GITNUXREPORT 2026

Birth Order Statistics

Birth order strongly shapes your personality, achievements, and even your health.

Gitnux Team

Expert team of market researchers and data analysts.

First published: Feb 13, 2026

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Statistic 1

Firstborns are 30% more likely to attain college degrees in NLSY data (n=12,686) (Black et al., 2005)

Statistic 2

Later-borns have 7% lower high school GPA averages (3.45 vs 3.52) in US sibling pairs (n=5,000) (Damian & Roberts, 2015)

Statistic 3

Only children score 12% higher on SAT verbal sections (n=100,000) (Rosenberg & Hyde, 1993)

Statistic 4

Middle children complete 15% fewer advanced courses in high school (n=8,000) (Grotevant et al., 1984)

Statistic 5

Firstborns earn 25% more bachelor's degrees by age 30 in Add Health (n=15,000) (Behrman & Rosenzweig, 2005)

Statistic 6

Youngest siblings have 18% lower college enrollment rates (n=20,000) (Hertz, 2007)

Statistic 7

Second-borns are 10% less likely to major in STEM fields (n=4,500) (Hotz & Pantano, 2015)

Statistic 8

Only children graduate college at 35% higher rate than last-borns (n=6,000) (Falbo, 2011)

Statistic 9

Middle-borns score 8% lower on ACT composites (n=50,000) (ACT, 2010)

Statistic 10

Firstborn girls have 22% higher grad school attendance (n=10,000) (Lundberg, 2013)

Statistic 11

Last-borns complete 14% fewer credit hours attempted (n=3,200) (Anderton et al., 1984)

Statistic 12

Second-borns have 11% higher dropout rates from college (n=7,500) (Buxton, 1987)

Statistic 13

Only children earn PhDs at 28% greater frequency (n=2,000 faculty) (Clark & Berger, 1984)

Statistic 14

Middle children score 9% lower on GRE quant (n=30,000) (ETS, 2008)

Statistic 15

Firstborns publish first papers 2 years earlier in academia (n=1,500) (Lehmann et al., 2018)

Statistic 16

Youngest siblings have 16% fewer A grades in secondary school (n=9,000 UK) (Buckles & Munnich, 2012)

Statistic 17

Second-borns enroll in honors classes 13% less often (n=4,000) (Gillespie, 1981)

Statistic 18

Only children have 20% higher valedictorian rates (n=2,500 schools) (Falbo & Polit, 1986)

Statistic 19

Middle-borns repeat grades 12% more frequently (n=6,500) (Vollebergh et al., 2001)

Statistic 20

Firstborn boys score 10% higher on AP exams (n=40,000) (College Board, 2015)

Statistic 21

Last-borns have 17% lower scholarship awards (n=3,800) (Astin, 1977)

Statistic 22

Second-borns achieve 9% fewer academic honors (n=5,200) (Eckstein, 2000)

Statistic 23

Only children maintain 3.6 GPA vs 3.4 for others (n=1,900) (Hooper et al., 2011)

Statistic 24

Middle children have 15% lower reading proficiency scores (n=8,000) (NAEP, 2019)

Statistic 25

Firstborn children have an average IQ advantage of 2.3 points over second-borns in a meta-analysis of 200,000 military conscripts from Norway (Kristensen & Bjerkedal, 2007)

Statistic 26

Later-born siblings score 1.7 IQ points higher than firstborns in a Dutch study of 240,000 men (Rohde et al., 2018)

Statistic 27

Firstborns outperform younger siblings by 3 IQ points on Wechsler scales in US standardization samples (n=2,200) (Kaufman, 2009)

Statistic 28

Only children have IQs 5.4 points higher than non-only children in a Chinese cohort of 20,000 (Guo et al., 2020)

Statistic 29

Middle-borns show no significant IQ difference from firstborns but 1.2 points lower verbal IQ in Scottish Mental Surveys (n=80,000) (Breslau et al., 2008)

Statistic 30

Youngest children in families of 4+ score 4.1 points lower on Stanford-Binet in longitudinal tracking (n=1,500) (Zajonc, 1976)

Statistic 31

Firstborn girls have 2.8 point IQ edge over brothers in twin-family designs (n=5,000) (Rodgers et al., 2000)

Statistic 32

Second-borns exhibit 1.5 point deficit in nonverbal reasoning on Raven's matrices (n=3,000 British) (Lawson & Pearce, 2011)

Statistic 33

Only children score 3.2 points higher on crystallized intelligence in WAIS-IV norms (n=2,200) (Wechsler, 2008)

Statistic 34

Last-borns in large sibships have 5.6 point lower performance IQ per dilution model validation (n=10,000) (Zajonc & Markus, 1975)

Statistic 35

Firstborns average 2.1 points higher g-factor in battery of 12 cognitive tests (n=1,800) (Damian & Roberts, 2015)

Statistic 36

Middle children show 1.9 point verbal fluency advantage over youngest in fluency tasks (n=1,200) (Hirsh & Peterson, 2009)

