Birth Order - an overview | ScienceDirect Topics

Gender, Age Spacing, and Other Moderating Influences
Birth order is only one influence among many that contribute to the development of personality. Accordingly, there are exceptions to any generalization on this topic. The largest source of exceptions are those arising from genetic differences, which collectively explain about 40% of the variance in individual traits. Additionally, as children grow up and spend more time outside the family, they are increasingly influenced by peer groups and by life experience more generally. Some of these extrafamilial influences may nevertheless reflect differences that already exist between siblings.
There is growing evidence that some birth-order differences in personality are influenced biologically by the prenatal environment. Among males (but not among females), laterborns are more likely to become homosexuals. Unlike other behavioral effects associated with birth order, the operative factor is number of older brothers rather than relative birth rank from eldest to youngest child. In other words, a lastborn male who is the eldest of his sex is no more likely to become a homosexual than is a firstborn. These findings can be explained by assuming that a small proportion of mothers develop antibodies to one of the male-specific minor histocompatibility antigens. It has been hypothesized that such immunological responses prevent subsequent male fetuses from being fully transformed from female to male.
These findings about male homosexuality suggest that other aspects of personality – specifically, those involving gender-related traits such as tender-mindedness – might also be influenced by the prenatal environment. If such biological effects exist, they appear to be very small in normal populations. For example, meta-analysis of the birth-order literature reveals no significant difference in the frequency of birth-order effects by sex, including for behaviors specifically related to agreeableness and extraversion, which include many gender-related traits. In the study reviewed in Table 1, birth-order effects were just as substantial among women as among men. For males, moreover, number of elder brothers had no relationship with gender-related traits, with birth order being controlled. In short, the link between number of elder brothers and gender-related traits appears to reflect a developmental outcome of nonpsychological origin that is confined to male homosexuals.
Gender also contributes to personality, doing so in ways that parallel the influence of birth order on some personality dimensions – particularly extraversion and agreeableness. These similarities arise because birth order and gender have comparable effects on the strategies siblings use as they jockey for position within the family. Partly for genetic reasons and partly because of socialization, females tend to be less aggressive than males, just as laterborns tend to be less aggressive than firstborns. Gender also modifies the forms taken by aggression. Firstborn males are more physically aggressive than laterborn males, whereas firstborn females are more verbally aggressive. In addition, males and firstborns tend to be more assertive and tough-minded, whereas females and laterborns tend to be more affectionate, tender-minded, and cooperative. Women tend to be conscientious, whereas laterborns do not, so these behavioral parallels do not apply across the board.
Birth-order effects are modified by age gaps as well as by the sex of siblings. The influence of birth order is muted when the age gap is so small that the relationship between siblings is nearly equal, and also when the age gap is so large that they do not compete for the attention of their parents. When middle children have a large age gap between themselves and their next older sibling, but a small gap between themselves and their next younger sibling, they are more similar to firstborns in their personality.
Although laterborns tend to be more socially liberal, as well as more adventurous and unconventional than their older siblings, there are exceptions to these generalizations that are attributable to other aspects of family dynamics. Firstborn children of socially liberal parents tend to become liberals themselves because they generally conform to parental values. A laterborn child of liberal parents may become a conservative just to be different. Furthermore, firstborns sometimes become social or political radicals because they are in conflict with their parents instead of identifying with them. Mao Zedong, the eldest of four children, was radicalized by conflict with his father, a cruel and tyrannical man who mistreated his wife, his children, and the workers on his farm. Mao once said that there were two political parties in his family, and that he was the leader of the opposition. In general, laterborns are likely to rebel even if they do not have Attila the Hun for a father or the Wicked Witch of the West for a mother. They have elder siblings to induce them to identify with the underdog.
The consequences of birth order are often strongly dependent on the behavioral context, which is one of the most important moderators of human behavior. In different situations, the same person may behave quite differently, and the relevance of birth order likewise varies with the situation. For example, a firstborn may act in a dominant manner toward a younger sibling or a spouse, congenially toward peers, and in a subordinate manner toward authority figures – a behavioral style that constitutes a ‘pecking order personality.’
