Poster #17 - Birth Order and Intelligence in Young Adulthood: A Register-Based Study on the Danish Conscription Register

• In Event: Poster Session 09

Abstract

Background: Several early-life factors influence children’s cognitive development and subsequent intelligence, which is a strong predictor of various social and health-related outcomes (Guez, 2021). Birth order has been proposed to influence intelligence later in life. However, current research on birth order and intelligence is characterized by disagreements on the nature of the association and the mechanisms explaining the apparent negative association between birth order and intelligence. A biological explanation suggests that maternal antibody levels tend to accumulate with each subsequent pregnancy, thus increasing the risk of immune attack upon the fetal brain with increasing birth order (Gualtieri & Hicks, 1985). Contrary, another hypothesis suggests that differences in social stimulation and access to parental resources cause an association between birth order and intelligence (Zajonc, 1975).

Aim: To investigate the association between birth order and intelligence, including whether the biological or social hypothesis can explain the potential association, in a large, representative Danish cohort.

Data: The study was based on Danish register data, with data on intelligence test scores (Børge Priens Prøve) drawn from the Danish Conscription Registry Data for men invited to draft board examinations in Denmark from 2006 to 2019 (N=461,920).

Methods: To assess the hypotheses, the analyses differentiate between men’s biological birth order and social order within the family. Biological birth order refers to the sequence in which children are born, including stillbirths and children who died in infancy. The social order within the family ranks children from first to last, excluding children who died in infancy. Thus, individuals with deceased elder siblings rank higher in the social order within the family than their biological birth order suggests. Separate effects of both aspects of birth order were estimated using general linear models adjusted for sibship size, parental educational levels, maternal age at birth, birth weight and conscription year. Adjusted IQ means for men defined by both aspects of birth order were estimated using a model with an interaction between biological birth order and social order within the family.

Results: There was a negative association between biological birth order and intelligence in young adulthood. However, the apparent dose-response relationship disappeared after adjusting for the social order of children (Figure 1). Regardless of biological birth order, IQ scores declined with increasing social order, indicating a negative dose-response relationship between social order and intelligence in young adulthood (Figure 2). Men who ranked first in the social order within the family had 1.34, 3.30, and 5.41 IQ points higher intelligence than men who ranked second, third and fourth, respectively (Figure 1).

Conclusion: Men with higher rank in the social order within the family have higher intelligence in young adulthood compared with men who rank lower in the social order, irrespective of biological birth order. Thus, it seems that social dynamics within the family and the order in which children are raised, and not the order in which they are born, influence intelligence. The findings indicate that social stimulation in the early stages of life is critical for children’s cognitive development and subsequent intelligence.

Authors

• Frederik Palmgren Kjaer, University of Copenhagen
  Presenting Author

• Gunhild Tidemann Okholm, University of Copenhagen

• Cathrine Lawaetz Wimmelmann, University of Copenhagen

• Merete Osler, Bispebjerg and Frederiksberg Hospitals

• Erik Lykke Mortensen, University of Copenhagen

• Trine Flensborg-Madsen, University of Copenhagen