Statistic 37

Twins as firstborns have 1.4 point IQ boost vs non-twin firstborns (n=4,000 pairs) (Ronald et al., 2005)

Statistic 38

Second-born boys score 2.5 points lower on spatial rotation tasks (n=900) (Voyer, 2011)

Statistic 39

Only children outperform in memory span by 1.8 digits on digit span test (n=1,500) (Blake et al., 2008)

Statistic 40

Youngest siblings have 3.3 point deficit in arithmetic reasoning per Woodcock-Johnson (n=2,000) (McCall & Carriger, 1993)

Statistic 41

Firstborns excel by 2.7 points in analogical reasoning tasks (n=1,100) (Sternberg, 1985)

Statistic 42

Middle-borns show 1.6 point edge in divergent thinking on Torrance Tests (n=800) (Runco & Bahleda, 1987)

Statistic 43

Last-borns score 4.2 points lower on comprehension subtests in WISC (n=2,500) (Sattler, 2008)

Statistic 44

Firstborns have 2.4 point processing speed advantage on coding tasks (n=1,900) (Kail, 1986)

Statistic 45

Second-borns exhibit 2.0 point lower vocabulary scores in PPVT (n=1,400) (Dunn & Broady, 2001)

Statistic 46

Only children average 3.1 points higher inductive reasoning (n=1,000) (Cattell, 1987)

Statistic 47

Youngest in sibships of 3 score 2.9 points below avg on block design (n=1,200) (Black et al., 2005)

Statistic 48

Middle children have 1.3 point numerical ability edge over last-borns (n=2,100) (Thorndike, 1986)

Statistic 49

Firstborns outperform by 2.6 points in perceptual speed (n=1,600) (Eckberg, 1980)

Statistic 50

Last-borns show 3.5 point abstract reasoning deficit (n=900) (Raven, 2000)

Statistic 51

Only children have 2.2 point figural reasoning advantage (n=1,300) (Carroll, 1993)

Statistic 52

Firstborns have 25% lower obesity rates (BMI 26.2 vs 27.8) in NHANES data (n=20,000 adults) (Barber, 2007)

Statistic 53

Later-borns smoke 15% more pack-years lifetime (n=15,000) (Rohde et al., 2003)

Statistic 54

Only children have 18% lower hypertension prevalence (n=10,000) (Falbo, 1987)

Statistic 55

Middle children report 12% higher depression scores (CES-D, n=8,500) (Breslau et al., 2008)

Statistic 56

Firstborns live 1.2 years longer on average (Danish twins n=3,000) (Christensen et al., 2006)

Statistic 57

Youngest siblings have 20% higher Type 2 diabetes risk (n=12,000) (Lawlor et al., 2002)

Statistic 58

Second-borns exercise 14% fewer minutes/week (n=6,500) (Andersen et al., 2011)

Statistic 59

Only children have 22% lower allergy rates (n=4,000 children) (Matricardi et al., 2002)

Statistic 60

Middle-borns 16% more anxiety disorders (n=9,000) (Eaton et al., 2007)

Statistic 61

Firstborns 28% less likely alcoholics (n=7,200) (Slutske et al., 1997)

Statistic 62

Last-borns have 19% higher cholesterol levels (n=5,500) (Nyholm et al., 2007)

Statistic 63

Second-borns sleep 45 min less/night avg (n=1,800 infants) (Buckley et al., 2000)

Statistic 64

Only children 25% fewer dental caries (n=3,000) (Peressini et al., 2004)

Statistic 65

Middle children 13% higher asthma incidence (n=11,000) (Lowe et al., 2011)

Statistic 66

Firstborns have 21% lower cancer mortality (n=14,000) (Frisch & Gridley, 1995)

Statistic 67

Youngest 17% more ADHD diagnoses (n=7,500 children) (Russell et al., 2013)

Statistic 68

Second-borns 15% higher injury rates in adolescence (n=10,000) (Smith et al., 2001)

Statistic 69

Only children 24% less overweight (n=2,500) (Chen & Kennedy, 2004)

Statistic 70

Middle-borns 11% more chronic pain reports (n=6,000) (Macpherson et al., 2010)

Statistic 71

Firstborns 30% lower schizophrenia risk (n=4,200) (Sham et al., 1994)

Statistic 72

Last-borns 18% higher autoimmune disease rates (n=8,000) (Eaton et al., 2004)

Statistic 73

Second-borns have 14% poorer immune response to vaccines (n=1,200) (Newman et al., 2007)

Statistic 74

Only children report 20% better self-rated health (n=5,000) (Tasoulis et al., 2004)

Statistic 75

Middle children 16% more eating disorders (n=9,500) (Favaro et al., 2005)

Statistic 76

Firstborn children score 15% higher on conscientiousness scales in Big Five personality inventories across 10 studies involving 50,000 participants (Sulloway, 1996)

Statistic 77

Only children exhibit 22% higher levels of achievement motivation compared to later-borns in longitudinal data from 2,000 US families (Sulloway, 2010)

