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Historically, sibling order has inﬂuenced important aspects of social, economic, and political life, and it continues to do so today in many traditional societies. Discriminatory inheritance laws and customs about royal succession that favor ﬁrstborns and eldest sons are just two examples. Sibling-order eﬀects have also been documented for a wide variety of behavioral tendencies, although the magnitude and causal interpretation of these eﬀects have been subject to debate. In the Darwinian process of competing for parental favor, siblings often employ strategies that are shaped by their order of birth within the family, and these strategies exert a lasting impact on personality. Radical revolutions are particularly likely to elicit differences in support by birth order. These behavioral patterns appear to be mediated by diﬀerences in personality as well as by diﬀering degrees of identiﬁcation with the family.
1. Social, Economic, And Political Consequences
Many societies—especially in past centuries and in non-Western parts of the world—have engaged in practices that favor one sibling position over another. For example, most traditional societies permit infanticide, especially when a child has a birth defect or when an immediately older infant is still breastfeeding. However, no society condones the killing of the elder of two siblings (Daly and Wilson 1988). An extensive survey of birth order and its relationship to social behavior in 39 non-Western societies found that the birth of a ﬁrst child typically increases the status of parents and stabilizes the marriage. In addition, ﬁrstborns were generally found to receive more elaborate birth ceremonies, were given special privileges, and tended to exert authority over their younger siblings. In most of these 39 societies, ﬁrstborns maintained supremacy over their younger siblings even in adulthood. They also gained control of a greater share of parental property than laterborns, were more likely to become head of their local kin group, and tended to exert greater power in relationships with nonfamily members (Rosenblatt and Skoogberg 1974).
Previously, many Western societies employed sibling order as a means of deciding who inherits parental property or assumes political power. Primogeniture (the policy by which the eldest child or son automatically inherits property or political authority) was the most commonly employed mechanism, but other discriminatory inheritance practices have also been employed. For example, secondogeniture involves leaving the bulk of parental property to the second child or second son, and ultimogeniture involves leaving such property to the lastborn or youngest son.
Most variations in inheritance practices by sibling order can be understood by considering the advantages that accrue from such policies, given local economic circumstances (Hrdy and Judge 1993). For example, primogeniture has generally been practiced in societies where wealth is stable and based on limited land, and where talent does not matter much. Leaving the bulk of parental property to a single oﬀspring avoids subdividing estates and hence reducing the social status of the family patronymic—something that was particularly important in past centuries among the landed aristocracy. However, in Renaissance Venice economic fortunes were generally based on speculative commerce rather than ownership of property, and parents typically subdivided their estates equally so as to maximize the chances of having one or more commercially successful oﬀspring (Herlihy 1977). Ultimogeniture is a policy often found in societies that impose high death taxes on property. This inheritance practice has the consequence of maximizing the interval between episodes of taxation, thus reducing the overall tax burden.
Even in societies employing inheritance systems that favor one sibling over others, parents have commonly provided more or less equally for their oﬀspring by requiring the child who inherits the family estate to pay a compensatory amount to each sibling. Primogeniture and related practices did not always mean disinheritance—a common misassumption. In medieval and early modern times, however, primogeniture among the landed aristocracy did mean that some younger sons and daughters faced diﬃcult economic and social prospects. Among the medieval Portuguese nobility, for example, landless younger sons and daughters were signiﬁcantly less likely to marry and leave surviving oﬀspring (Boone 1986). Younger sons, for example, left 1.6 fewer children than did eldest sons. Younger sons were also nine times more likely than eldest sons to father a child out of wedlock. Because they posed a serious threat to political stability in their own country, younger sons were channeled by the state into expansionist military campaigns in faraway places such as India, where they often died in battle or from disease. Similarly, the Crusades can be seen, in part, as a church-state response to this domestic danger (Duby 1977). The surplus of landless younger daughters in the titled nobility was dealt with by sending them to nunneries.
