Birth Order and Parental Investment
KeywordsBirth Order Parental Investment Young Sibling Major League Baseball Cognitive Stimulation
Parents may differentially invest limited resources in offspring who have a higher likelihood of survival, which is often older children; and equal division of parental investment across multiple children tends to have differing effects across firstborns, middleborns, and lastborns.
During his visit to the Galápagos Islands in 1835, Charles Darwin was puzzled by the extensive diversity he observed among 13 species of Galápagos finches. These birds, he later realized, were actually members of a single, closely related avian group that had evolved its remarkable diversity through competition over time. Human siblings are a lot like Darwin’s finches, as they also tend to diversify in various ways – for example in intellectual performance, personality, and social attitudes (Sulloway 2010). Some of this diversity among siblings is genetic. On average, full siblings share only half of their genes unless they are identical twins. Furthermore, even though siblings are brought up within the same family, they do not fully share the same family environment. And their differing family experiences are systematically related to birth order and parental investment.
From the perspective of evolutionary biology, Trivers (1972) defined parental investment as any investment in individual offspring by a parent that increases the offspring’s chances of survival and reproduction at the cost of the parent’s ability to invest in other offspring. Parental investment can take various forms, including material resources such as food and other basic necessities; financial support for education and professional training; cognitive stimulation (for example, time spent reading to children or helping them with their homework); and interpersonal investment, such as attention, affection, and general encouragement (Hertwig et al. 2002). Intentionally or unintentionally, parents often invest unevenly in their offspring according to birth order. Such differences in parental investment are one of the mechanisms that drive offspring to compete with one another.
How Parental Investment Is Related to Birth Order
Before the beginning of the nineteenth century, nearly half of all children died before the age of 5, and even today infant mortality rates are still very high in some developing countries. When resources are limited, parents may invest more heavily in offspring who have a higher likelihood of survival and hence of passing their genes onto future generations. Older children are in a more favorable position as they have already survived some of the lethal diseases of childhood and therefore possess a greater prospect of future survival than do their younger siblings (Sulloway 1996). In addition, firstborns are often granted a greater role in perpetuating the family line, wealth, and traditions, especially in preindustrial societies. A good example is the custom of primogeniture, leaving inheritance to the oldest son, a practice that prevailed during much of the premodern period (Hrdy and Judge 1992). Apart from being favored by a greater share of family inheritance, firstborns – regardless of sex – are often given privileges through other social customs, including birth ceremonies, leadership roles, teknonymy (renaming a parent after a child), and having authority over their siblings in adulthood (Rosenblatt and Skoogberg 1974).
In modern society, as resources become more plentiful, parents tend to become increasingly egalitarian and to invest more or less equally in their offspring. Even when this is the case, however, the cumulative pattern of parental investment still tends to differ among siblings, and often produces inequality that is related to birth order (Hertwig et al. 2002). Because firstborns and lastborns both experience a period of exclusive parental investment when other siblings are not yet born or have grown up and left the home, they generally receive a greater share of resources compared with middleborns. Thus, the relationship between parental investment and birth order tends to be quadratic or U-shaped. Nevertheless, for those aspects of parental investment that are particularly important for human development during infancy or early childhood – for example, vaccinations and cognitive stimulation – lastborns never obtain an advantage over middleborns. In such cases, an egalitarian motive among parents results in linear relationships between parental investment and birth order because each additional child dilutes the available family resources.
Theories about how birth order influences parental investment are well supported by empirical studies. Horton (1988), for instance, examined the relationship between birth order and nutritional status in 1,903 households in the Bicol region of the Philippines. He found that older children were more nutritionally advantaged over their younger siblings. Numerous other studies have shown that firstborns are more likely to receive vaccinations than are laterborns (Hertwig et al. 2002). A study of 14,192 children born between 1915 and 1929 in Sweden found that laterborn children had a considerably higher mortality risk compared with their firstborn counterparts (Modin 2002). Price (2008) reported that firstborns in two-child families received, each day, about 25 more minutes of quality time engaging with their parents compared with laterborns. In an investigation of 11,420 children of respondents from the National Longitudinal Survey of Youth 1979, Lehmann et al. (2014) found that mothers were more likely to breastfeed and to provide cognitive stimulation for their firstborn children. These empirical findings exemplify the linear relationship that is expected between birth order and some forms of parental investment. We need to bear in mind, however, that other forms of parental investment that are confined to adolescence and early adulthood have not been adequately investigated and, in some cases, are likely to manifest U-shaped relationships between birth order and parental investment.