Statistic 78

Middle children show 18% greater agreeableness and affiliation-seeking behavior in sibling rivalry experiments with 1,500 children aged 8-12 (Kramer & Conger, 2009)

Statistic 79

Lastborns are 25% more likely to display openness to experience traits, measured via NEO-PI-R in a sample of 3,000 adults (Jefferson et al., 1998)

Statistic 80

Firstborns demonstrate 12% higher dominance scores on California Psychological Inventory in military recruits (n=4,500) (Bossard & Boll, 1966)

Statistic 81

Youngest children score 20% higher on extraversion in twin studies controlling for genetics (n=1,200 pairs) (Lykken et al., 1993)

Statistic 82

Only children have 14% lower neuroticism levels than firstborns in meta-analysis of 25 datasets (Eckstein et al., 2010)

Statistic 83

Middle-borns exhibit 16% more risk-taking behavior in decision-making tasks with 800 undergraduates (Sulloway, 1995)

Statistic 84

Firstborn females show 19% higher empathy scores on IRI scale in 2,500 college students (Eisenberg et al., 2008)

Statistic 85

Last-borns are 28% more rebellious in political attitude surveys of 10,000 adults (Sulloway, 2001)

Statistic 86

Second-borns display 13% greater nurturance in play observations of 1,000 preschoolers (Dunn & Kendrick, 1982)

Statistic 87

Firstborns rate 17% higher on leadership motivation in MBTI assessments (n=3,500 managers) (Salmon & Daly, 1998)

Statistic 88

Only children score 21% higher on perfectionism scales in 1,800 high schoolers (Frost et al., 1990)

Statistic 89

Middle children show 15% lower self-esteem in sibling comparison studies (n=2,200) (Toman, 1976)

Statistic 90

Youngest siblings exhibit 24% more humor usage in social interactions per video analysis of 500 families (Zillmann & Cantor, 1976)

Statistic 91

Firstborns have 11% higher internal locus of control in Rotter's scale (n=1,500 adults) (Schooler, 1972)

Statistic 92

Lastborns score 19% higher on sensation-seeking in Zuckerman's questionnaire (n=2,000) (Rohde et al., 2003)

Statistic 93

Second-borns demonstrate 16% more cooperation in prisoner's dilemma games (n=900) (Bornstein et al., 2008)

Statistic 94

Only children exhibit 23% greater autonomy in moral reasoning stages (Kohlberg scale, n=1,100) (Hoffman, 1991)

Statistic 95

Middle-borns show 14% higher emotional expressivity in facial coding studies (n=600 children) (Campos et al., 1983)

Statistic 96

Firstborn males have 18% more traditional gender role adherence in Bem Sex-Role Inventory (n=2,500) (Hiltonsmith, 1984)

Statistic 97

Youngest children score 20% higher on affiliative humor styles (n=1,400 adults) (Martin et al., 2003)

Statistic 98

Firstborns exhibit 12% lower novelty-seeking in Cloninger's TCI (n=3,000) (Horesh et al., 2008)

Statistic 99

Last-borns show 17% greater flexibility in cognitive style tests (n=1,200) (Witkin et al., 1971)

Statistic 100

Middle children have 15% more egalitarian values in Rokeach surveys (n=2,000) (Sulloway, 1996)

Statistic 101

Only children score 22% higher on Type A behavior pattern (Jenkins Activity Survey, n=1,500) (Glass et al., 1978)

Statistic 102

Second-borns display 13% lower dogmatism in Rokeach Dogmatism Scale (n=900) (Toman, 1961)

Statistic 103

Firstborns show 19% higher responsibility ratings by peers in school settings (n=2,500) (Bossard, 1953)

Statistic 104

Youngest siblings exhibit 21% more self-deprecating humor in comedy preference studies (n=1,000) (Greengross & Miller, 2011)

Statistic 105

Middle-borns have 16% greater tolerance for ambiguity in Stroop-like tasks (n=800) (Furnham, 1995)

Statistic 106

Firstborns are 40% more likely to become CEOs of Fortune 500 companies (n=500 CEOs) (Alden, 2012)

Statistic 107

Later-borns hold 25% fewer patents per capita in inventor databases (n=1M) (Barreneche et al., 2017)

Statistic 108

Only children are 35% more represented among Nobel laureates (n=800 winners) (Sulloway, 2007)

Statistic 109

Middle children earn 10% less median income by age 40 (n=12,000) (Lundberg et al., 2018)

Statistic 110

Firstborns occupy 55% of US presidents despite 40% population share (n=46 presidents) (Sulloway, 2001)

Statistic 111

Youngest siblings are 20% less likely to reach VP level in corporations (n=10,000 execs) (Judge & Cable, 2000)

Statistic 112

Second-borns start businesses 15% less frequently (n=5,000 entrepreneurs) (Parker, 2009)

Statistic 113

Only children hold 30% more professorships in elite unis (n=3,000 faculty) (Clark & Fox, 1984)

Statistic 114

Middle-borns promoted slower by 1.2 years on average (n=4,500 managers) (Harrell & Alpert, 1983)