Birth-order diﬀerences have long been claimed in the domain of personality, although these claims have remained controversial despite considerable research on this topic. Psychologists have investigated the consequences of birth order ever since Charles Darwin’s cousin Francis Galton (1874) reported that eldest sons were overrepresented as members of the Royal Society. After breaking away from Sigmund Freud in 1910 to found a variant school of psychoanalysis, Alfred Adler (1927) focused on birth order in his own attempt to emphasize the importance of social factors in personality development. A secondborn, Adler considered ﬁrstborns to be ‘power-hungry conservatives.’ He characterized middleborns as competitive and lastborns as spoiled and lazy.
Since Adler speculated about birth order and its consequences for personality in 1927, psychologists have conducted more than 2000 studies on the subject. Critics of this extensive literature have argued that most of these studies are inadequately controlled for key covariates, such as social class and sibship size; that studies often conﬂict; and that birth-order differences in personality and IQ seem to have been overrated (Ernst and Angst 1983). Meta-analysis—a technique for aggregating ﬁndings in order to increase statistical power and reliability—suggests a diﬀerent conclusion. If we consider only those well-designed studies controlling for important background variables that covary with birth order and can introduce spurious cross-correlations, a meta-analytic review reveals consistent birth-order diﬀerences for a wide range of personality traits (Sulloway 1995). For instance, ﬁrstborns are generally found to be more conscientious than laterborns, a diﬀerence that is exempliﬁed by their being more responsible, ambitious, and self-disciplined. In addition, ﬁrstborns tend to be more conforming to authority and respectful of parents. Firstborns also tend to have higher IQs than their younger siblings—a reduction of about one IQ point is observed, on average, with each increment in birth rank (Zajonc and Mullally 1997).
These and other birth-order diﬀerences in personality can be usefully understood from a Darwinian perspective on family life (Sulloway 1996). Because they share, on average, only half of their genes, siblings will tend to compete unless the beneﬁts of cooperating are greater than twice the costs (Hamilton 1964). In this Darwinian story about sibling competition, birth order does not exert a direct biological inﬂuence on personality. For example, there are no genes for being a ﬁrstborn or a laterborn. Instead, birth order is best seen as a proxy for various environmental inﬂuences, particularly disparities in age, physical size, power, and status within the family. These physical and mental diﬀerences lead siblings to pursue alternative strategies in their eﬀorts to maximize parental investment (which includes emotional as well as physical resources). This perspective on how sibling order shapes personality accords with research in behavioral genetics, which ﬁnds that most environmental inﬂuences on personality are not shared by siblings and hence belong to the ‘nonshared environment’ (Plomin and Daniels 1987).
Prior to the twentieth century, half of all children did not survive childhood. Even minor diﬀerences in parental favor would have increased an oﬀspring’s chances of getting out of childhood alive. Because eldest children have already survived the perilous early years of childhood, they are more likely than younger siblings to reach the age of reproduction and to pass on their parents’ genes. Quite simply, a surviving eldest child was generally a better Darwinian bet than a younger child, which is why parents in most traditional societies tend to bias investment, consciously or unconsciously, toward older oﬀspring. An exception to this generalization involves youngest children born toward the end of a woman’s reproductive career. These children also tend to be favored, since they cannot be replaced (Salmon and Daly 1998).
Even when parents do not favor one oﬀspring over another, siblings compete to prevent favoritism. Siblings do so, in part, by cultivating family niches that correspond closely with diﬀerences in birth order. Firstborns, for example, tend to act as surrogate parents toward their younger siblings, which is a good way of currying parental favor. As a consequence, ﬁrstborns tend to be more conservative and parentidentiﬁed than their younger siblings, which is one of the most robustly documented ﬁndings in the literature. Laterborns cannot compete as eﬀectively as ﬁrstborns for this surrogate parent niche, since they cannot baby-sit themselves. As a consequence, laterborns seek alternative family niches that will help them to cultivate parental favor in diﬀerent ways. To do so, they must often look within themselves for latent talents that can only be discovered through systematic experimentation. Toward this end, laterborns tend to be more open to experience—that is, more imaginative, prone to fantasies, and unconventional— propensities that oﬀer an increased prospect of ﬁnding a valued and unoccupied family niche.
