A multilevel approach to the relationship between birth order and intelligence

Many studies show relationships between birth order and intelligence but use cross-sectional designs or manifest other threats to internal validity. Multilevel analyses with a control variable show that when these threats are removed, two major results emerge: (a) birth order has no significant influence on children's intelligence and (b) earlier reported birth order effects on intelligence are attributable to factors that vary between, not within, families. Analyses on 7- to 8 - and 13- to 14-year-old children from the National Longitudinal Survey of Youth support these conclusions. When hierarchical data structures, age variance of children, and within-family versus between-family variance sources are taken into account, previous research is seen in a new light.     

The family dynamics of intellectual development.

Birth order effects on intellectual performance show both positive and negative results. The confluence model reconciles these conflicting data by proving that these effects interact with the age of participants at testing, such that young children should show negative or no effects, whereas older individuals (past age 11 +/- 2 years) should show positive effects. Birth order studies strongly support this prediction. Some writers have claimed the apparent relation between birth order and intelligence is an artifact created by applying a cross-sectional analysis to data that should have been analyzed by comparing siblings within families. However, if siblings within the same family are compared all at the same time, their ages are necessarily different. As a result, birth order effects are confounded with age effects. Moreover, within-family data conceal patterns of aggregate effects that cross-sectional data reveal.     

What causes birth order-intelligence patterns? The admixture hypothesis, revived.

Recent evidence shows that the relation between birth order and intelligence is not the same in cross-sectional and within-family data. This simple empirical observation invalidates the conclusions from hundreds of previous birth order studies that relied on cross-sectional data. Simultaneously, the empirical foundation disappears from underneath theories like dilution and the confluence model that use explanatory processes occurring within the family. A theory proposed almost 25 years ago--the admixture hypothesis--effectively accounts for these empirical patterns. In this article, the author describes why birth order is of such intense interest to both parents and researchers (the birth order trap), discusses past birth order-intelligence patterns, shows that the admixture hypothesis accounts for those patterns, and reframes the original argument to support future productive research efforts.     

Intelligence, birth order, and family size

The analysis of the National Child Development Study in the United Kingdom (n = 17,419) replicates some earlier findings and shows that genuine within-family data are not necessary to make the apparent birth-order effect on intelligence disappear. Birth order is not associated with intelligence in between-family data once the number of siblings is statistically controlled. The analyses support the admixture hypothesis, which avers that the apparent birth-order effect on intelligence is an artifact of family size, and cast doubt on the confluence and resource dilution models, both of which claim that birth order has a causal influence on children's cognitive development. The analyses suggest that birth order has no genuine causal effect on general intelligence.     

Personality and life satisfaction in China: The birth order effect under the influence of national policy

Individuals’ development is a multilayered affair. The influence of family relationship on personality, such as  and  focusing on birth order, is subject to influence from other social systems in which the families are situated. The current research examined the relation of birth order to personality and life satisfaction in China, where only children have become the majority because of national policy. Across two studies with both between-family data (N = 1468) and within-family data (N = 171), onlyborns and laterborns surpassed firstborns on openness to experience. In addition, only-child participants were more satisfied with their own lives than were sibling participants, especially laterborns. The results offer new insights into the dynamic relations between ecology and personality.     

Birth order has no effect on intelligence: a reply and extension of previous findings

We address points raised by Zajonc and Sulloway, who reject findings showing that birth order has no effect on intelligence. Many objections to findings of null birth-order results seem to stem from a misunderstanding of the difference between study designs where birth order is confounded with true causal influences on intelligence across families and designs that control for some of these influences. We discuss some of the consequences of not appreciating the nature of this difference. When between-family confounds are controlled using appropriate study designs and techniques such as multilevel modeling, birth order is shown not to influence intelligence. We conclude with an empirical investigation of the replicability and generalizability of this approach.     

Between- versus within-family factor analyses of cognitive abilities

Confirmatory factor analyses of the 15 cognitive abilities tests from the Hawaii Family Study of Cognition were conducted on between-family (BF) means and within-family (WF) differences for 370 AEA (Americans of European ancestry) and 116 AJA (Americans of Japanese ancestry) sibling pairs. Difference chi-square significance tests, goodness-of-fit indices, and congruence coefficients for the estimated loadings on four specific cognitive abilities factors and on the second-order general intelligence factor nearly all indicated that the between-family factor structures were not significantly different from the within-family structures for both AEA and AJA siblings (the AEA and AJA structures were also not significantly different). The similarity of the BF and WF structures suggests that the genetic and environmental influences underlying cognitive abilities are “intrinsic” in nature, that is, not just due to between-family differences in culture, status, values, and fortuitous cross-assortative mating.     