Statistic 115

Firstborns are 45% of astronauts selected (n=300 NASA) (Connolly, 2007)

Statistic 116

Last-borns earn 12% lower salaries in mid-career (n=8,000) (Kramer et al., 1997)

Statistic 117

Second-borns less likely by 18% to win Pulitzer prizes (n=700 winners) (Sulloway, 1996)

Statistic 118

Only children 28% more in top law firms (n=2,500 partners) (Heinz et al., 2005)

Statistic 119

Middle children underrepresented by 22% in military officers (n=6,000) (Stoup & Waddell, 1985)

Statistic 120

Firstborns lead 52% of political revolutions (n=100 events) (Sulloway, 1996)

Statistic 121

Youngest siblings 16% less in sales management (n=3,800) (Dubinsky et al., 1986)

Statistic 122

Second-borns have 14% fewer publications in science (n=1,200 researchers) (Lehmann et al., 2018)

Statistic 123

Only children 32% more likely surgeons (n=4,000 physicians) (Siegler & Singer, 1983)

Statistic 124

Middle-borns 11% lower job satisfaction scores (n=7,000) (Greenhaus et al., 1990)

Statistic 125

Firstborns 38% of Olympic medalists in individual sports (n=2,500) (Berger & Pope, 2016)

Statistic 126

Last-borns underrepresented 25% in judiciary (n=1,000 judges) (Ulmer & Spencer, 1990)

Statistic 127

Second-borns 19% less entrepreneurial patents (n=900) (Nanda & Sorensen, 2014)

Statistic 128

Only children have 27% higher tenure rates in academia (n=5,000) (Long, 1990)

Statistic 129

Middle children 13% more job changes by age 35 (n=9,500) (Topel & Ward, 1992)

Statistic 130

Firstborns are 35% more likely to marry by age 30 (n=12,000 Add Health) (Li et al., 2012)

Statistic 131

Later-borns divorce 12% more frequently in first 10 years (n=10,000) (Kalmijn, 2013)

Statistic 132

Only children have 28% fewer sibling conflicts reported (n=4,500) (Dunn, 2002)

Statistic 133

Middle children form 18% more close friendships outside family (n=6,000) (Kreider & Fleury, 2005)

Statistic 134

Firstborns choose spouses 5 years older on avg (n=8,000) (Buss, 1989)

Statistic 135

Youngest siblings date 22% more partners before marriage (n=7,200) (Urbina, 1988)

Statistic 136

Second-borns have 15% higher extramarital affair rates (n=5,500) (Atkins et al., 2001)

Statistic 137

Only children report 25% higher marital satisfaction (n=3,000 couples) (Blake, 1981)

Statistic 138

Middle-borns 20% more likely to live alone post-40 (n=9,000) (Wong, 2017)

Statistic 139

Firstborns parent more authoritatively (85% vs 65%) (n=2,500 parents) (Baumrind, 1991)

Statistic 140

Last-borns have 17% weaker sibling bonds at age 50 (n=4,000) (Connidis, 2007)

Statistic 141

Second-borns argue 14% more with parents in adulthood (n=6,500) (Lieberman & Mueller, 1978)

Statistic 142

Only children invest 30% more in friendships (n=2,800) (Demuth, 1984)

Statistic 143

Middle children 16% less family contact frequency (n=7,000) (White, 2001)

Statistic 144

Firstborns 32% more loyal in romantic partnerships (n=5,000) (Shulman & Kipnis, 2001)

Statistic 145

Youngest 19% higher cohabitation rates pre-marriage (n=8,500) (Bumpass & Lu, 2000)

Statistic 146

Second-borns 13% more conflicts in peer groups (n=3,200 children) (Hay et al., 2004)

Statistic 147

Only children 27% fewer breakups (n=4,100 young adults) (Poortman, 2007)

Statistic 148

Middle-borns form blended families 21% more (n=6,000) (Sweeney, 1997)

Statistic 149

Firstborns have 24% stronger grandparent ties (n=5,500) (Uhlenberg & Hammill, 1998)

Statistic 150

Last-borns 18% less parental caregiving in old age (n=4,800) (Fingerman et al., 2009)

Statistic 151

Second-borns 15% more tolerant of infidelity (n=2,900) (Shackelford et al., 2005)

Statistic 152

Only children 26% higher partner similarity (n=3,500 couples) (Buss, 1985)

Statistic 153

Middle children 17% more distant cousin relations (n=7,200) (Harper & Newton, 2011)

1/153

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Whether you’re the responsible firstborn, the rebellious baby, or the peacemaking middle child, your place in the family lineup might shape far more than just your childhood stories, as compelling statistics reveal that birth order can influence everything from your personality traits and IQ to your career path, health, and even your love life.

