Part II    Psychodynamic Theories98
of gender, than laterborns (Barclay, 2015; Booth & Kee, 2008; de Haan, 2005; Eck -
stein et al., 2010; Lehmann, Nuevo-Chiquero, & Vidal-Fernandez, 2018; Paulhus, 
Trapnell, & Chen, 1999). An interesting study involving adopted versus non-adopted 
children examined the question of whether biological or social factors explained 
the relationship between birth order and educational attainment (Barclay, 2015). 
Because both adopted and non-adopted firstborns attained higher educational levels, 
Barclay argued this shows that the association is more social and family dynamic 
than biological in origin. Similarly, Lehmann and colleagues (2018) found that the 
birth order effect on educational attainment is due in large part to increased cognitive 
stimulation by the mother for firstborn children. 
Similarly, there is a small but consistent association showing that, regardless 
of gender, firstborns tend to score a few points higher than laterborns on IQ tests 
(Damian & Roberts, 2015; Rohrer, Eggloff, & Schmulke, 2015, 2017). To elaborate, 
Roher and colleagues (2015) examined more than 20,000 participants in national 
samples from the United States, Great Britain, and Germany and found small but 
robust effects with intelligence (IQ) and intellect. Intellect was measured with self-
reported questions such as “I am quick to understand things,” or “I am someone 
who is eager for knowledge.” In another large-scale national sample, Damian and 
Roberts (2015) analyzed longitudinal data from more than 260,000 U.S. high school 
students, excluding only children and twins. They also assessed age, sex, number of 
siblings, family structure (two- or one-parent houses) and parental socioeconomic sta -
tus to rule those variables out as explanations. Birth order was coded either as “first” 
or “later.” Damian and Roberts found very small to non-existent relationships (aver -
age r = .02) between birth order and self-rated personality traits such as sociability, 
impulsivity, calmness, self-confidence, and leadership. Differences in age, gender, the 
family structure, socio-economic status, and the number of siblings did not affect the 
results. The relationship with intelligence, however, was larger but still small (average 
r = .08), with firstborns having slightly higher IQ scores on average than laterborns. 
Moreover, research into the so-called “Big Five” dimensions of personality 
(Extraversion, Neuroticism, Openness, Agreeableness, and Conscientiousness) and 
birth order consistently reports small to no relationships (Bleske-Rechek & Kelley, 
2014; Damian & Roberts, 2015; Mozammil et al., 2018; Roher et al., 2015). In other 
words, Adler’s theory that firstborns should be highest in anxiety (Neuroticism) does 
not find support. A few studies, however, have supported Adler’s theory that later -
born children should be more socially interested or pro-social (Paulhus et al., 1999; 
Salmon, Cuthbertson, & Figuerdo, 2016).
Although research finds little effect of birth order on the five main dimensions 
of personality, other researchers have reported significant relationships with more 
specific aspects of personality, most notably rebelliousness (Paulhus et al., 1999). 
In 1996, for example, Frank Sulloway published  Born to Rebel: Birth Order, Family 
Dynamics and Creative Lives , in which he presented an evolutionary argument for 
birth order effects on personality. Siblings, he argued, compete for an important and 
often scarce resource: parental affection and attention. Children’s success in this com -
petition reflects strategies that impact their personalities, and our birth order position 
predicts these strategic personality traits. Lending support to Adler’s theory, Sulloway 
proposed that firstborns are likely to be achievement-oriented, anxious, and conform -
ist whereas laterborns tend to be more adventurous, open to experience, innovative, 
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