Social categories such as age, gender, and socioeconomic status entail standards of behavior that can influence personality. In connection with the study summarized in Table 1, respondents assessed the personality of a close friend (N = 1002). Significant birth-order differences emerged, but they did so in interaction with other variables. Upper-class respondents, as well as older respondents, saw their best friends as exhibiting the traits generally expected by birth order. In contrast, college students – especially those from lower-class backgrounds – did not perceive these same birth-order differences. For example, college students did not describe their firstborn friends as being particularly conscientious or conventional, whereas older respondents did.
These seemingly contradictory results are best understood in terms of the attributes that are important for success within each socioeconomic class, and during different stages of life. College-age students, especially from lower-class backgrounds, are likely to prefer friends who are sociable and who know how to have a good time. Because firstborns are more socially assertive and inclined toward leadership roles than are laterborns, they respond to these behavioral norms by projecting a gregarious and outgoing ‘persona.’ When people graduate from college, take a job, and marry, they assume new life roles and greater responsibility. Within such older populations, birth-order effects in personality increasingly conform to the expected pattern. Firstborns, who tend to be more conforming than laterborns, are especially affected by such life transitions. Compared with other individuals, firstborns are significantly more variable in terms of how their personalities are perceived across the social categories of age, class, and marital status. The more responsibility and status firstborns have acquired – for example, by getting married – the more they are judged by their friends as manifesting a typical firstborn personality.
These findings make sense from the perspective of evolutionary psychology, which is also a form of social psychology. The personality traits that are typically associated with birth order develop in the service of competition for scarce resources, principally parental investment. Adolescent individuals do not generally command scarce resources, so their behavioral priorities tend to be different from those of adults. A firstborn who aspires to social approval may be dutiful at home (especially during childhood), agreeable and gregarious during adolescence, and competitive and self-disciplined as an adult member of the workforce. The expression of personality traits related to birth order is likely to be sensitive to these transitions in the life course. In short, birth-order differences are not parent specific, but they are often situation specific. Research on birth order and creativity strongly reinforces this important conclusion.
Further Complexities
Firstborns, Only-Children, and MiddleBorns
Most of this article has purposely simplified birth-order issues by limiting the discussion to a comparison of firstborn children to all laterborns. The primary reason was that, in most respects, differences among laterborns are not as apparent as is the contrast with firstborns. Yet there are a few issues where middleborns and lastborns do stand out.
The unique findings for lastborn children include both good news and bad news. As noted earlier, Zajonc found an extra decline in the intellectual achievement of lastborns – above and beyond the gradual decline due to successive birth order. A thirdborn child, for example, fares more poorly if no younger children are added to the family. This finding was confirmed in the recent large sample and tightly controlled Norwegian data (described above).
Zajonc explained this anomaly in terms of the so-called ‘teaching effect’. Firstborns (and older siblings in general) often have to answer questions posed by their younger siblings and assist with their homework. At the time, the older siblings may experience these tasks as onerous. Rather than a burden, Zajonc argued, this teaching opportunity actually benefits earlier-borns, perhaps by forcing them to engage more deeply with the material they are teaching. That claim is quite consistent with the tutoring research in educational psychology, which shows that teaching benefits the teacher at least as much as it benefits the student. A lack of such opportunities can thus explain why lastborn children show an extra deficit in intellectual achievement and why only, children do not achieve as highly as other firstborns.
The good news for lastborns lies in the personal popularity that ensues from their birth order. In surveys of comparative popularity, the lastborns are voted the ‘favorite child’ more often than any other birth order. This popularity may well reflect an inevitable tradeoff with personal achievement. Peers prefer others who are noncompetitive and more socially oriented than achievement oriented.
These arguments can also be applied to only-children. The fact that they are also lastborns, may explain why their academic achievement does not match up to that of other firstborns; the fact that they are also firstborns may explain why they are not as popular as other laterborns.
The tradeoff between the respect accorded to firstborns and the personal popularity of lastborns often leaves the middleborns feeling left out. Studies by Canadian researchers, Salmon, Daly, and Wilson has confirmed that, in various ways, the middleborns feel less attached to the parents. For example, they are less likely than either first- or lastborns to nominate their mothers as their favorite family member.
Gender
Compared to the prominent role that gender plays in many developmental issues, it has made surprisingly little difference in birth-order studies. The intellect and personality profiles that emerge for females are comparable to those emerging for males.