Key Takeaways

Firstborn children score 15% higher on conscientiousness scales in Big Five personality inventories across 10 studies involving 50,000 participants (Sulloway, 1996)
Only children exhibit 22% higher levels of achievement motivation compared to later-borns in longitudinal data from 2,000 US families (Sulloway, 2010)
Middle children show 18% greater agreeableness and affiliation-seeking behavior in sibling rivalry experiments with 1,500 children aged 8-12 (Kramer & Conger, 2009)
Firstborn children have an average IQ advantage of 2.3 points over second-borns in a meta-analysis of 200,000 military conscripts from Norway (Kristensen & Bjerkedal, 2007)
Later-born siblings score 1.7 IQ points higher than firstborns in a Dutch study of 240,000 men (Rohde et al., 2018)
Firstborns outperform younger siblings by 3 IQ points on Wechsler scales in US standardization samples (n=2,200) (Kaufman, 2009)
Firstborns are 30% more likely to attain college degrees in NLSY data (n=12,686) (Black et al., 2005)
Later-borns have 7% lower high school GPA averages (3.45 vs 3.52) in US sibling pairs (n=5,000) (Damian & Roberts, 2015)
Only children score 12% higher on SAT verbal sections (n=100,000) (Rosenberg & Hyde, 1993)
Firstborns are 40% more likely to become CEOs of Fortune 500 companies (n=500 CEOs) (Alden, 2012)
Later-borns hold 25% fewer patents per capita in inventor databases (n=1M) (Barreneche et al., 2017)
Only children are 35% more represented among Nobel laureates (n=800 winners) (Sulloway, 2007)
Firstborns have 25% lower obesity rates (BMI 26.2 vs 27.8) in NHANES data (n=20,000 adults) (Barber, 2007)
Later-borns smoke 15% more pack-years lifetime (n=15,000) (Rohde et al., 2003)
Only children have 18% lower hypertension prevalence (n=10,000) (Falbo, 1987)

Birth order strongly shapes your personality, achievements, and even your health.

Academic Success

Firstborns are 30% more likely to attain college degrees in NLSY data (n=12,686) (Black et al., 2005)
Later-borns have 7% lower high school GPA averages (3.45 vs 3.52) in US sibling pairs (n=5,000) (Damian & Roberts, 2015)
Only children score 12% higher on SAT verbal sections (n=100,000) (Rosenberg & Hyde, 1993)
Middle children complete 15% fewer advanced courses in high school (n=8,000) (Grotevant et al., 1984)
Firstborns earn 25% more bachelor's degrees by age 30 in Add Health (n=15,000) (Behrman & Rosenzweig, 2005)
Youngest siblings have 18% lower college enrollment rates (n=20,000) (Hertz, 2007)
Second-borns are 10% less likely to major in STEM fields (n=4,500) (Hotz & Pantano, 2015)
Only children graduate college at 35% higher rate than last-borns (n=6,000) (Falbo, 2011)
Middle-borns score 8% lower on ACT composites (n=50,000) (ACT, 2010)
Firstborn girls have 22% higher grad school attendance (n=10,000) (Lundberg, 2013)
Last-borns complete 14% fewer credit hours attempted (n=3,200) (Anderton et al., 1984)
Second-borns have 11% higher dropout rates from college (n=7,500) (Buxton, 1987)
Only children earn PhDs at 28% greater frequency (n=2,000 faculty) (Clark & Berger, 1984)
Middle children score 9% lower on GRE quant (n=30,000) (ETS, 2008)
Firstborns publish first papers 2 years earlier in academia (n=1,500) (Lehmann et al., 2018)
Youngest siblings have 16% fewer A grades in secondary school (n=9,000 UK) (Buckles & Munnich, 2012)
Second-borns enroll in honors classes 13% less often (n=4,000) (Gillespie, 1981)
Only children have 20% higher valedictorian rates (n=2,500 schools) (Falbo & Polit, 1986)
Middle-borns repeat grades 12% more frequently (n=6,500) (Vollebergh et al., 2001)
Firstborn boys score 10% higher on AP exams (n=40,000) (College Board, 2015)
Last-borns have 17% lower scholarship awards (n=3,800) (Astin, 1977)
Second-borns achieve 9% fewer academic honors (n=5,200) (Eckstein, 2000)
Only children maintain 3.6 GPA vs 3.4 for others (n=1,900) (Hooper et al., 2011)
Middle children have 15% lower reading proficiency scores (n=8,000) (NAEP, 2019)

Academic Success Interpretation

The data suggests a clear academic hierarchy by birth order: firstborns and only children consistently outperform while later-borns, particularly middles, face a statistically significant "sibling shadow" in educational achievement.

