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Related Experiment Video

Updated: Jul 2, 2026

Use of a Video Scoring Anchor for Rapid Serial Assessment of Social Communication in Toddlers

Use of a Video Scoring Anchor for Rapid Serial Assessment of Social Communication in Toddlers

Published on: March 14, 2018

Twin-singleton differences in intelligence: a meta-analysis.

Martin Voracek¹, Tanja Haubner

¹Department of Basic Psychological Research, School of Psychology, University of Vienna, Vienna, Austria. martin.voracek@univie.ac.at

Psychological Reports

|September 4, 2008

Summary

This summary is machine-generated.

Twin studies reveal intelligence differences compared to singletons. On average, twins score 4.2 IQ points lower, a gap narrowing in recent birth cohorts.

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Using Cholesky Decomposition to Explore Individual Differences in Longitudinal Relations between Reading Skills

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Using Cholesky Decomposition to Explore Individual Differences in Longitudinal Relations between Reading Skills

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Published on: September 17, 2019

Area of Science:

Behavioral Genetics
Developmental Psychology
Psychometrics

Background:

The question of intelligence differences between twins and singletons has persisted since the advent of twin studies in the 1920s.
Previous research has yielded varied results, necessitating a comprehensive synthesis of existing data.

Purpose of the Study:

To conduct a meta-analysis to quantitatively assess the intelligence difference between twins and singletons.
To investigate factors influencing this difference, including birth cohort and potential causes.

Main Methods:

Meta-analysis of published studies comparing intelligence test scores of twins and singletons.
Inclusion of studies from six countries, encompassing over 30,000 twins and 1.6 million singletons.
Analysis of birth cohorts spanning most of the 20th century, with a focus on children and adolescents.

Main Results:

Twins, on average, scored 4.2 IQ points lower than singletons, a difference less than one-third of a standard deviation.
A significant decline in this IQ difference was observed between earlier (5.1 IQ points) and more recent birth cohorts (0.5 IQ points).
The effect was robust, generalizable across various intelligence domains, and not attributable to publication bias or influential studies.

Conclusions:

A small but consistent IQ deficit exists for twins compared to singletons, likely attributable to prenatal and perinatal factors such as reduced fetal growth and shorter gestation.
The intelligence gap between twins and singletons has diminished in more recent birth cohorts.
Further research is needed to determine if this effect persists across the lifespan or within families.