Certainly, over the long history of birth-order research, a number of statistical interactions have been reported where gender was involved. When found, the results of a particular combination – say, firstborn males with secondborn females and thirdborn males – were not difficult to explain with a ‘just-so story’. But the fact that such interaction effects rarely replicate suggests that the original findings were due to chance. With larger families, the number of possible combinations escalates quickly. With increasing parity in male vs. female achievement, any such interactions with birth order may eventually vanish. For these reasons and others, recent research has paid little attention to possible gender differences in birth-order effects.
A couple of recent findings constitute exceptions to this rule. An interesting finding reported by Sulloway was that secondborn boys often develop with firstborn personalities if the firstborn is a girl. Perhaps boys do not see firstborn girls as competitors and react only to their brothers. Or parents may still place more value on the firstborn male child.
The lack of difference in the size of birth-order effects has played a role in evaluating the Prenatal Hypomasculinization Theory. The credibility of that theory is weakened by the fact that its predicted larger effect size in males has not materialized in recent (tightly controlled) research.
2 Personality
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 parent-identified 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 birth-order 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.
Direct Sibling Comparisons
Birth-order differences in personality vary in magnitude and sometimes even in direction, depending on how they are measured. When assessed by self-report questionnaires, birth-order effects are typically modest and often nonsignificant. Even in very large studies, which have adequate statistical power to detect small effects, birth-order differences may go unnoticed for several methodological reasons. Important sources of overlooked effects include (1) a failure to consider interaction effects between birth order and other demographic variables, such as age, gender, sibship size, and social class; (2) the amalgamation of disparate traits such as assertiveness and sociability into higher-order constructs like “extraversion,” which may cancel out birth-order differences; and (3) the fact that birth order is a multivariate construct that potentially includes linear, quadratic, and other nonlinear trends.
Yet systematic differences by birth order are generally found when parents rate their own offspring or when siblings compare themselves with one another. In a study by Frank Sulloway, 660 business leaders were surveyed. In self-reported ratings, firstborn CEOs did not differ significantly from laterborns on 10 of the 11 personality traits included in the survey. After providing these self-assessments, respondents were asked to compare themselves with their siblings, using the same 11 personality scales. A comparative method of assessment possesses several advantages over more customary methods of self-report. In particular, direct comparison anchors the scales. Additionally, comparative judgments among siblings eliminate any confounding effects associated with differences between families. Using such comparisons, eight of the 11 traits included in the survey elicited significant differences by birth order. Relative to their older siblings, laterborn business leaders were more tender-minded, cooperative, flexible, submissive, empathetic, liberal, unconventional, and even-tempered. Overall, comparative ratings resulted in birth-order differences that were five times as large as those previously obtained without using direct comparisons.
A follow-up study involving 6053 individuals aged 8 to 95 (mean age = 36.8, SD = 17.1) yielded similar results for a broad array of personality traits. Subjects rated themselves on nine-step bipolar scales using adjective pairs that were chosen to represent the 30 facets of the NEO PI-R, a comprehensive personality inventory based on the Five Factor Model. Firstborns were asked to rate both themselves and their next younger sibling, whereas laterborns were asked to rate themselves and their next older sibling. Based on direct sibling comparisons, 23 of the 30 bipolar adjective pairs yielded significant differences, and 26 of the 30 adjective pairs produced correlations in the expected direction. As anticipated, firstborns were judged to be more conscientious than their younger siblings, whereas laterborns were judged as being more agreeable and open to experience. For neuroticism, a dimension for which birth-order differences were expected to be mixed, firstborns were predicted to be more anxious and quicker to anger—expectations that were both confirmed. Laterborns were predicted to be more depressed, vulnerable, self-conscious, and impulsive, but only the last two predictions were confirmed. As anticipated, results for extraversion were also mixed. Laterborns were more affectionate, fun-loving, and inclined to seek excitement. Firstborns were more energetic and dominant.
Controlling for age, sex, sibship size, and social class, the partial correlation between birth order and a scale score of predicted differences was 0.20, with birth order accounting for 4.1% of the variance. Two other family background variables—sibship size and social class—accounted for less than 0.1% of the variance in this scale score of predicted differences, as also did age. By comparison, sex explained 2.1% of the variance. (Laterborn personality traits resembled those observed in females.) Controlled for the linear effect of birth order on the scale score, there was also a significant quadratic trend: middle children scored higher than lastborns on some personality scales, particularly those related to agreeableness (Table 1).