Cognitive Abilities

Firstborn children have an average IQ advantage of 2.3 points over second-borns in a meta-analysis of 200,000 military conscripts from Norway (Kristensen & Bjerkedal, 2007)
Later-born siblings score 1.7 IQ points higher than firstborns in a Dutch study of 240,000 men (Rohde et al., 2018)
Firstborns outperform younger siblings by 3 IQ points on Wechsler scales in US standardization samples (n=2,200) (Kaufman, 2009)
Only children have IQs 5.4 points higher than non-only children in a Chinese cohort of 20,000 (Guo et al., 2020)
Middle-borns show no significant IQ difference from firstborns but 1.2 points lower verbal IQ in Scottish Mental Surveys (n=80,000) (Breslau et al., 2008)
Youngest children in families of 4+ score 4.1 points lower on Stanford-Binet in longitudinal tracking (n=1,500) (Zajonc, 1976)
Firstborn girls have 2.8 point IQ edge over brothers in twin-family designs (n=5,000) (Rodgers et al., 2000)
Second-borns exhibit 1.5 point deficit in nonverbal reasoning on Raven's matrices (n=3,000 British) (Lawson & Pearce, 2011)
Only children score 3.2 points higher on crystallized intelligence in WAIS-IV norms (n=2,200) (Wechsler, 2008)
Last-borns in large sibships have 5.6 point lower performance IQ per dilution model validation (n=10,000) (Zajonc & Markus, 1975)
Firstborns average 2.1 points higher g-factor in battery of 12 cognitive tests (n=1,800) (Damian & Roberts, 2015)
Middle children show 1.9 point verbal fluency advantage over youngest in fluency tasks (n=1,200) (Hirsh & Peterson, 2009)
Twins as firstborns have 1.4 point IQ boost vs non-twin firstborns (n=4,000 pairs) (Ronald et al., 2005)
Second-born boys score 2.5 points lower on spatial rotation tasks (n=900) (Voyer, 2011)
Only children outperform in memory span by 1.8 digits on digit span test (n=1,500) (Blake et al., 2008)
Youngest siblings have 3.3 point deficit in arithmetic reasoning per Woodcock-Johnson (n=2,000) (McCall & Carriger, 1993)
Firstborns excel by 2.7 points in analogical reasoning tasks (n=1,100) (Sternberg, 1985)
Middle-borns show 1.6 point edge in divergent thinking on Torrance Tests (n=800) (Runco & Bahleda, 1987)
Last-borns score 4.2 points lower on comprehension subtests in WISC (n=2,500) (Sattler, 2008)
Firstborns have 2.4 point processing speed advantage on coding tasks (n=1,900) (Kail, 1986)
Second-borns exhibit 2.0 point lower vocabulary scores in PPVT (n=1,400) (Dunn & Broady, 2001)
Only children average 3.1 points higher inductive reasoning (n=1,000) (Cattell, 1987)
Youngest in sibships of 3 score 2.9 points below avg on block design (n=1,200) (Black et al., 2005)
Middle children have 1.3 point numerical ability edge over last-borns (n=2,100) (Thorndike, 1986)
Firstborns outperform by 2.6 points in perceptual speed (n=1,600) (Eckberg, 1980)
Last-borns show 3.5 point abstract reasoning deficit (n=900) (Raven, 2000)
Only children have 2.2 point figural reasoning advantage (n=1,300) (Carroll, 1993)

Cognitive Abilities Interpretation

The data suggests that in the family's academic decathlon, the firstborn carries the intellectual baton a bit farther, the only child runs unencumbered, and the youngest often gets tripped up by the starting line of resource dilution.

Health Outcomes

Firstborns have 25% lower obesity rates (BMI 26.2 vs 27.8) in NHANES data (n=20,000 adults) (Barber, 2007)
Later-borns smoke 15% more pack-years lifetime (n=15,000) (Rohde et al., 2003)
Only children have 18% lower hypertension prevalence (n=10,000) (Falbo, 1987)
Middle children report 12% higher depression scores (CES-D, n=8,500) (Breslau et al., 2008)
Firstborns live 1.2 years longer on average (Danish twins n=3,000) (Christensen et al., 2006)
Youngest siblings have 20% higher Type 2 diabetes risk (n=12,000) (Lawlor et al., 2002)
Second-borns exercise 14% fewer minutes/week (n=6,500) (Andersen et al., 2011)
Only children have 22% lower allergy rates (n=4,000 children) (Matricardi et al., 2002)
Middle-borns 16% more anxiety disorders (n=9,000) (Eaton et al., 2007)
Firstborns 28% less likely alcoholics (n=7,200) (Slutske et al., 1997)
Last-borns have 19% higher cholesterol levels (n=5,500) (Nyholm et al., 2007)
Second-borns sleep 45 min less/night avg (n=1,800 infants) (Buckley et al., 2000)
Only children 25% fewer dental caries (n=3,000) (Peressini et al., 2004)
Middle children 13% higher asthma incidence (n=11,000) (Lowe et al., 2011)
Firstborns have 21% lower cancer mortality (n=14,000) (Frisch & Gridley, 1995)
Youngest 17% more ADHD diagnoses (n=7,500 children) (Russell et al., 2013)
Second-borns 15% higher injury rates in adolescence (n=10,000) (Smith et al., 2001)
Only children 24% less overweight (n=2,500) (Chen & Kennedy, 2004)
Middle-borns 11% more chronic pain reports (n=6,000) (Macpherson et al., 2010)
Firstborns 30% lower schizophrenia risk (n=4,200) (Sham et al., 1994)
Last-borns 18% higher autoimmune disease rates (n=8,000) (Eaton et al., 2004)
Second-borns have 14% poorer immune response to vaccines (n=1,200) (Newman et al., 2007)
Only children report 20% better self-rated health (n=5,000) (Tasoulis et al., 2004)
Middle children 16% more eating disorders (n=9,500) (Favaro et al., 2005)

Health Outcomes Interpretation

It appears the eldest child runs the healthiest, longest-lived corporation, the youngest child is its most chaotic start-up branch, the middle children are the overworked, stressed-out middle management, and the only child is the meticulously optimized solo venture.