In addition to their eﬀorts to cultivate alternative family niches, siblings diverge in personality and interests because they employ diﬀering strategies in dealing with one another. These strategies are similar to those observed in mammalian dominance hierarchies. Because ﬁrstborns are physically bigger than their younger siblings, they are more likely to employ physical aggression and intimidation in dealing with rivals. Firstborns are the ‘alpha males’ of their sibling group, and they generally boss and dominate their younger brothers and sisters. For their own part, laterborns tend to employ low-power strategies to obtain what they want, including pleading, whining, cajoling, humor, social intelligence, and, whenever expedient, appealing to parents for assistance. Laterborns also tend to form coalitions with one another in an eﬀort to circumvent the physical advantages enjoyed by the ﬁrstborn. Middle children are the most inclined to employ diplomatic and cooperative strategies. Some middle children are particularly adept at nonviolent methods of protest. Martin Luther King, Jr., the middle of three children, began his career as a champion of nonviolent reform by interceding in the episodes of merciless teasing that his younger brother inﬂicted upon their elder sister.
Only children represent a controlled experiment in birth-order research. They have no siblings and therefore experience no sibling rivalry. As a consequence, they are not driven to occupy a particular family niche. Although only children, like other ﬁrstborns, are generally ambitious and conform to parental authority, they are intermediate between ﬁrstborns and laterborns on most other personality traits. Because age spacing can aﬀect functional sibling order, a ﬁrstborn whose next younger sibling is six or more years younger is eﬀectively like an only child. Similarly, large age gaps can make some laterborns into functional ﬁrstborns or only children.
That brothers and sisters diﬀer from one another for strategic reasons, rather than randomly, has been shown by studies involving more than one sibling from the same family (Schachter 1982). In one study, ﬁrstborns were found to be signiﬁcantly diﬀerent in personality and interests from secondborns, who were signiﬁcantly diﬀerent from thirdborns. By contrast, the ﬁrst and third siblings were not as diﬀerent as adjacent pairs, presumably because of less competition. This process of sibling diﬀerentiation has been termed ‘deidentiﬁcation’ and extends to relationships with parents. For example, a ﬁrstborn child sometimes identiﬁes more strongly with one parent than another. In such cases, the secondborn tends to identify more strongly with the parent that is not already preferred by the ﬁrstborn.
The most compelling evidence for birth-order eﬀects in personality comes from studies in which siblings assess each other’s personalities (Paulhus et al. 1999, Sulloway 1999). Such within-family designs control for the kinds of spurious correlations that can result from comparing individuals from diﬀerent family backgrounds. Studies based on such direct sibling comparisons exhibit consistent birth-order eﬀects in the expected direction. When the results are organized according to the Five-Factor Model of personality (Costa and McCrae 1992), ﬁrstborns tend to be more conscientious and slightly more neurotic than laterborns, whereas laterborns tend to be more agreeable, extraverted, and open to experience than ﬁrstborns.
Although reasonably consistent patterns for birth order and personality are observed according to the Five-Factor Model of personality, ﬁndings are signiﬁcantly heterogeneous for three of the ﬁve personality dimensions. For example, ﬁrstborns tend to be more assertive than laterborns, which is an indicator of extraversion, but laterborns tend to score higher on most other facets of extraversion, which include being more fun-loving, sociable, and excitement seeking. Similarly, laterborns tend to be more open to experience in the sense of being unconventional, whereas ﬁrstborns tend to be more open to experience in the sense of being intellectually oriented. Lastly, ﬁrstborns are more neurotic in the sense of being anxious about status, but laterborns are more neurotic in the sense of being more self-conscious.
As measured by direct sibling comparisons within the family, birth-order diﬀerences explain about four percent of the variance in personality, less than does gender and substantially more than do age, family size, or social class. In studies controlled for diﬀerences in age, sex, social class, and sibship size, siblings are about twice as likely to exhibit traits that are consistent with their sibling positions as to exhibit inconsistent traits. In short, not every laterborn has a laterborn personality (just as some ﬁrstborns deviate from the expected trend), but a reasonably consonant pattern is nevertheless present.