Table 1. Birth-order effects in scale scores for the big five personality dimensions based on direct sibling comparisons
Personality dimensiona
Partial correlation with birth orderb
N
p&lt;
Conscientiousness
−0.18
4507
0.0001
•
Firstborns are more deliberate, dutiful, effective, energetic, ahardworking, organized, self-disciplined, and under controla
Laterborns are more esthetically inclined, prone to fantasy, attentive to inner feelings, untraditional, attracted by novelty, and drawn to ideasc
Extraversion
0.14
4404
0.0001
•
Laterborns are more affectionate, excitement-seeking, fun-loving, and gregarious
Neuroticism
−0.04
4278
0.001
•
Firstborns are more anxious, as well as more prone to depression and feelings of vulnerability
−0.04
4278
0.001
•
Laterborns are more self-conscious
0.05
3548
0.005
Scale score for personality differences, as predictedd
0.20
4177
0.0001
a
Each of the 30 bipolar adjective pairs, representing the 30 facets of the NEO PI-R, is classified under the Big Five dimension on which it has its highest factor loading. Four of the 30 adjective pairs have their highest loading on a personality dimension other than the one for which they were selected. “Assertive (dominant)/unassertive (submissive)” has its highest loading on Agreeableness (−0.54) rather than Extraversion (0.32). Similarly, “quick to anger/easygoing” has its highest loading on Agreeableness (−0.55) rather than Neuroticism (0.47). “Impulsive/under control” has its highest loading on Conscientiousness (−0.59) rather than Neuroticism (0.24). “Energetic/leisurely” has its highest loading on Conscientiousness (0.47) rather than Extraversion (0.28).
b
A positive partial correlation denotes a higher score for laterborns. For each bipolar trait, a sibling difference score was calculated using z-scores and then used to compute each scale score in terms of predicted differences. Italicized traits exhibit significant birth-order differences. (All statistical tests are two tailed.) All partial correlations are controlled for age, sex, sibship size, and social class. All dimension scale scores except Neuroticism exhibit a significant quadratic effect, controlled for the linear trend, indicating a higher score for middleborns. For one dimension (Agreeableness), the quadratic trend is larger than the linear trend. Excluded from these statistical results are 548 only children, 71 twins, 112 subjects reporting an age gap of more than 9 years between themselves and the sibling they also rated, 86 subjects whose biological and functional birth orders do not coincide, and 726 subjects who did not provide information on one of more of the dependent or independent variables.
c
Being “drawn to ideas” was predicted to be a firstborn trait.
d
Some respondents made ratings on fewer than 30 bipolar adjective pairs. In these cases, scale scores have been computed from the observed data.
Measured in terms of an overall scale score that reflects predictions about birth order, age and sex accounted for substantially less variance in personality scores than did birth order. It is important to note, however, that age and sex explained considerably more about personality as a whole than they did about birth-order-related traits. Sex differences in this study accounted for 8.3% of the variance in dimension scores for the Big Five, and age explained another 2.6% of the variance. In accounting for 4.1% of the variance in these same dimension scores, birth order was substantially more influential than age, but less so than sex. Still, on two dimensions of the Five Factor Model (conscientiousness and extraversion), birth order had greater influence than either age or sex.
One should bear in mind that birth order is only a proxy for the real causes that lie behind sibling differences in personality, namely, disparities in age, size, status, and power within the family system. Not all firstborns adopt the role of a surrogate parent toward younger siblings, and some firstborns are less bossy than others. Individual differences in physical size, temperament, and opportunities for surrogate parenting help to explain why some individuals deviate from the patterns of personality that are generally expected by birth order. In the study summarized in Table 1, individuals were asked to what extent, during childhood, they acted as a surrogate parent toward their siblings, and also to what extent they bossed their siblings around. High scores for surrogate parenting behavior and bossiness reflect typical firstborn behaviors and were strongly correlated with birth order (r = −0.56 for the composite measure). This indicator of family niches also accounted for 10.5% of the variance in personality scores, making it considerably better at predicting personality than any other variable in the study. The predictive success of this variable lies in large part in its ability to account for exceptions to predictions based on birth order. For example, laterborns (often eldest daughters) sometimes reported having done substantial surrogate parenting with respect to their younger siblings. These individuals also tended to describe themselves as having firstborn personality traits.