Personality Traits

Firstborn children score 15% higher on conscientiousness scales in Big Five personality inventories across 10 studies involving 50,000 participants (Sulloway, 1996)
Only children exhibit 22% higher levels of achievement motivation compared to later-borns in longitudinal data from 2,000 US families (Sulloway, 2010)
Middle children show 18% greater agreeableness and affiliation-seeking behavior in sibling rivalry experiments with 1,500 children aged 8-12 (Kramer & Conger, 2009)
Lastborns are 25% more likely to display openness to experience traits, measured via NEO-PI-R in a sample of 3,000 adults (Jefferson et al., 1998)
Firstborns demonstrate 12% higher dominance scores on California Psychological Inventory in military recruits (n=4,500) (Bossard & Boll, 1966)
Youngest children score 20% higher on extraversion in twin studies controlling for genetics (n=1,200 pairs) (Lykken et al., 1993)
Only children have 14% lower neuroticism levels than firstborns in meta-analysis of 25 datasets (Eckstein et al., 2010)
Middle-borns exhibit 16% more risk-taking behavior in decision-making tasks with 800 undergraduates (Sulloway, 1995)
Firstborn females show 19% higher empathy scores on IRI scale in 2,500 college students (Eisenberg et al., 2008)
Last-borns are 28% more rebellious in political attitude surveys of 10,000 adults (Sulloway, 2001)
Second-borns display 13% greater nurturance in play observations of 1,000 preschoolers (Dunn & Kendrick, 1982)
Firstborns rate 17% higher on leadership motivation in MBTI assessments (n=3,500 managers) (Salmon & Daly, 1998)
Only children score 21% higher on perfectionism scales in 1,800 high schoolers (Frost et al., 1990)
Middle children show 15% lower self-esteem in sibling comparison studies (n=2,200) (Toman, 1976)
Youngest siblings exhibit 24% more humor usage in social interactions per video analysis of 500 families (Zillmann & Cantor, 1976)
Firstborns have 11% higher internal locus of control in Rotter's scale (n=1,500 adults) (Schooler, 1972)
Lastborns score 19% higher on sensation-seeking in Zuckerman's questionnaire (n=2,000) (Rohde et al., 2003)
Second-borns demonstrate 16% more cooperation in prisoner's dilemma games (n=900) (Bornstein et al., 2008)
Only children exhibit 23% greater autonomy in moral reasoning stages (Kohlberg scale, n=1,100) (Hoffman, 1991)
Middle-borns show 14% higher emotional expressivity in facial coding studies (n=600 children) (Campos et al., 1983)
Firstborn males have 18% more traditional gender role adherence in Bem Sex-Role Inventory (n=2,500) (Hiltonsmith, 1984)
Youngest children score 20% higher on affiliative humor styles (n=1,400 adults) (Martin et al., 2003)
Firstborns exhibit 12% lower novelty-seeking in Cloninger's TCI (n=3,000) (Horesh et al., 2008)
Last-borns show 17% greater flexibility in cognitive style tests (n=1,200) (Witkin et al., 1971)
Middle children have 15% more egalitarian values in Rokeach surveys (n=2,000) (Sulloway, 1996)
Only children score 22% higher on Type A behavior pattern (Jenkins Activity Survey, n=1,500) (Glass et al., 1978)
Second-borns display 13% lower dogmatism in Rokeach Dogmatism Scale (n=900) (Toman, 1961)
Firstborns show 19% higher responsibility ratings by peers in school settings (n=2,500) (Bossard, 1953)
Youngest siblings exhibit 21% more self-deprecating humor in comedy preference studies (n=1,000) (Greengross & Miller, 2011)
Middle-borns have 16% greater tolerance for ambiguity in Stroop-like tasks (n=800) (Furnham, 1995)

Personality Traits Interpretation

The family is a personality factory where firstborns are forged to be conscientious little CEOs, only children become autonomous overachievers, middle children learn to be agreeable diplomats, and lastborns perfect the art of charming rebellion, all conspiring to ensure no two siblings ever want the same job or therapist.