Intelligence and Education
Birth order differences in parental investment can have profound influences on siblings in various ways. Abundant empirical evidence has shown that laterborn children tend to have lower IQs than their firstborn counterparts. For example, studies using large Norwegian administrative datasets (N > 241,000) have found that, compared with secondborns, firstborns have IQ scores that were around 0.2 standard deviations, or 3 points, higher (Black et al. 2011; Kristensen and Bjerkedal 2007). In the same studies, these findings were confirmed by within-family comparisons, which cannot be confounded by socioeconomic or other differences in family background. Of particular interest in the Norwegian data is the fact that when birth rank changed as a result of the early death of an older sibling, the IQ scores of younger siblings increased, showing that IQ corresponds with functional rather than biological birth rank. Another large within-family study using Swedish administrative data (N = 222,034) also found that intelligence declined with increasing birth order (Barclay 2015). However, the overall evidence for the relationship of birth order to intelligence is mixed, as some studies have found no significant relationship. Most of these exceptional instances involve small sample sizes that often lacking adequate statistical power, or studies carried out with young children, for whom the influence of birth order is not yet fully manifested (Sulloway 2007; Zajonc and Sulloway 2007). Still, well-designed studies have also shown that lower-IQ parents tend to have larger families, and lower-IQ offspring, compared with higher-IQ parents (Rodgers et al. 2000; Wichman et al. 2006). This relationship has been ascribed to genetics and its influence on social status, which in turn correlates with family size. This confound has not always been adequately addressed in studies of birth order and intelligence, although it is not a relevant factor in within-family studies, such as the large Norwegian study conducted by Kristensen and Bjerkedal (2007).
Despite debate about birth order and intelligence, there is consistent evidence indicating that firstborns have an advantage over their laterborn siblings in educational performance and attainment. The following are just a few examples. In a study using sibling data from the UK, Bu (2016) found that firstborns not only had higher educational aspirations but also were more likely to gain post-sixteen qualifications allowing access to higher-level courses within the UK educational system. Using data that included the entire Norwegian population, Black et al. (2005) also found a negative relationship between birth order and educational attainment. They reported that the difference in educational levels between the first and the fifth child in a five-child family was comparable to the black-white racial difference. Apart from the indirect influence on educational attainment through IQ, some birth order differences in education may also be explained by disparities in parental investment arising from differences in available financial resources as family size increases.
Personality and Social Attitudes
Birth order differences in parental investment, as well as closeness to parents, influence personality, self-esteem, parental identification, and the overall parent–child relationship. In one study, siblings who were favored by their parents, or who reported low levels of parent-offspring conflict, were more conscientious, agreeable, and extraverted; whereas those not favored by parents, or who reported high levels of parent-offspring conflict, were more open to experience and tended to identify less strongly with parental values and to question parental authority (Sulloway 1996, 2001). In a study asking participants to compare themselves with their siblings on various personality measures, Paulhus et al. (1999) found that firstborns were more conscientious and achieving; laterborns, by contrast, tended to be more agreeable, liberal, and rebellious. Such within-family studies are particularly instructive because there cannot be any confounds caused by uncontrolled socioeconomic disparities between different families. Sulloway (2001, 2010) performed a meta-analysis of 10 different within-family studies (N = 7,218), including that by Paulhus et al. (1999). These meta-analytic results showed that firstborns scored higher than laterborns on measures of conscientiousness, one aspect of extraversion (being assertive), and one aspect of neuroticism (anxiety). By contrast, laterborns scored higher on traits relating to agreeableness, most other aspects of extraversion, one aspect of neuroticism (self-consciousness), and several indicators of openness to experience that are associated with unconventionality and open-mindedness. In another meta-analysis involving 8,340 participants in 24 different studies of athletic participation, Sulloway and Zweigenhaft (2010) showed that laterborns were more likely to engage in dangerous sports and to take more risks in professional athletics. For example, among 700 brothers who played Major League baseball in the United States, younger brothers were 1.7 times more likely to attempt to steal bases than their older brothers, and they were 1.4 times more likely to successfully complete a stolen base attempt.