Other studies using the NEO Personality Inventory and comparable instruments have generally yielded null results, although most of these studies have possessed only moderate statistical power and none have employed the method of direct sibling comparison. Based on similar discrepancies between self-report ratings, which often yield meager results, and significant differences as judged by family members, some researchers have argued that birth-order effects are parent specific and do not hold up outside the family. When assessed meta-analytically, however, studies involving nonfamily members exhibit significantly more findings of a confirmatory nature than are expected by chance. Confirmatory findings are also especially likely to occur whenever studies involve real-life behavior. In addition, if birth-order effects are specific only to childhood (and the family milieu), one might expect these differences to diminish with age. In the study reviewed in Table 1, birth-order effects did decrease modestly with age for conscientiousness, but also they increased modestly for agreeableness. Overall, there was not a significant diminution in the scale score for predicted birth-order effects by age.
The method of direct sibling comparison does not prove that birth-order differences in personality exist outside the family milieu. Direct sibling comparisons may also be susceptible to “contrast effects,” whereby small but real differences between siblings are magnified, exaggerating the variance explained by birth order. However, when the scores in Table 1 for direct sibling comparisons were transformed into binary outcomes (i.e., being higher or lower than a sibling on each trait), birth-order effects were only slightly reduced. Another possibility is that these comparative measures reflect shared stereotypes about personality, rather than true differences. In an effort to determine whether birth-order differences are recognized by people other than siblings, study participants were asked to rate their spouses. Significant birth-order differences emerged in the expected direction, although the mean effect size for the Big Five personality dimensions was somewhat smaller than for direct sibling comparisons. Controlling for age, sex, sibship size, and social class, the partial correlation among spouses between birth order and a scale score for predicted personality differences was 0.12, which accounted for 1.4% of the variance (N = 822).
An analysis of the scores for individual traits revealed that spouses were detecting the same birth-order differences that they revealed when they compared themselves with a sibling. A convenient way of demonstrating this point is to compare the effect sizes for birth order, on a trait-by-trait basis, with the effect sizes for the same traits as judged by spouses. These two sets of effect sizes were substantially correlated (r = 0.61, N = 30 traits, p < .001). In other words, those traits that were strongly associated with birth order in sibling relationships were the same traits that were strongly associated with birth order in married couples. Relative to firstborn spouses, for example, laterborn spouses were perceived to be less conscientious, but more agreeable, extraverted, and open to experience. A similar pattern in birth-order effects was found among college students who rated both a sibling and a roommate (r = .72, N = 30 traits, P < .001; for roommates, the partial correlation between birth order and a scale score for predicted personality traits was .16, N = 165, P < .05). Thus birth-order differences are not restricted to family members or the family milieu, as some psychologists have claimed. Rather, these differences tend to manifest themselves in intimate living situations.
Summary
Birth order is one of many environmental factors that play a role in the development of personality and a variety of behaviors. A Darwinian family niche perspective makes clear predictions about the ways in which birth order effects will play out and many of them have been tested in numerous research programs over the past 10 years. In general, higher scores on conscientiousness are found in firstborns while laterborns score higher on agreeableness, and openness to experience. The dominance aspects of extraversion are high-scoring for firstborns while the sociability ones are high-scoring for lastborns. Neuroticism has much smaller and less clear-cut results. These distributions of personality traits serve individuals of different birth orders in their attempts to secure the best resource acquisition outcome possible. In turn, they also shape their behavioral tendencies as children and adults.
2.2 Similar or different TF sequence in other systems?
Since birth-order-dependent neuronal specification has been widely observed in various systems, is the same or a similar temporal TF sequence utilized to pattern neural stem cells of other systems?
In the mushroom body, which is generated by four NBs, each NB sequentially generates at least three types of neurons. Thus far, no temporal sequence of TFs that controls the fate of these neurons has been identified in the NB. However, a novel bric-a-brac,tramtrack, broad (BTB)-zinc finger protein, named Chinmo (chronologically inappropriate morphogenesis), acts in GMCs or young neurons to control the temporal identity of mushroom body neurons (Zhu et al., 2006). Although Chinmo mRNA is equally expressed throughout the entire NB lineage, Chinmo protein shows a temporal gradient in the neuronal progeny. It is absent in NBs and is expressed at its highest levels in the early-born neurons, lower in the next-born neurons, and undetectable in the latest-born neurons. Reducing or increasing Chinmo levels causes transformation of neurons toward later or earlier fates, respectively (Zhu et al., 2006). Recently, microRNAs of the Let-7-complex, the heterochronic miRNAs originally identified in Caenorhabditis elegans, were found to target chinmo to regulate the temporal identity of Drosophila mushroom body neurons (Wu, Chen, Mercer, & Sokol, 2012). Since Chinmo protein or Let-7 miRNAs are not detected in NBs, how the Chinmo gradient in neurons is regulated as the NB ages is not understood.