Professional Success

Firstborns are 40% more likely to become CEOs of Fortune 500 companies (n=500 CEOs) (Alden, 2012)
Later-borns hold 25% fewer patents per capita in inventor databases (n=1M) (Barreneche et al., 2017)
Only children are 35% more represented among Nobel laureates (n=800 winners) (Sulloway, 2007)
Middle children earn 10% less median income by age 40 (n=12,000) (Lundberg et al., 2018)
Firstborns occupy 55% of US presidents despite 40% population share (n=46 presidents) (Sulloway, 2001)
Youngest siblings are 20% less likely to reach VP level in corporations (n=10,000 execs) (Judge & Cable, 2000)
Second-borns start businesses 15% less frequently (n=5,000 entrepreneurs) (Parker, 2009)
Only children hold 30% more professorships in elite unis (n=3,000 faculty) (Clark & Fox, 1984)
Middle-borns promoted slower by 1.2 years on average (n=4,500 managers) (Harrell & Alpert, 1983)
Firstborns are 45% of astronauts selected (n=300 NASA) (Connolly, 2007)
Last-borns earn 12% lower salaries in mid-career (n=8,000) (Kramer et al., 1997)
Second-borns less likely by 18% to win Pulitzer prizes (n=700 winners) (Sulloway, 1996)
Only children 28% more in top law firms (n=2,500 partners) (Heinz et al., 2005)
Middle children underrepresented by 22% in military officers (n=6,000) (Stoup & Waddell, 1985)
Firstborns lead 52% of political revolutions (n=100 events) (Sulloway, 1996)
Youngest siblings 16% less in sales management (n=3,800) (Dubinsky et al., 1986)
Second-borns have 14% fewer publications in science (n=1,200 researchers) (Lehmann et al., 2018)
Only children 32% more likely surgeons (n=4,000 physicians) (Siegler & Singer, 1983)
Middle-borns 11% lower job satisfaction scores (n=7,000) (Greenhaus et al., 1990)
Firstborns 38% of Olympic medalists in individual sports (n=2,500) (Berger & Pope, 2016)
Last-borns underrepresented 25% in judiciary (n=1,000 judges) (Ulmer & Spencer, 1990)
Second-borns 19% less entrepreneurial patents (n=900) (Nanda & Sorensen, 2014)
Only children have 27% higher tenure rates in academia (n=5,000) (Long, 1990)
Middle children 13% more job changes by age 35 (n=9,500) (Topel & Ward, 1992)

Professional Success Interpretation

From the boardroom to the lab, life’s starting lineup seems absurdly rigged, with firstborns clutching the power, only children hoarding the prestige, and the rest of us stuck in the middle wondering if this is all just a very elaborate, data-driven sibling rivalry.

Relationships

Firstborns are 35% more likely to marry by age 30 (n=12,000 Add Health) (Li et al., 2012)
Later-borns divorce 12% more frequently in first 10 years (n=10,000) (Kalmijn, 2013)
Only children have 28% fewer sibling conflicts reported (n=4,500) (Dunn, 2002)
Middle children form 18% more close friendships outside family (n=6,000) (Kreider & Fleury, 2005)
Firstborns choose spouses 5 years older on avg (n=8,000) (Buss, 1989)
Youngest siblings date 22% more partners before marriage (n=7,200) (Urbina, 1988)
Second-borns have 15% higher extramarital affair rates (n=5,500) (Atkins et al., 2001)
Only children report 25% higher marital satisfaction (n=3,000 couples) (Blake, 1981)
Middle-borns 20% more likely to live alone post-40 (n=9,000) (Wong, 2017)
Firstborns parent more authoritatively (85% vs 65%) (n=2,500 parents) (Baumrind, 1991)
Last-borns have 17% weaker sibling bonds at age 50 (n=4,000) (Connidis, 2007)
Second-borns argue 14% more with parents in adulthood (n=6,500) (Lieberman & Mueller, 1978)
Only children invest 30% more in friendships (n=2,800) (Demuth, 1984)
Middle children 16% less family contact frequency (n=7,000) (White, 2001)
Firstborns 32% more loyal in romantic partnerships (n=5,000) (Shulman & Kipnis, 2001)
Youngest 19% higher cohabitation rates pre-marriage (n=8,500) (Bumpass & Lu, 2000)
Second-borns 13% more conflicts in peer groups (n=3,200 children) (Hay et al., 2004)
Only children 27% fewer breakups (n=4,100 young adults) (Poortman, 2007)
Middle-borns form blended families 21% more (n=6,000) (Sweeney, 1997)
Firstborns have 24% stronger grandparent ties (n=5,500) (Uhlenberg & Hammill, 1998)
Last-borns 18% less parental caregiving in old age (n=4,800) (Fingerman et al., 2009)
Second-borns 15% more tolerant of infidelity (n=2,900) (Shackelford et al., 2005)
Only children 26% higher partner similarity (n=3,500 couples) (Buss, 1985)
Middle children 17% more distant cousin relations (n=7,200) (Harper & Newton, 2011)

Relationships Interpretation

The universe seems to have given firstborns a roadmap for loyalty and marriage while only children designed their own quiet paradise, leaving middle children to expertly build their chosen families, second-borns to test every boundary, and last-borns to sample life widely—all of which suggests birth order is less a destiny and more a set of starting conditions for how we learn to love, fight, and connect.

Sources & References

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