can be usefully summarized by the Five Factor Model of personality, which encompasses the dimensions of conscientiousness, openness to experience, agreeableness, extraversion, and neuroticism. As reflected by their frequent role as surrogate parents, firstborns tend to be more conscientious than laterborns. By comparison, laterborns tend to be more open to experience than firstborns, especially in those facets of this personality dimension that involve the questioning of family values or the authority of their elders. Laterborns are also somewhat more agreeable than firstborns, since they generally adopt low-power strategies, including cooperation and acquiescence, that accord with their lesser age, power, and physical size. In addition, laterborns are more extraverted than firstborns in the specific sense of being fun-loving, excitement seeking, and sociable. Finally, firstborns and laterborns both manifest aspects of neuroticism, but in different ways. Firstborns are more neurotic in the sense of being anxious about loss of power and status, whereas laterborns are more neurotic in the sense of being self-conscious–an attribute that probably stems from their tendency to compare themselves with older and more accomplished sibling models.
Compared with birth-order differences in personality that are measured within the family, those documented in extrafamilial contexts tend to be less pronounced. Still, there is considerable evidence that birth-order differences in personality and behavior manifest themselves in nonfamilial contexts–especially when these behavioral contexts resemble those previously encountered within the family. To cite an example documented by Catherine Salmon, firstborns and laterborns respond differently to political speeches that use the terms brother and sister as opposed to friend.
Extensive research indicates that firstborns tend to have higher IQS than laterborns, although this difference is small (IQ is reduced about one point with each successive birth rank in the family). Explanations for these findings have generally focused on the consequences of increasing family size, since children from large families have lower IQs than children from small families. According to Robert Zajonc's confluence model, the addition of younger siblings impoverishes the family's intellectual environment because children are less intellectually proficient than adults. This theory predicts that firstborns will tend to have higher IQs than laterborns because firstborns spend more time alone with their parents, and more time in smaller sibling groups.
Differences in personality and behavior related to birth order sometimes reflect themselves onto the stage of world history. The tendency for firstborns to receive greater parental investment than laterborns, and in turn to be more conscientious and have higher IQs, fosters greater achievement and has led to an overrepresentation of firstborns among people listed in Who's Who and other biographical encyclopedias. Similarly, firstborns have tended to be over-represented among world leaders, successful writers, and famous scientists (including Nobel Prize winners). By contrast, laterborns have been historically prominent as explorers and as leaders of radical political and social upheavals, including the Protestant Reformation, the French Revolution, and other noteworthy episodes of radical social and political change. The expression of these birth-order differences is generally context sensitive–that is, dependent on local circumstances. For example, in Catholic countries during the Reformation, laterborns were more likely than firstborns to undergo martyrdom for supporting the Reformed faith; but in Protestant countries such as England, firstborns were more likely than laterborns to be martyred for their resistance to Protestant reforms.
In the history of science, birth order has often played a role during times of radical theory change. Laterborns such as Nicholas Copernicus (the youngest of four children) and Charles Darwin (the fifth of six children) pioneered revolutions in science that challenged both the scientific status quo and associated religious dogma. Other laterborns, such as Francis Bacon and René Descartes, led the seventeenth-century assault on scholastic learning and Aristotelian dogma, culminating in the Scientific Revolution. Even when firstborns have initiated major revolutions in science–such as those led by Isaac Newton, Antoine Lavoisier, and Albert Einstein–the earliest supporters of these revolutions have tended to be laterborns. Laterborns nevertheless possess a decided Achilles heel when it comes to initiating and supporting revolutionary science. They have generally promoted radical but failed revolutions such as phrenology (the nineteenth-century notion that bumps on the head reveal character and personality) just as eagerly as they have endorsed successful revolutions such as Darwinism, quantum mechanics, and plate tectonics. For this reason, there is no evidence that laterborns are more creative than firstborns. Rather, firstborns and laterborns are each capable of creativity, but in different ways. In particular, firstborns tend to create within the system, whereas laterborns are more likely to create by questioning the status quo.
Owing to its influence on inheritance practices as well as social and political life, birth order appears to have exerted greater impact on people's lives in past centuries than it does today. Still, birth order continues to shape personality and behavior by influencing parental investment, as well as by affecting sibling strategies for increasing parental investment. In large part through behavioral genetic studies, psychologists have learned that the family is not primarily a shared environment. Most environmental influences on personality appear to owe themselves to nonshared experiences, including some that are attributable to birth order. In addition to shaping personality and behavior, birth order also exerts an influence on familial sentiments. Individual differences in family sentiments mediate loyalties to the family, degree of contact with parents and other close relatives, and attitudes toward parental authority. In past centuries, these birth-order differences have often played themselves out during radical revolutions, providing a link between the formative experiences of childhood and the course of world history. Even today birth order continues to shape differences in personality and behavior that, in meaningful ways, affect overall life experience.
See also: Family Patterns.
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FRANK J. SULLOWAY