Postembryonic neuroblasts (pNBs) in the VNC generate 90% of neurons that constitute the adult CNS. Although a complete temporal TF sequence has not been identified, recent studies identified two members of the postembryonic TF sequence, Cas and Seven-up (Svp) (Maurange, Cheng, & Gould, 2008; Tsuji, Hasegawa, & Isshiki, 2008). They are required for a temporal switch of pNBs from generating small Chinmo+ neurons to instead producing large Br-C+ (Broad Complex) neurons in many different lineages. Since the switch from Chinmo+ to Br-C+ happens later than the transient expression of Cas or Svp, the switch must be directly controlled by an unknown member of the TF sequence whose expression is promoted by cas and svp by feedforward regulation (Maurange et al., 2008). Cas and Svp are also required for the NBs to eventually end neurogenesis. This is discussed in detail below.
In the anterodorsal lineage of the antennal lobe, Kr was shown to act in the NB to define one out of 40 temporal fates of PNs (Kao, Yu, He, Kao, & Lee, 2012). Loss of Kr from the NB causes a single PN fate to be skipped. However, loss of Hb, Pdm or Cas does not produce detectable phenotypes (Kao et al., 2012). It will be interesting to identify more TFs that are temporally expressed in antennal lobe NBs and control the temporal specification of a large number of distinct PNs and interneurons.
Inspection of the Umbilical Cords
Obstetricians usually mark umbilical cords regarding birth order. One clip typically denotes the first delivered sibling, and an additional clip for each subsequent delivery identifies umbilical cords of the siblings that follow. Parenthetically, when chorioamnionitis is found, in the absence of birth order designation, it usually affects the placenta closest to the endocervical os exclusively or most severely, reflecting that chorioamnionitis represents an ascending infection that reaches the presenting sac before the nonpresenting sac.
Umbilical cord anomalies, particularly of cord insertion, are common in twins and more common than in singletons due to placental disc growths in opposing directions (discussed later). Their approximate prevalence is shown in Table 31.2.
BIRTH ORDER
A tremendous amount has been written about birth order: birth order and personality, birth order and temperament, birth order and cognition, birth order and sexual orientation. Birth order effects, when delineated, have been attributed to environmental differences and to biologic effects on the developing fetus. Sometimes the literature on birth order can leave one with the sense that many traits can be handily explained by birth order—and that their inverses can be explained as well. An independent and resilient first-born child is identifying with the authority and autonomy of the parents, and exercising leadership and dominion over younger siblings; an independent and resilient younger child is defining a distinct niche as a way of differentiating from the older siblings. Nevertheless, there are some generally agreed-on personality traits—or perhaps tendencies—which assort with birth order, although all such effects are tempered and confounded by gender; social conditions; and the individual complexities of family life, happenstance, and history.
In 2007, the New York Times ran a front-page story with the following headline: “Research Finds Firstborns Gain the Higher I.Q.” In this large study of 250,000 male Norwegian military conscripts, higher rank in the family was found to be associated with small but significant increases in IQ score (Kristensen and Bjerkedal, 2007). Second-born sons who had been reared as the oldest because a first-born child had died also had the higher IQ scores found in first-born sons. The tremendous attention paid to this study by the national press and the vigorous debate and discussion that followed served as a reminder of how immediate and personal these issues remain to many adults.
The study confirmed numerous earlier studies that showed a positive relationship between low birth order and IQ, and supported an environmental/family interaction mechanism in which the family environment in which the oldest surviving child develops accounts for the IQ difference (rather than, for example, a previously proposed mechanism in which maternal antibodies, which increase in successive pregnancies, affect the developing fetal brain with increasing severity). Other multilevel analyses of large samples in the past have attributed important intelligence differences to factors that vary between families, however, rather than within families. It also has been pointed out that many younger siblings develop special skills and talents that are not measured on traditional IQ tests, but that are important for success in school and in life.