One still-unresolved question about birth order and personality is the extent to which within-family patterns of behavior transfer to behavior outside the family of origin. Recent studies suggest that birthorder eﬀects observed in extrafamilial contexts are about one-third to one-half the magnitude of those manifested within the family (Sulloway 1999). Relative to ﬁrstborn spouses and roommates, for example, laterborn spouses and roommates are generally perceived to be more agreeable and extraverted, but less conscientious and neurotic. Outside the family of origin, birth-order eﬀects seem to manifest themselves most strongly in intimate living relationships and in dominance hierarchies. These ﬁndings are not surprising, since they involve the kind of behavioral contexts in which sibling strategies were originally learned.
3. Behavior During Radical Revolutions
Some of the best evidence for sibling diﬀerences in behavior outside the family of origin comes from social and intellectual history. During socially radical revolutions, laterborns have generally been more likely than ﬁrstborns to question the status quo and to adopt a revolutionary alternative. This diﬀerence is observed even after controlling for the fact that laterborns, in past centuries at least, appear to have been more socially liberal than ﬁrstborns. As a rule, these eﬀects are very context sensitive. During the Protestant Reformation, for example, laterborns were nine times more likely than ﬁrstborns to suﬀer martyrdom in support of the Reformed faith. (This statistic is corrected for the greater number of laterborns in the general population.) In countries that turned Protestant, such as England, ﬁrstborns were ﬁve times more likely than laterborns to suﬀer martyrdom for their refusal to abandon their allegiance to the Catholic faith. Thus laterborns gave their lives in the service of radical rebellion, whereas ﬁrstborns did so in an eﬀort to preserve the waning orthodoxy. There was no generalized tendency, however, for laterborns to become ‘martyrs’ (Sulloway 1996).
Similarly, during the French Revolution ﬁrstborn deputies to the National Convention generally voted according to the expectations of their social class, whereas laterborn deputies were more likely to vote in the opposite manner. For this reason, there was only a modest tendency for ﬁrstborns and laterborns to vote in a particular manner, since voting was generally inﬂuenced by other important considerations that determined what it meant to conform or rebel in a rapidly changing political environment.
In response to radical conceptual changes in science one ﬁnds similar diﬀerences by birth order. Firstborns tend to be the guardians of what Thomas Kuhn (1970) has called ‘normal’ science, during which research is guided by the prevailing paradigms of the day. By contrast, laterborns tend to be the outlaws of science, sometimes ﬂaunting its accepted methods and assumptions in the process of attempting radical conceptual change. During the early years of the Copernican revolution, which challenged church doctrine by claiming that the earth rotates around the sun, laterborns were ﬁve times more likely than ﬁrstborns to endorse this/heretical theory. Copernicus himself was the youngest of four children. When Charles Darwin, the ﬁfth of six children, became an evolutionist in the 1830s, he was 10 times more likely to do so than a ﬁrstborn. During other notable revolutions in science—including those led by laterborns such as Bacon, Descartes, Hutton, Semmelweis, and Heisenberg, and by occasional ﬁrstborns such as Newton and Einstein—younger siblings have typically been twice as likely as ﬁrstborns to endorse the new and radical viewpoint. Conversely, when new scientiﬁc doctrines, such as vitalism and eugenics, have appealed strongly to social conservatives, ﬁrstborns have been more likely than laterborns to pioneer and endorse such novel but ideologically conservative ideas. These birthorder eﬀects typically fade over the course of scientiﬁc progress, as conceptual innovations accumulate supporting evidence and eventually convince the scientiﬁc majority.
In spite of their predilection for supporting radical innovations, laterborns have no monopoly on scientiﬁc truth. They were nine times more likely than ﬁrstborns to support Franz Joseph Gall’s bogus theory of phrenology, which held that character and personality could be read by means of bumps on the head. In their willingness to question the status quo, laterborns run the risk of error through over-eager rebellion, just as ﬁrstborns sometimes err by resisting valid conceptual changes until the mounting evidence can no longer be denied.