Testing between-family associations in within-family comparisons

Using behaviorally discordant siblings to test for gene-behavior associations is a common tool in molecular genetics, because the within-family contrast offers a research design that avoids confounds inevitable in all between-family comparisons of unrelated individuals. We propose a similar strategy to assess the behavior-behavior associations on which much of psychological science is built. Between-family correlations of personality test scores (e.g., sensation seeking) and behavioral outcomes (e.g., substance use) may be mediated by variables that differ between families (e.g., social class or religiosity) and correlate with both personality and outcome. Contrasting twin and nontwin siblings who were highly discordant for behavioral correlates of substance use, we tested whether between-family behavioral correlations replicated within families. Some, but not all, did. Within-family analyses of behaviorally discordant siblings may find wide application in efforts to clarify the meaning of correlational research data.     

Birth order differences in normal personality traits: Perspectives from within and outside the family

The hypotheses of Sulloway (1996) regarding birth order differences in five-factor model personality traits were tested in a sample of 231 college students with the NEO Five-Factor Inventory (NEO-FFI; Costa & McCrae, 1992). Data were collected from three sources (self, peer, and parent) to systematically evaluate previous observations that birth order differences are more commonly found when ratings are obtained from family members than from observers outside the family (Ernst & Angst, 1983). Using a between-family design, students were selected only from families with two or three full biological siblings and no half-siblings, step-siblings, or adopted siblings. Firstborn (n = 103) and laterborn (n = 128) students were compared using NEO-FFI ratings by the self, by a same-sex college peer, and by a biological parent. No birth order differences were found for any of the five NEO-FFI scores using any of the three rating sources. Effect sizes (Cohen’s d) were less than .20 for all comparisons. Suggestions for new approaches to the study of birth order differences are considered, including the use of within-family designs and more contextualized personality variables.     

The Role of Harsh Discipline in Explaining Sex Differences in Conduct Disorder: a Study of Opposite-Sex Twin Pairs

In the current study, two hypotheses about the role of harsh discipline (HD) in explaining the sex difference in the prevalence of conduct disorder (CD) were evaluated: that boys exhibit more CD than girls because (1) they are exposed to more HD and/or (2) there is a greater association between HD and CD in boys. These hypotheses were evaluated in a sample of male and female adult twins from different families (N = 3,502) as well as a sample of adult twin brothers and sisters (N = 655) in order to examine the extent to which sex differences remained after controlling for between-family differences. Retrospective reports of HD experienced between ages 6–13 and DSM-IV CD symptoms experienced before age 18 were obtained via structured psychiatric telephone interviews. Boys reported higher mean levels of HD and CD than girls, both between and within families, and the results of model-fitting analyses suggested that differences in the use of harsh disciplinary practices for sons versus daughters may partially explain the sex difference in the prevalence of CD. Between families, the relation between HD and CD was greater for girls than boys, but within families, there was no evidence of a sex difference in the relation between HD and CD. Inconsistent between-family and within-family results suggest that factors that differ between families are confounded with sex differences in the relation between HD and CD. A more stringent test of sex differences involves eliminating these between-family differences by studying boys and girls within the same family.     

The confluence model: birth order as a within-family or between-family dynamic?

The confluence model explains birth-order differences in intellectual performance by quantifying the changing dynamics within the family. Wichman, Rodgers, and MacCallum (2006) claimed that these differences are a between-family phenomenon--and hence are not directly related to birth order itself. The study design and analyses presented by Wichman et al. nevertheless suffer from crucial shortcomings, including their use of unfocused tests, which cause statistically significant trends to be overlooked. In addition, Wichman et al. treated birth-order effects as a linear phenomenon thereby ignoring the confluence model's prediction that these two samples may manifest opposing results based on age. This article cites between- and within-family data that demonstrate systematic birth-order effects as predicted by the confluence model. The corpus of evidence invoked here offers strong support for the assumption of the confluence model that birth-order differences in intellectual performance are primarily a within-family phenomenon.     