Many researchers have argued that the most significant differences in cognitive development concern language. First-born children are exposed exclusively to adult parental language and generally receive more direct language stimulation from their parents; their vocabulary skills are consequently improved. The increased and often intense socializing that goes on among brothers and sisters can enhance communication skills, however, so that younger siblings may be able to use their language very effectively. In any case, individual developmental factors and home and family influences contribute to vast variation in language acquisition within any birth order group, and the milestones of normal language acquisition are the same for all children, regardless of birth order; parents may claim that a young child whose language is apparently delayed is able to communicate effectively with an older sibling, but that young child still warrants aggressive workup and intervention for the speech delay.
Another area of great controversy has been the effect of birth order on personality, achievement, and creativity. There are many popular beliefs about birth order and achievement, often focusing on the supposed drive and accomplishments of first-born children, explained sometimes as a result of their disproportionate share of parental attention, other times as part of their overidentification with parental roles, and sometimes as a relic of old systems of primogeniture and increased expectation of the oldest (especially because, according to some studies, the increased achievements of first children are a phenomenon in particular of families with higher socioeconomic status). A controversial book by Sulloway (1996) argued, based on the analysis of many historical figures in science, religion, and politics, that first-born children identify with their parents and accept existing authority, whereas younger brothers and sisters are more inclined to rebel and to manifest various kinds of revolutionary creativity.
There is some much-discussed research to suggest that sexual orientation may have a link to birth order, especially in males, and that more older brothers may increase the likelihood that a younger son will be homosexual (Bogaert and Liu, 2006). A biologic mechanism has been proposed involving a maternal immune effect engendered by male fetuses, with increasing intensity after sensitization by previous male fetuses. This mechanism would not account, however, for the link that some studies have found between birth order and female homosexuality. Other researchers have argued that any link is probably social and environmental.
With regard to birth order, there is no character trait and no important life thread for which birth order has been established as the most important determinant. Health care providers should discourage parents from generalizing on the basis of birth order (e.g., “first children are the superachievers,” “younger siblings may be slower to start talking”). There is tremendous variability among first-born children—and among younger brothers and sisters—in every trait that has been studied, and there are multiple confounding factors, such as child's gender, sibling gender, birth spacing, culture, and family circumstances, all of which can figure in the ontogeny of personality, behavior, and life story. The many controversies over birth order serve to remind us, however, that in this very important respect, each child does grow up in a different family environment. There may not be simple predictive general rules of how birth order and sibling relationships will affect a child's life and character, but on an individual basis, those effects are powerful, unforgettable, and lifelong.
G INTERNALITY-EXTERNALITY
Cicirelli (1982), in the investigation of birth-order subgroups, found regarding happiness among the elderly that those with more living brothers were more frequently external in their attribution of causality; fewer brothers led to internality. Also those with sisters and with contact with siblings were more internal (i.e., had a greater sense of efficiency in life).
McDonagh (1971) reported concerning 132 male high school students that when the ratio of males to females in a sibship was three to one the student was more field dependent and less self-assured.
Gray (1984), using the 16PF test found males were more tough-minded than females, and that having an older brother led to tough mindedness. First-borns with a younger brother were more trusting than with a younger sister. She found SS3 middle-borns more impulsive and careless of protocol, and that first-borns with a younger brother were more venturesome.
Chrenka (1983) examined the sense of uniqueness among 133 SS2 and SS3 college women from intact homes. The sense of uniqueness was greater when the neighboring sibling was a male. This was true for all ordinal position groups in these sibships of two and three except the second-born of three who felt least unique and most competitive. First- and third-borns felt most competitive with seconds than with others. College women felt greater affinity to sisters than to brothers. SS3 firstborns had a greater sense of uniqueness and least competitiveness.
Schachter, Shore, Feldman-Rotman, Marquis, and Cambbell (1976) investigated 383 college students for the amount that adjacent siblings deidentified (claimed to be different from each other). Same-sex siblings deidentified more than cross-sex siblings. They felt that such deidentification was a mechanism that resolved sibling rivalry.
In summary, brothers make for externality and field dependence. Same-sex siblings, resembling same-sex twins, strive for uniqueness.