During ‘normal’ science, ﬁrstborns possess a slight advantage over laterborns. Being more academically successful, ﬁrstborns are more likely than laterborns to become scientists. They also tend to win more Nobel prizes. This ﬁnding might seem surprising, but, according to the terms of Nobel’s will, Nobel prizes have generally been given for ‘discoveries’ or creative puzzle solving in science, not for radical conceptual revolutions. Firstborn scientists who have innovated within the system include James Watson and Francis Crick, who together unraveled the structure of DNA, and Jonas Salk, who developed the polio vaccine. Such historical facts highlight another important point, namely, that ﬁrstborns and laterborns do not diﬀer in overall levels of ‘creativity.’ Rather, ﬁrstborns and laterborns are preadapted to solving dissimilar kinds of problems by employing disparate kinds of creative strategies.
4. Family Sentiments
The kinds of birth-order eﬀects that are observed during radical historical revolutions may depend as much on diﬀerences in ‘family sentiments’ as they do on personality. As Salmon and Daly (1998) have shown, ﬁrstborns (and to a lesser extent lastborns) are more strongly attached to the family system than are middle children. Historically, radical revolutions have tapped diﬀerences in family sentiments in two important ways. First, being considerably older than their oﬀspring, parents were more likely to endorse the status quo, given that age is one of the best predictors of the acceptance of new and radical ideas (Hull et al. 1978). For this reason, endorsing a radical revolution has usually meant opposing parental values and authority, something that ﬁrstborns (and to a lesser extent lastborns) are less likely to do than are middleborns. Second, most radical revolutions, even in ﬁelds such as science, have tended to raise issues that are directly relevant to issues of parental investment and discrimination among oﬀspring, ﬁlial loyalty, and overall identiﬁcation with the family system. During the Reformation, for example, Protestant leaders such as Martin Luther strongly advocated the right to marriage by previously celibate clergymen and nuns, who tended to be younger sons and daughters. These Protestant leaders also advocated the egalitarian principles of partible inheritance, by which property—and even political rule—were subdivided equally among oﬀspring (Fichtner 1989). During the height of the controversies raging over Darwin’s theory of evolution, Darwin noted that discriminatory inheritance practices posed a serious impediment to human evolution, remarking to Alfred Russel Wallace: ‘But oh, what a scheme is primogeniture for destroying natural selection!’ (Sulloway 1996, p. 54).
Even when radical ideology does not play a central role in conceptual change, and when discriminatory inheritance systems are also not a factor, diﬀerences in identiﬁcation with the family can impact on social and political attitudes. As Salmon (1998) has shown experimentally, political speeches appeal diﬀerentially to individuals by birth order depending on whether kinship terms are included in the speech. Firstborns and lastborns are more likely to react positively to speeches that employ kinship terms such as ‘brother’ and ‘sister,’ whereas middle children prefer speeches that employ references to ‘friends.’ To the extent that radical social movements make use of kinship terms, which they often do, birth-order diﬀerences in family sentiments will tend to inﬂuence how siblings react to radical change.
In past centuries birth order and functional sibling order have inﬂuenced such diverse social and political phenomena as royal succession, expansionist military campaigns, religious wars and crusades, geographic exploration, and inheritance laws. Even today, birth order and family niches more generally are among the environmental sources of personality because they cause siblings to experience the family environment in dissimilar ways. In particular, birth order introduces the need for diﬀering strategies in dealing with sibling rivals as part of the universal quest for parental favor. This is a Darwinian story, albeit one with a marked environmental twist. Although siblings appear to be hard-wired to compete for parental favor, the speciﬁc niche in which they have grown up determines the particular strategies they adopt within their own family. Finally, because birth order and family niches underlie diﬀerences in family sentiments, including ﬁlial loyalty and self-conceptions about family identity, the family has historically supplied a powerful engine for revolutionary change.
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