The associations of birth order with personality and intelligence in a representative sample of U.S. high school students

We tested birth order associations with personality traits and intelligence using Project Talent, a representative sample (N = 377,000) of U.S. high school students. Using a between-family design and several background factors (i.e., age, sex, sibship size, parental socio-economic status, and family structure), we were able to control for potential confounds, and estimate the links between birth order and outcomes across several different social categories. In addition to differences between firstborns and laterborns across the entire sample, we also tested birth rank trends in a sub-sample of targets from sibships of three, raised by two parents. Overall, the average absolute association between birth order and personality traits was .02, whereas the one between birth order and intelligence was .04.     

Implications of Family Cohesion and Conflict for Adolescent Mood and Well-Being: Examining Within- and Between-Family Processes on a Daily Timescale

Family-level conflict and cohesion are well-established predictors of adolescent mental health. However, traditional approaches focusing on between-family differences in cohesion and conflict may overlook daily intrafamily variability that might provide important new information. We used data from a 21-day daily diary protocol in a sample of 151 caregivers (95.3% female) and their adolescent child (61.5% female) in two-caregiver families to test whether daily changes in family functioning are associated with daily changes in adolescent well-being and whether adolescent well-being depends on average levels of family functioning. We examined family cohesion and conflict in relation to adolescent angry, depressed, and anxious mood, as well as happiness, life satisfaction, and meaning and purpose in life in multilevel models. Both cohesion and conflict exhibited meaningful daily variation. Adolescent-reported cohesion and conflict had unique within-family associations with all six adolescent outcomes. Models using parent reports of family functioning yielded fewer associations than models with adolescent reports; however, several findings remained. Cross-level interactions indicated that within-family variations in cohesion were only associated with adolescent depression in families with lower average levels of cohesion across days. In sum, this study provides compelling evidence that families exhibit meaningful variability from day to day and that daily variation has important implications for adolescent well-being.     

Intergenerational relationships and marriage in China: Within-family longitudinal associations and between-family differences

Marriage is embedded in the web of spouses' broader social ties, and relationship quality with parents and parents-in-law is associated with marital quality. Guided by Family Systems theory and using three waves of dyadic data from 268 Chinese different-sex couples across the first several years of marriage, we first conducted a Random-Intercept Actor-Partner Interdependence Cross-lagged Panel Model (RI-APIM-CLPM) to examine the within-family longitudinal associations among husbands' and wives' relationship quality with parents, parents-in-law, and spouse. Then, husbands' and wives' filial obligations were added as predictors of between-family differences in their own and their partner's relationship quality in the three social ties. Among husbands, increased relationship quality in one social tie (e.g., with parents) predicted reductions in relationship quality in the other social ties (e.g., relationships with parents-in-law and marital quality). Our examination of between-family differences demonstrated that high levels of filial obligations predicted higher intergenerational relationship quality and marital quality. By simultaneously considering the within-family associations of multiple social ties and how filial obligations account for between-family differences in relationship quality, we contribute to a nuanced understanding of how Chinese couples' romantic partnerships are embedded in their broader family system.     

Pubertal timing and substance use: associations between and within families across late adolescence

In the present study, between-family analyses of data from adolescent twin girls offer new evidence that early menarche is associated with earlier initiation and greater frequency of smoking and drinking. The role of personality factors and peer relationships in that association was investigated, and little support was found for their involvement. Novel within-family analyses replicating associations of substance use with pubertal timing in contrasts of twin sisters selected for extreme discordance for age at menarche are reported. Within-family replications demonstrated that the association of pubertal timing with substance use cannot be explained solely by between-family confounds. Within-family analyses demonstrated contextual modulation of the influence of pubertal timing: Its impact on drinking frequency is apparent only among girls in urban settings. Sibling comparisons illustrate a promising analytic tool for studying diverse developmental outcomes.     

Birth Order Effects on Personalityand Achievement Within Families

We investigated birth order effects on personality and achievement in four studies (N = 1,022 families) including both student and adult samples. Control over a wide range of variables was effected by collecting within-family data: Participants compared their siblings (and themselves) on a variety of personality and achievement dimensions. Across four diverse data sets, first-borns were nominated as most achieving and most conscientious. Later-borns were nominated as most rebellious, liberal, and agreeable. The same results obtained whether or not birth order was made salient (to activate stereotypes) during the personality ratings. Overall, the results support predictions from Sulloway's niche model of personality development, as well as Zajonc's confluence model of intellectual achievement.     