Middleborns, who generally receive the least amount of parental investment, are reported to have lower self-esteem and to be more self-conscious compared with firstborns and lastborns (Kidwell 1982; Sulloway 2001). Studies have also shown that middleborns are not as close to their parents as are their siblings. For example, middleborns are less likely to turn to parents for emotional support in response to traumatic events (Rohde et al. 2003; Salmon and Daly 1998). This finding is a good example of the U-shaped relationship that is expected between birth order and some cognitive and behavioral outcomes.
Although within-family studies are nearly always supportive of birth-order differences in personality and social behavior, some very large between-family studies have found only small effects (Damian and Roberts 2015; Rohrer et al. 2015). Recent research involving more than 500,000 participants in an Internet personality survey using the Big Five Inventory has shown that reasonably impressive birth order effects exist in between-family samples when a variety of methodological precautions are observed, such as taking into account interactions between birth order and other demographic variables, including age, gender, sibship size, social class, race, and nationality (Sulloway et al. 2016). In addition, analyses need to be performed at the trait level, rather than at the broader dimension level to prevent differences in some aspects of personality from canceling themselves out (for example, assertiveness – a firstborn attribute, and being fun-loving – a laterborn attribute, would both be scored under the dimension of extraversion). Some birth order effects are also relationship specific, in that the personality differences obtained when study participants rated only themselves were different from the results obtained when participants rated themselves and another person, which in turn differed depending on the specific identity of the other person being rated (for example, a sibling, a friend, or a romantic partner). One should also keep in mind that birth order is only an imperfect proxy, or mediator, of the real causes of sibling differences in personality. Three examples of important influences that are mediated by birth order are differences in parental investment, involvement in surrogate parenting, and physical size. Because researchers rarely examine these other causal variables, investigations typically overlook influences on personality that are revealed by more carefully designed approaches to the study of “family niches” (Sulloway 2001; Sulloway et al. 2016).
The influence of parental investment on parental identification and the parent–child relationship extends to social attitudes. More specifically, higher levels of parent-offspring conflict are associated with more liberal social attitudes among offspring (Sulloway 1996). In a study of 569 Chinese Tokok families in Indonesia, older siblings were more obedient to their parents’ wishes and more conservative, socially and politically, compared with their younger siblings (Skinner 1992). Among Japanese-American immigrant families, Manaster et al. (1998) found that firstborns were more likely than laterborns to carry on family traditions, including speaking the Japanese language, and were also more likely to be political and religious conservatives. In a meta-analysis of 20 previous studies, Sulloway (2001) reported that laterborns were 20 % more likely than firstborns to endorse a liberal political viewpoint, which was roughly equivalent to the gender gap in voting behavior in the United States. These results are consistent with another series of studies based on historical data, which found that laterborns were twice as likely as firstborns to support radical revolutions in science and various social and political domains (Sulloway 1996).
In sum, considerable theoretical and empirical research has shown that parents invest unevenly in their offspring in relation to birth order. As a result, siblings with differing birth orders tend to compete for parental favor and to diversify in various ways, including in their intelligence, education, personality, parental identification, and social attitudes. Under most circumstances, birth order effects are relatively modest, but it is likely that these effects will be greater in magnitude whenever they have been elicited by an appropriate stimulus that taps patterns of behavior learned in childhood. Parental investment is just one among several potential mechanisms that generate and explain these observed birth order differences. Much still remains to be learned about other psychological mechanisms that underlie sibling differences. One conclusion, however, is now well established, namely, that the family is not a uniform environment for offspring, and differences in within-family experiences cause siblings to diverge in a multiplicity of ways.
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