Did Mom Really Love You Best? Developmental Histories,Status Transitions,and Parental Favoritism in Later Life Families

In this paper, we draw upon insights from the developmental psychological literature on younger families to investigate within-family differences in parent–adult child relations in later life families. Using data from 30 families, we examine whether mothers aged 65–75 report greater closeness to particular adult children, and the extent to which patterns of closeness can be explained by within-family differences in the children's status transitions and developmental histories. Eighty percent of the mothers reported being closer to at least one of their adult children. Further, mothers were more likely to report that the children to whom they were closest had experienced nonnormative status transitions or other problematic events that had been involuntary and had made the children more dependent on their mothers. In contrast, mothers were least likely to name children who experienced voluntary problematic events. Comparison of the data from mothers with that from adult children revealed substantial discrepancies in reporting developmental histories, thus demonstrating the limitations imposed by collecting data from only one generation. Taken together, these findings suggest that examining within-family variations in social structural transitions and developmental histories, in combination with collecting data from multiple generations, may shed light on the quality of intergenerational relations.     

Twin-singleton differences in intelligence: a meta-analysis

Since the emergence of twin studies in the 1920s, time and again the question of possible twin-singleton differences in intelligence has been posed. This study addressed the issue through a meta-analysis of published studies on this theme. Twins on the average seem to have lower IQs than singletons. The best estimate for this group difference is 4.2 IQ points (less than one-third of a standard deviation), with a great divide between study outcomes of less vs more recent birth cohorts (5.1 vs 0.5 IQ points, respectively). The evidence is based on studies from six countries (including population-based ones of entire birth cohorts), a massive database (comparisons of more than 30,000 twins with nearly 1.6 million singletons), a variety of intelligence tests, and birth cohorts spanning most of the 20th century, but, for the most part, on an age range limited to children and adolescents. The effect shows considerable between-study heterogeneity but appears robust (fail-safe N calculations), not due to influential individual studies (sensitivity analysis) or publication bias, was present since the very first published studies (cumulative meta-analysis), and appears generalizable across sex, zygosity, and other intelligence domains beyond the verbal. There are insufficient data as to whether the effect persists over the lifespan, exists as well within families, or has ceased in recent birth cohorts of highly developed countries (Denmark and The Netherlands). Likely causes of the effect appear to be prenatal and perinatal factors (reduced fetal growth and shorter gestation for twins).     

Intelligence and executive functioning in adult age: Effects of sibship size and birth order

Several studies have demonstrated that social influences from having been brought up in a family with few siblings and early in birth order result in higher scores on intelligence tests in childhood and adolescence as compared to having been brought up in large sibships. The present study examined whether influences of such social factors would have long-lasting effects on intelligence (block design and word comprehension) and executive functions (working memory and verbal fluency) in adulthood and old age, i.e., long after the individuals had moved out of the family structure they were born in. After having controlled for socioeconomic status and a variety of health conditions affecting cognition in adult life, a sibship size effect was demonstrated for executive functions but not for intelligence. The social influences of birth order affected only executive functions and working memory in particular; earlier born individuals performed better than later born individuals in tests assessing executive function but not in tests assessing intelligence. Implications for the relationship between executive functioning and intelligence, and implications for the Confluence and Resource Dilution Models (Blake, 1981; Downey, 1995, 2001; Zajonc, 1976) are discussed.     

The influence of rearing order on personality development within two adoption cohorts

There is an extensive literature on the relationship between birth order and psychological traits, but no previous study has investigated the influence of ordinal position on personality development within adoptive siblings. Such a design is important because it effectively separates the effects of biological birth order and rearing order. Here we report data from two adoption cohorts in which subjects were biological first-borns reared in various ordinal positions. Data were analyzed with reference to Sulloway's (1996) evolutionarily based sibling rivalry theory of birth order effects. Between- and within-family analyses indicated that rearing order's influence on personality was very weak. The only clear difference was for conscientiousness, on which first-reared siblings scored higher. We draw possible implications for Sulloway's theory and speculate upon an alternative, prenatal biological process that may produce birth order differences.